Chapter IV: Anatomy1 (2)

FIG. 14.--Chondrocranium of a young Lepidosiren, showing the suspension of the lower jaw by the upper portion of the mandibular arch. (After Agar.)

H, Hyoid arch.
M, Mandibular arch.
o.a, Occipital arch.
ot, Auditory capsule.
q, Quadrate = upper end of mandibular arch.
tr, Trabecula.

The palatopterygoid bar (p.pt) is represented by a faint vestige which disappears before the stage figured.]

(B) _Chondrocranium[28]._--In front of the vertebral column lies the cartilaginous trough, the chondrocranium, which protects the brain. This consists of a praechordal portion--developed out of a pair of lateral cartilaginous rods--the _trabeculae cranii_--and a parachordal portion lying on either side of the anterior end of the notochord. This arises in development from a cartilaginous rod (parachordal cartilage) lying on each side of the notochord and possibly representing a fused row of dorsal arcualia. The originally separate parachordals and trabeculae become connected to form a trough-like, primitive cranium, complete or nearly so laterally and ventrally but open dorsally. With the primitive cranium there are also connected cartilaginous capsules developed round the olfactory and auditory organs. There also become fused with the hinder end of the cranium a varying number of originally distinct neural arches.

_B._ After Gegenbaur, _Untersuchungen zur verg. Anat. der Wirbeltiere_, by permission of Wilhelm Engelmann.

_C._ After Hubrecht, Brown's _Tierreich_, by permission of Gustav Fischer.

FIG. 15.--Chondrocranium, &c. of _Scyllium_ (A), _Notidanus cinereus_ (B) and _Chimaera_ (C).

Br.A, Branchial arches. olf, Olfactory capsule.
c.h, Ceratohyal. ot, Auditory capsule.
e.p.l, Ethmopalatine ligament. p.pt, Palato-pterygoid bar.
Hm, Hyomandibular. p.s.l, Prespiracular ligament.
M, Meckel's cartilage. r, Rostrum.]
o, Orbit.

(C) _Visceral Arches._--The skeleton of the visceral arches consists essentially of a series of half-hoops of cartilage, each divided in the adult into a number of segments and connected with its fellow by a median ventral cartilage. The skeleton of arches I. and II. (mandibular and hyoidean) undergoes modifications of special interest (figs. 14 and 15). The lower portion of the mandibular arch becomes greatly thickened to support the lower or hinder edge of the mouth. It forms the primitive lower jaw or "Meckel's cartilage." Dorsal to this an outgrowth arises from the anterior face of the arch which supports the upper or anterior margin of the mouth: it is the primitive upper jaw or palato-pterygoquadrate cartilage. The portion of the arch dorsal to the palato-pterygo-quadrate outgrowth may form the suspensorial apparatus of the lower jaw, being fused with the cranium at its upper end. This relatively primitive con-arrangement (_protostylic_, as it may be termed) occurs in Dipneusti among fishes (cf. fig. 14). More usually this dorsal part of the mandibular arch becomes reduced, its place being occupied by a ligament (pre-spiracular) uniting the jaw apparatus to the chondrocranium, the upper jaw being also attached to the chondrocranium by the ethmopalatine ligament situated more anteriorly. The main attachment, however, of the jaws to the chondrocranium in such a case, as holds for the majority of fishes, is through the enlarged dorsal segment of the hyoid arch (hyomandibular) which articulates at its dorsal end with the chondrocranium, while its ventral end is attached to the hinge region of the jaw by stout ligamentous bands. A skull in which the jaws are suspended in this manner is termed a hyostylic skull (e.g. _Scyllium_ in fig. 15).

In _Notidanus_ (fig. 15, B) there is a large direct articulation of
the upper jaw to the chondrocranium in addition to the indirect one
through the hyomandibular: such a skull is amphistylic. In
_Heterodontus_ the upper jaw is firmly bound to the cranium throughout
its length, while in Holocephali (fig. 15, C) complete fusion has
taken place, so that the lower jaw appears to articulate directly with
the cranium ("auto stylic" condition). In Dipneusti[29] (_Lepidosiren_
and _Protopterus_) the cartilaginous upper jaw never develops (except
in its hinder quadrate portion) beyond the condition of a faint
rudiment, owing doubtless to its being replaced functionally by
precociously developed bone.

(D) _Appendicular Skeleton._--The skeleton of the free part of the limb is attached to the limb girdle which lies embedded in the musculature of the body. Each limb girdle is probably to be looked upon as consisting, like the skeleton of the visceral arches, of a pair of lateral half-hoops of cartilage. While in _Pleuracanthus_ the lateral halves are distinct (and segmented like the branchial arches), in living Selachians generally the two halves are completely fused ventrally with one another. The part of the girdle lying dorsal to the articulation of the limb is termed scapular in the case of the pectoral limb, iliac in the case of the pelvic, while the ventral portions are known respectively as coracoid and ischio-pubic.

In most Teleostomes the primitive pelvic girdle does not develop; in
the Dipneusti it is represented by a median unpaired cartilage.

The skeleton of the free limb is probably seen in its most archaic form amongst existing fishes in the biserial archipterygium of _Ceratodus_ (fig. 16). This is indicated by the relative predominance of this type of fin amongst the geologically more ancient fishes. The biserial archipterygium consists of a segmented axial rod, bearing a praeaxial and a postaxial series of jointed rays.

In _Protopterus_ and _Lepidosiren_ the limbs are reduced and the
lateral rays have less (_Protopterus_) or more (_Lepidosiren_)
completely disappeared.

FIG. 18.--Skeleton of Pectoral Limb of _Polypterus_. a, 30 mm. larva. b, Adult.]

FIG. 19.--Skeleton of Pectoral Fin of _Amia_.]

In such an archaic Selachian as _Pleuracanthus_ the fin is clearly of the biserial archipterygial type, but the lateral rays are reduced (pectoral) or absent (pelvic) (fig. 17, a) on one side of the axis. In a typical adult Selachian the pectoral fin skeleton has little apparent resemblance to the biserial archipterygium--the numerous outwardly directed rays springing from a series of large basal cartilages (_pro_-, _meso_- and _metapterygium_). The condition in the young (e.g. fig. 17, b, _Acanthias_) hints strongly, however, at the possibility of the fin skeleton being really a modified biserial archipterygium, and that the basal cartilages represent the greatly enlarged axis which has become fixed back along the side of the body. In Crossopterygians (_Polypterus_) the highly peculiar fin skeleton (fig. 18) while still in the embryonic cartilaginous stage is clearly referable to a similar condition. In the Actinopterygians--with the increased development of dermal fin rays--there comes about reduction of the primitive limb skeleton. The axis becomes particularly reduced, and the fin comes to be attached directly to the pectoral girdle by a number of basal pieces (Teleosts) probably representing vestigial rays (cf. fig. 19).

Views on the general morphology of the fin skeleton are strongly
affected by the view held as to the mode of evolution of the fins. By
upholders of the lateral fold hypothesis the type of fin skeleton
described for _Cladoselache_[30] is regarded as particularly
primitive. It is, however, by no means clear that the obscure basal
structures figured (Fig. 20) in this fin do not really represent the
pressed back axis as in _Pleuracanthus_.

The pelvic fin skeleton, while built obviously on the same plan as the pectoral, is liable to much modification and frequently degeneration.

FIG. 20.--Skeleton of Pectoral Fin of _Cladoselache_.]

_Osseous or Bony Skeleton._--The most ancient type of bony skeleton appears to be represented in the _placoid_ elements such as are seen in the skin of the Selachian (fig. 21). Each placoid element consists of a spine with a broadly expanded base embedded in the dermis. The base is composed of bone: the spine of the somewhat modified bone known as dentine. Ensheathing the tip of the spine is a layer of extremely hard enamel formed by the inner surface of the ectoderm which originally covered it. Such typical placoid scales are well seen on any ordinary skate. In the groups of fishes above the Selachians, the coating of placoid elements shows various modifications. The spines disappear, though they may be present for a time in early development. The bony basal plates tend to undergo fusion--in certain cases they form a continuous bony cuirass (various Siluroids, trunk-fishes) formed of large plates jointed together at their edges. More usually the plates are small and regular in size. In Crossopterygians and _Lepidosteus_ and in many extinct forms the scales are of the ganoid type, being rhomboidal and having their outer layer composed of hard glistening ganoine. In other Teleostomes the scales are as a rule thin, rounded and overlapping--the so-called cycloid type (fig. 22, A); where the posterior edge shows toothlike projections the scale is termed ctenoid (fig. 22, B). In various Teleosts the scales are vestigial (eel); in others (as in most electric fishes) they have completely disappeared.

_Teeth._--Certain of the placoid elements belonging to that part of the skin which gives rise to the lining of the stomodaeum have their spines enlarged or otherwise modified to form teeth. In the majority of fishes these remain simple, conical structures: in some of the larger sharks (_Carcharodon_) they become flattened into trenchant blades with serrated edges: in certain rays (_Myliobatis_) they form a pavement of flattened plates suited for crushing molluscan shells. In the young _Neoceratodus_[31] there are numerous small conical teeth, the bases of which become connected by a kind of spongework of bony trabeculae. As development goes on a large basal mass is formed which becomes the functional tooth plate of the adult, the original separate denticles disappearing completely. In the other two surviving Dipnoans, similar large teeth exist, though here there is no longer trace in ontogeny of their formation by the basal fusion of originally separate denticles. In the Selachians the bony skeleton is restricted to the placoid elements. In the Teleostomes and the Dipnoans the original cartilaginous skeleton becomes to a great extent unsheathed or replaced by bony tissue. It seems highly probable that the more deeply seated osseous elements occurring in these as in the higher groups arose in the course of evolution by the spreading inwards of bony trabeculae from the bases of the placoid elements. Such a method has been demonstrated as occurring in individual development in the case of certain of the more superficially placed bones.[32]

The placoid element with its cap of enamel secreted by the ectoderm is
probably originally derived from a local thickening of the basement
membrane which with the external cuticle may be looked on as the most
ancient skeletal structure in the Metazoa. The basal plate appears to
have been a later development than the spine; in the palaeozoic
_Coelolepidae_[33] the basal plate is apparently not yet developed.

Only a brief summary can be given here of the leading features in the osteology of fishes. Care must be taken not to assume that bony elements bearing the same name in fishes and in other groups, or even in the various sub-divisions of the fishes, are necessarily strictly homologous. In all probability bony elements occupying similar positions and described by the same anatomical name have been evolved independently from the ancestral covering of placoid elements.

_Teleostei._--It will be convenient to take as the basis of our description the bony skeleton of such a Teleostean fish as the salmon. In the vertebral column all the cartilaginous elements are replaced by bone. The haemal spines of the turned-up tip of the tail are flattened (hypural bones) and serve to support the caudal fin rays.

In _Argyropelecus_ and in one or two deep-sea forms the vertebral column remains cartilaginous.

FIG. 23.--One of the radialia of the salmon, consisting of three segments, ptg¹, ptg², ptg³, and supporting a dermal fin ray. _D.F.R._]

Apart from the ossification of the radialia which takes place in the adults of bony fishes there exist special supporting structures in the fins (paired as well as median) of all the gnathostomatous fishes and apparently in nature independent of the cartilaginous skeleton. These are known as dermal fin-rays.[34] Morphologically they are probably to be looked on (like placoid elements) as local exaggerations of the basement membrane.

In their detailed characters two main types of dermal fin-ray may be
recognized. The first of these are horny unjointed rays and occur in
the fins of Selachians and at the edge of the fins of Teleostomes
(well seen in the small posterior dorsal or "adipose" fin,
particularly in Siluroids). The second type of dermal fin-ray is
originally arranged in pairs and forms the main supports of the fin in
the adult Teleost (fig. 23). The members of each pair are in close
contact except proximally where they separate and embrace the tip of
one of the radialia. The fin-rays of this second type are frequently
branched and jointed: in other cases they form unbranched rigid
spines.

In the angler or fishing-frog (_Lophius_) the anterior rays of the
dorsal fin become greatly elongated to form small fishing-rods, from
which depend bait-like lures for the attraction of its prey.

FIG. 24.--Chondrocranium of Salmon, seen from the right side.

alsph, Alisphenoid. orbsph, Orbitosphenoid.
basocc, Basioccipital. proot, Prootic.
ekteth, Lateral ethmoid. psph, Parasphenoid.
epiot, Epiotic. ptero, Pterotic.
exocc, Exoccipital. socc, Supra occipital.
fr, Frontal. sphot, Sphenotic.
opisth, Opisthotic. vo, Vomer.]

In the skull of the adult salmon it is seen that certain parts of the chondrocranium (fig. 24) have been replaced by bone ("cartilage bones") while other more superficially placed bones ("membrane bones") cover its surface (fig. 25). Of cartilage bones four are developed round the foramen magnum--the basioccipital, supraoccipital and two exoccipitals. In front of the basioccipital is the basisphenoid with an alisphenoid on each side. The region (presphenoidal) immediately in front of the basisphenoid is unossified, but on each side of it an orbitosphenoid is developed, the two orbitosphenoids being closely approximated in the mesial plane and to a certain extent fused, forming the upper part of the interorbital septum. In the anterior or ethmoidal portion of the cranium the only cartilage bones are a pair of lateral ethmoids lying at the anterior boundary of the orbit. A series of five distinct elements are ossified in the wall of the auditory or otic capsule, the prootic and opisthotic more ventrally, and the sphenotic, pterotic and epiotic more dorsally. The roof of the cranium is covered in by the following dermal bones--parietals (on each side of the supraoccipital), frontals, dermal ethmoid and small nasals, one over each olfactory organ. The floor of the cranium on its oral aspect is ensheathed by the large parasphenoid and the smaller vomer in front of and overlapping it. The cartilaginous lower jaw is ossified posteriorly to form the articular (fig. 25) with a small membrane bone, the angular, ventral to it, but the main part of the jaw is replaced functionally by a large membrane bone which ensheaths it--the dentary--evolved in all probability by the spreading outwards of bony tissue from the bases of the placoid elements (teeth) which it bears. The original upper jaw (palatopterygoid bar) is replaced by a chain of bones--palatine in front, then pterygoid and mesopterygoid, and posteriorly metapterygoid and quadrate, the latter giving articulation to the articular bone of the lower jaw. These representatives of the palatopterygoid bar no longer form the functional upper jaw. This function is performed by membrane bones which have appeared external to the palatopterygoid bar--the premaxilla and maxilla--which carry teeth--and the small scale-like jugal behind them. The quadrate is suspended from the skull as in the Selachians (hyostylic skull) by the upper portion of the hyoid arch--here represented by two bones--the hyomandibular and symplectic. The ventral portion of the hyoid arch is also represented by a chain of bones (stylohyal, epihyal, ceratohyal, hypohyal and the ventral unpaired basihyal), as is also each of the five branchial arches behind it. In addition to the bony elements belonging to the hyoid arch proper a series of membrane bones support the opercular flap. Ventrally there project backwards from the ceratohyal a series of ten overlapping branchiostegal rays, while more dorsally are the broader interopercular, subopercular and opercular.

FIG. 25.--Complete Skull of Salmon from left side.

art, Articular. op, Opercular.
branchiost, Branchiostegal. pal, Palatine.
dent, Dentary. par, Parietal.
epiot, Epiotic. pmx, Premaxilla.
eth, Dermal ethmoid. preop, Preopercular.
fr, Frontal. pt, Pterygoid.
hyom, Hyomandibular. pter, Pterotic.
intop, Interopercular. Quad, Quadrate.
Jug, Jugal. socc, Supraoccipital.
mpt, Mesopterygoid. sphot, Sphenotic.
mtpt, Metapterygoid. subop, Subopercular.
mx, Maxilla. sympl, Symplectic.
nas, Nasal. Zunge, Tongue.]

In addition to the bones already enumerated there is present a ring of circumorbital bones, a preopercular, behind and external to the hyomandibular and quadrate, and squamosal, external to the hinder end of the auditory capsule.

In the salmon, pike, and various other Teleosts, extensive regions of
the chondrocranium persist in the adult, while in others (e.g. the
cod) the replacement by bone is practically complete. Bony elements
may be developed in addition to those noticed in the salmon.

In the sturgeon the chondrocranium is ensheathed by numerous membrane
bones, but cartilage bones are absent. In the Crossopterygians[35] the
chondrocranium persists to a great extent in the adult, but portions
of it are replaced by cartilage bones--the most interesting being a
large sphenethmoid like that of the frog. Numerous membrane bones
cover the chondrocranium externally. In the Dipneusti[36] the
chondrocranium is strengthened in the adult by numerous bones. One of
the most characteristic is the great palatopterygoid bone which
develops very early by the spreading of ossification backwards from
the tooth bases, and whose early development probably accounts for the
non-development of the palatopterygoid cartilage.

_Appendicular Skeleton._--The primitive pectoral girdle, which in the Dipneusti is strengthened by a sheath of bone, becomes in the Teleostomes reduced in size (small scapula and coracoid bones) and replaced functionally by a secondary shoulder girdle formed of superficially placed membrane bones (supraclavicular and cleithrum or "clavicle," with, in addition in certain cases, an infraclavicular and one or two postclavicular elements), and connected at its dorsal end with the skull by a post-temporal bone.

The pelvic girdle is in Teleostomes completely absent as a rule.

The skeleton of the free limb undergoes ossification to a less or greater extent in the Teleostomes.

In _Polypterus_ the pectoral fin (fig. 18, B) shows three
ossifications in the basal part of the fin--pro-, meso- and
metapterygium. Of these the metapterygium probably represents the
ossified skeletal axis: while the propterygium and also the numerous
diverging radials probably represent the lateral rays of one side of
the archipterygium.

In the _Teleostomes_ the place of the pelvic girdle is taken
functionally by an element apparently formed by the fusion of the
basal portions of several radials.

_Vascular System._--The main components of the blood vascular system in the lower vertebrates are the following: (1) a single or double dorsal aorta lying between the enteron and notochord; (2) a ventral vessel lying beneath the enteron; and (3) a series of paired hoop-like aortic arches connecting dorsal and ventral vessels round the sides of the pharynx. The blood-stream passes forwards towards the head in the ventral vessel, dorsalwards through the aortic arches, and tailwards in the dorsal aorta.

The dorsal aorta is single throughout the greater part of its extent, but for a greater or less extent at its anterior end (_circulus cephalicus_) it consists of two paired aortic roots. It is impossible to say whether the paired or the unpaired condition is the more primitive, general morphological conditions being in favour of the latter, while embryological evidence rather supports the former. The dorsal aorta, which receives its highly oxygenated blood from the aortic arches, is the main artery for the distribution of this oxygenated blood. Anteriorly the aortic roots are continued forwards as the dorsal carotid arteries to supply the head region. A series of paired, segmentally-arranged arteries pass from the dorsal aorta to supply the muscular body wall, and the branches which supply the pectoral and pelvic fins (subclavian or brachial artery, and iliac artery) are probably specially enlarged members of this series of segmental vessels. Besides these paired vessels a varying number of unpaired branches pass from dorsal aorta to the wall of the alimentary canal with its glandular diverticula (coeliac, mesenteric, rectal).

The ventral vessel undergoes complicated changes and is represented in the adults of existing fishes by a series of important structures. Its post-anal portion comes with the atrophy of the post-anal gut to lie close under the caudal portion of the dorsal aorta and is known as the caudal vein. This assumes a secondary connexion with, and drains its blood into, the posterior cardinal veins (see below). In the region between cloaca and liver the ventral vessel becomes much branched or even reticular and--serving serving to convey the food-laden blood from the wall of the enteron to the capillary network of the liver--is known as the hepatic portal vein. The short section in front of the liver is known as the hepatic vein and this conveys the blood, which has been treated by the liver, into a section of the ventral vessel, which has become highly muscular and is rhythmically contractile. This enlarged muscular portion, in which the contractility--probably once common to the main vessels throughout their extent--has become concentrated, serves as a pump and is known as the heart. Finally the precardiac section of the ventral vessel--the ventral aorta--conveys the blood from heart to aortic arches.

In addition to the vessels mentioned a large paired vein is developed in close relation to the renal organ which it serves to drain. This is the posterior cardinal. An anterior prolongation (anterior cardinal) serves to drain the blood from the head region. From the point of junction of anterior and posterior cardinal a large transverse vessel leads to the heart (_ductus Cuvieri_).

Fig. 26.--Diagram to illustrate the condition of the Conus in an Elasmobranch (A), _Amia_ (B) and a typical Teleost (C).

a, Atrium. v,v´, Valves.
b.a, Bulbus aortae. v.a, Ventral aorta.
c.a, Conus arteriosus. vt, Ventricle.]
s.v, Sinus venosus.

_Heart._--Originally a simple tube curved into a somewhat S-shape, the heart, by enlargements, constrictions and fusions of its parts, becomes converted into the complex, compact heart of the adult. In this we recognize the following portions--(1) _Sinus venosus_, (2) _Atrium_, (3) _Ventricle_. A fourth chamber, the _conus arteriosus_, the enlarged and contractile hinder end of the ventral aorta, is also physiologically a part of the heart. The sinus venosus receives the blood from the great veins (ductus Cuvieri and hepatic veins). It--like the atrium which it enters by an opening guarded by two lateral valves--has thin though contractile walls. The atrium is as a rule single, but in the Dipnoans, in correlation with the importance of their pulmonary breathing, it is incompletely divided into a right and a left auricle. In Neoceratodus the incomplete division is effected by the presence of a longitudinal shelf projecting into the atrial cavity from its posterior wall. The opening of the sinus venosus is to the right of this shell, that of the pulmonary vein to the left. In _Prototerus_ and _Lepidosiren_ a nearly complete septum is formed by the fusion of trabeculae, there being only a minute opening in it posteriorly. The atrium opens by a wide opening guarded by two or more flap valves provided with chordae tendineae into the ventricle.

The ventricle, in correspondence with it being the main pumping apparatus, has its walls much thickened by the development of muscular trabeculae which, in the lower forms separated by wide spaces in which most of the blood is contained, become in the Teleostomes so enlarged as to give the wall a compact character, the spaces being reduced to small scattered openings on its inner surface. In the Dipnoans the ventricle, like the atrium, is incompletely divided into a right and left ventricle. In _Ceratodus_ this is effected by an extension of the interauricular shelf into the ventricle. In _Lepidosiren_ the separation of the two ventricles is complete but for a small perforation anteriorly, the heart in this respect showing a closer approximation to the condition in the higher vertebrates than is found in any Amphibians or in any reptiles except the Crocodilia. The conus arteriosus is of interest from the valvular arrangements in its interior to prevent regurgitation of blood from ventral aorta into ventricle. In their simplest condition, as seen e.g. in an embryonic Selachian, these arrangements consist of three, four or more prominent longitudinal ridges projecting into the lumen of the conus, and serving to obliterate the lumen when jammed together by the systole of the conus. As development goes on each of these ridges becomes segmented into a row of pocket valves with their openings directed anteriorly so that regurgitation causes them to open out and occlude the lumen by their free edges meeting. Amongst the Teleostomes the lower ganoids show a similar development of longitudinal rows of valves in the conus. In _Amia_ (fig. 26, B), however, the conus is shortened and the number of valves in each longitudinal row is much reduced. This leads to the condition found in the Teleosts (fig. 26, O), where practically all trace of the conus has disappeared, a single circle of valves representing a last survivor of each row (save in a few exceptional cases, e.g. _Albula_, _Tarpen_, _Osteoglossum_, where two valves of each row are present).

FIG. 27.--Venous System of _Protopterus_, as seen from ventral side.

a, Atrium. k, Kidney.
ac, Anterior cardinal. l, Liver.
an.v, Anastomotic vein. ov.v, Ovarian veins.
c, Intestine. p, Pericardium.
c.v, Caudal vein. p.c.v, Left posterior cardinal.
f.v, Femoral vein. p.v´, Parietal veins.
g.b, Gall-bladder. r.p.v, Renal portal.
h.v, Hepatic vein. s, Stomach.
i.j.v, Inferior jugular vein. s.b.v, Subclavian.]
i.v.c, Posterior vena cava.

In Front of the conus vestige of the Teleost there is present a thick
walled _bulbus aortae_ differing from the conus in not being
rhythmically contractile, its walls being on the contrary richly
provided with elastic tissue.

The Dipnoans[37] show an important advance on the conus as in atrium and ventricle. The conus has a characteristic spiral twist. Within it in _Neoceratodus_ are a number of longitudinal rows of pocket valves. One of these rows is marked out by the very large size of its valves and by the fact that they are not distinct from one another but even in the adult form a continuous, spirally-running, longitudinal fold. This ridge projecting into the lumen of the conus divides it incompletely into two channels, the one beginning (i.e. at its hinder end) on the _left_ side and ending in front _ventrally_, the other beginning on the _right_ and ending _dorsally_. In _Protopterus_ a similar condition occurs, only in the front end of the conus a second spiral fold is present opposite the first and, meeting this, completes the division of the conus cavity into two separate parts. The rows of pocket valves which do not enter into the formation of the spiral folds are here greatly reduced.

These arrangements in the conus of the Dipnoans are of the highest morphological interest, pointing in an unmistakable way towards the condition found in the higher lung-breathing vertebrates. Of the two cavities into which the conus is partially divided in the Dipneusti the one which begins posteriorly on the right receives the (venous) blood from the right side of the heart, and ending up anteriorly dorsal to the other cavity communicates only with aortic arches V. and VI. In the higher vertebrates this cavity has become completely split off to form the root of the pulmonary arteries, and a result of aortic arch V. receiving its blood along with the functionally much more important VI. (the pulmonary arch) from this special part of the conus has been the almost complete disappearance of this arch (V.) in all the higher vertebrates.

_Arterial System._--There are normally six aortic arches laid down corresponding with the visceral arches, the first (mandibular) and second (hyoidean) undergoing atrophy to a less or greater extent in post-embryonic life. Where an external gill is present the aortic arch loops out into this, a kind of short-circuiting of the blood-stream taking place as the external gill atrophies. As the walls of the clefts assume their respiratory function the aortic arch becomes broken into a network of capillaries in its respiratory portion, and there is now distinguished a ventral afferent and a dorsal efferent portion of each arch. Complicated developmental changes, into which it is unnecessary to enter,[38] may lead to each efferent vessel draining the two sides of a single cleft instead of the adjacent walls of two clefts as it does primitively. In the Crossopterygians and Dipnoans as in the higher vertebrates the sixth aortic arch gives off the pulmonary artery to the lung. Among the Actinopterygians this, probably primitive, blood-supply to the lung (swimbladder) persists only in _Amia_.

a.c.v, Anterior cardinal vein.
d.C, Ductus Cuvieri.
h.v, Hepatic vein.
i.j.v, Inferior jugular vein.
ir.v, Inter-renal vein.
l.v, Lateral cutaneous vein.
p.c.v, Posterior cardinal vein.
p.n, Pronephros.
p.v, Pulmonary vein.
s, Subclavian vein.
s.v, Sinus venosus.
th, Thyroid.
v, Vein from pharyngeal wall.
* Anterior portion of left posterior cardinal vein.]

_Venous System._--The most interesting variations from the general plan outlined have to do with the arrangements of the posterior cardinals. In the Selachians these are in their anterior portion wide and sinuslike, while in the region of the kidney they become broken into a sinusoidal network supplied by the postrenal portion now known as the renal portal vein. In the Teleostomes the chief noteworthy feature is the tendency to asymmetry, the right posterior cardinal being frequently considerably larger than the left and connected with it by transverse anastomotic vessels, the result being that most of the blood from the two kidneys passes forwards by the right posterior cardinal. The Dipnoans (fig. 27) show a similar asymmetry, but here the anterior end of the right posterior cardinal disappears, being replaced functionally by a new vessel which conveys the blood from the right posterior cardinal direct to the sinus venosus instead of to the outer end of the ductus Cuvieri. This new vessel is the posterior vena cava which thus in the series of vertebrates appears for the first time in the Dipneusti.

_Pulmonary Veins._--In _Polypterus_ (fig. 28) the blood is drained from the lungs by a pulmonary vein on each side which unites in front with its fellow and opens into the great hepatic vein behind the heart. In the Dipnoans the conjoined pulmonary veins open directly into the left section of the atrium as in higher forms. In the Actinopterygians with their specialized air-bladder the blood passes to the heart via posterior cardinals, or hepatic portal, or--a probably more primitive condition--directly into the left ductus Cuvieri (_Amia_).

_Lymphatics._--More or less irregular lymphatic spaces occur in the fishes as elsewhere and, as in the Amphibia, localized muscular developments are present forming lymph hearts.

_Central Nervous System._--The neural tube shows in very early stages an anterior dilated portion which forms the rudiment of the brain in contradistinction to the hinder, narrower part which forms the spinal cord. This enlargement of the brain is correlated with the increasing predominance of the nerve centres at the anterior end of the body which tend to assume more and more complete control over those lying behind.

_Spinal Cord._--A remarkable peculiarity occurs in the sun fishes (_Molidae_), where the body is greatly shortened and where the spinal cord undergoes a corresponding abbreviation so as to be actually shorter than the brain.

_Brain._--It is customary to divide the brain into three main regions, fore-, mid-, and hind-brain, as in the most familiar vertebrates there is frequently seen in the embryo a division of the primitive brain dilatation into three vesicles lying one behind the other. A consideration of the development of the brain in the various main groups of vertebrates shows that these divisions are not of equal importance. In those archaic groups where the egg is not encumbered by the presence of a large mass of yolk it is usual for the brain to show in its early stages a division into two main regions which we may term the primitive fore-brain or cerebrum and the primitive hind-brain or rhombencephalon. Only later does the hinder part of the primitive fore-brain become marked off as mid-brain. In the fully developed brain it is customary to recognize the series of regions indicated below, though the boundaries between these regions are not mathematical lines or surfaces any more than are any other biological boundaries:--

Rhombencephalon (Hind-brain) / Myelencephalon (Medulla oblongata).
\ Metencephalon (Cerebellum).

/ Mesencephalon (Mid-brain).
Cerebrum (Primitive Fore-brain) < Thalamencephalon (Diencephalon).
\ [Hemispheres (Telencephalon).]

The myelencephalon or medulla oblongata calls for no special remark, except that in the case of _Torpedo_ there is a special upward bulging of its floor on each side of the middle line forming the electric lobe and containing the nucleus of origin of the nerves to the electric organ.

FIG. 29.--Brain of _Scyllium_ (A), _Salmo_ (B) and _Lepidosiren_ (C). The three figures are not drawn to the same scale.

cer, Cerebellum.
c.h, Cerebral hemisphere.
th, Thalamencephalon.
f.b, Primitive fore-brain (in B the line points to the thickened
wall of the fore-brain, the so-called "basal ganglia").
G.p, Pineal body.
m.b, Roof of mid-brain, optic lobes, _tectum opticum_.
o.l, Olfactory lobe.
IV.v, Fourth ventricle.]

The cerebellum occurs in its simplest form in lampreys and Dipnoans (fig. 29, C), where it forms a simple band-like thickening of the anterior end of the roof of the hind-brain. In Selachians it is very large and bulges upwards, forming a conspicuous organ in a dorsal view of the brain (fig. 29, A). In Teleosts (fig. 29, B) the cerebellum is also large. It projects back as a great tongue-like structure over the roof of the fourth ventricle, while in front it dips downwards and projects under the roof of the mid-brain forming a highly characteristic _valvula cerebelli_. A _valvula cerebelli_ occurs also in ganoids, while in the Crossopterygians a similar extension of the cerebellum projects backwards into the IV. ventricle or cavity of the hind-brain (fig. 30).

The mesencephalon is a conspicuous structure in the fishes from its greatly developed roof (_tectum opticum_) which receives the end pencils of the optic nerve. Normally it projects upwards as a pair of large optic lobes, but in the Dipnoans (fig. 29, C) the lateral thickening is not sufficiently great to cause obvious lateral swellings in external view.

a.c, Anterior commissure. par, Paraphysis.
cer, Cerebellum. pin, Pineal body.
d.s, Dorsal sac. p.c, Posterior commissure.
g.h, Habenular ganglion. s.v, Saccus vasculosus.
h.c, Habenular commissure. t.o, Tectum opticum.
i.g, Infundibular gland. v.III, Third ventricle.
l.p, Lateral plexus. v.IV, Fourth ventricle.
o.c, Optic chiasma. vel, Velum transversum.]
pall, Pallium.

The thalamencephalon is one of the most interesting parts of the brain from its remarkable uniformity throughout the Vertebrata. Even in _Amphioxus_ the appearance of a sagittal section strongly suggests vestiges of a once present thalamencephalon.[39] The roof--like that of the myelencephalon--remains to a great extent membranous, forming with the closely applied _pia mater_ a vascular roof to the III. ventricle. Frequently a transverse fold of the roof dips down into the III. ventricle forming the _velum transversum_ (fig. 30).

The side walls of the thalamencephalon are greatly thickened forming the _thalamus_ (epithalamus and hypothalamus), while a ganglionic thickening of the roof posteriorly on each side forms the _ganglia habenulae_ which receive olfactory fibres from the base of the hemisphere. The habenular ganglia are unusually large in the lampreys and are here strongly asymmetrical, the right being the larger.

The floor of the thalamencephalon projects downwards and backwards as the infundibulum. The side walls of this are thickened to form characteristic _lobi inferiores_, while the blind end develops glandular outgrowths (infundibular gland, fig. 30) overlaid by a rich development of blood sinuses and forming with them the _saccus vasculosus_. The optic chiasma, where present, is involved in the floor of the thalamencephalon and forms a large, upwardly-projecting ridge. Farther forwards on the floor or anterior wall is the anterior commissure (see below).

Passing forwards from the mid-brain (cf. fig. 30) a series of interesting structures are found connected with the roof of the primitive fore-brain, viz.--posterior commissure (intercalary region), pineal organ, habenular commissure with anterior parietal organ, dorsal sac (= pineal cushion), _velum transversum_, paraphysis. The posterior commissure is situated in the boundary between thalamencephalon and mid-brain. It is formed of fibres connecting up the right and left sides of the tectum opticum (?). The habenular or superior commissure situated farther forwards connects the two ganglia habenulae. In the immediate neighbourhood of these ganglia there project upwards two diverticula of the brain-roof known as the pineal organ and the parapineal (or anterior parietal) organ. The special interest of these organs[40] lies in the fact that in certain vertebrates one (parapineal in _Sphenodon_ and in lizards) or both (_Petromyzon_) exhibit histological features which show that they must be looked on as visual organs or eyes. In gnathostomatous fishes they do not show any definite eye-like structure, but in certain cases (_Polyodon_, _Callichthys_, &c.) the bony plates of the skull-roof are discontinuous over the pineal organ forming a definite parietal foramen such as exists in lizards where the eye-like structure is distinct. It is also usual to find in the epithelial wall of the pineal organ columnar cells which show club-shaped ends projecting into the lumen (exactly as in the young visual cells of the retina[41]) and are prolonged into a root-like process at the other end. Definite nerve fibres pass down from these parietal organs to the brain. It is stated that the fibres from the pineal organ pass into the posterior commissure, those of the parapineal organ into the habenular commissure.

The facts mentioned render it difficult to avoid the conclusion that these organs either have been sensory or are sensory. Possibly they represent the degenerate and altered vestiges of eye-like organs present in archaic vertebrates, or it may be that they represent the remains of organs not eye-like in function but which for some other reason lay close under the surface of the body. It would seem natural that a diverticulum of brain-tissue exposed to the influence of light-rays should exhibit the same reaction as is shown frequently elsewhere in the animal kingdom and tend to assume secondarily the characters of a visual organ. The presence of the rod-like features in the epithelial cells is perhaps in favour of the latter view. In evolution we should expect these to appear before the camera-like structure of a highly developed eye, while in the process of degeneration we should expect these fine histological characters to go first.

Selachians.--No parapineal organ is present. The pineal body (except
in _Torpedo_ where it is absent) is in the form of a long slender tube
ending in front in a dilated bulb lying near the front end of the
brain in close contact with, or enclosed in, a definite foramen in the
cranial roof.

Holocephali and Crossopterygii.--Here also the pineal body is long and
tubular: at its origin it passes dorsalwards or slightly backwards
behind the large dorsal sac.

Actinopterygian Ganoids resemble Selachians on the whole. In _Amia_ a
parapineal organ is present, and it is said to lie towards the left
side and to be connected by a thick nerve with the _left_ habenular
ganglion (cf. _Petromyzon_, article CYCLOSTOMATA). This is adduced to
support the view that the pineal and parapineal bodies represent
originally paired structures.

Teleostei.--A parapineal rudiment appears in the embryo of some forms,
but in the adult only the pineal organ is known to exist. This is
usually short and club-shaped, its terminal part with much folded wall
and glandular in character. In a few cases a parietal foramen occurs
(_Callichthys_, _Loricaria_, &c.).

Dipneusti.--The pineal organ is short and simple. No parapineal organ
is developed.

The dorsal sac is formed by that part of the roof of the thalamencephalon lying between the habenular commissure and the region of the velum. In some cases a longitudinal groove is present in which the pineal organ lies (Dipneusti). In the Crossopterygians the dorsal sac is particularly large and was formerly mistaken for the pineal organ.

The _velum transversum_ is a transverse, inwardly-projecting fold of the roof of the primitive fore-brain in front of the dorsal sac. To those morphologists who regard the hemisphere region or telencephalon as a primitively unpaired structure the velum is an important landmark indicating the posterior limit of the telencephalon. Those who hold the view taken in this article that the hemispheres are to be regarded as paired outpushings of the side wall of the primitive fore-brain attribute less morphological importance to the velum. Physiologically the velum is frequently important from the plexus of blood-vessels which passes with it into the III. ventricle.

In _Petromyzon_ and _Chimaera_ the velum is not developed. In Dipnoans there are present in its place _paired_ transverse folds which are probably merely extensions backwards of the lateral plexuses.

The Paraphysis is a projection from the roof of the primitive fore-brain near its anterior end. It is well seen in Dipnoans[42] (_Lepidosiren_ and _Protopterus_) where in the larva (exactly as in the urodele larva) it forms a blindly ending tube sloping upwards and forwards between the two hemispheres. In the adult it becomes mixed with the two lateral plexuses and is liable to be confused with them. In the other groups--except the Teleosts where it is small (_Anguilla_) or absent (most Teleosts)--the paraphysis is by no means such a definite structure, but generally there is present a more or less branched and divided diverticulum of the brain wall, frequently glandular, which is homologized with the paraphysis. The morphological significance of the paraphysis is uncertain. It may represent the remains of an ancient sense organ, or it may simply represent the last connexion between the brain and the external ectoderm from which it was derived.

An important derivative of the primitive fore-brain is seen in the pair of cerebral hemispheres which in the higher vertebrates become of such relatively gigantic dimensions. The hemispheres appear to be primitively associated with the special sense of smell, and they are prolonged anteriorly into a pair of olfactory lobes which come into close relation with the olfactory organ. From a consideration of their adult relations and of their development--particularly in those groups where there is no disturbing factor in the shape of a large yolk sac--it seems probable that the hemispheres are primitively paired outpushings of the lateral wall of the primitive fore-brain[43]--in order to give increased space for the increased mass of nervous matter associated with the olfactory sense. They are most highly developed in the Dipneusti amongst fishes. They are there (cf. fig. 29, C) of relatively enormous size with thick nervous floor (corpus striatum) and side walls and roof (pallium) surrounding a central cavity (lateral ventricle) which opens into the third ventricle. At the posterior end of the hemisphere a small area of its wall remains thin and membranous, and this becomes pushed into the lateral ventricle by an ingrowth of blood-vessel to form the huge lateral plexus ( = _plexus hemisphaerium_). In this great size of the hemispheres[44] and also in the presence of a rudimentary cortex in the Dipnoi we see, as in many other features in these fishes, a distinct foreshadowing of conditions occurring in the higher groups of vertebrates. The Cyclostomes possess a distinct though small pair of hemispheres. In the Selachians the relatively archaic _Notidanidae_[45] possess a pair of thick-walled hemispheres, but in the majority of the members of the group the paired condition is obscured (fig. 29, A).

In the Teleostomes the mass of nervous matter which in other groups forms the hemispheres does not undergo any pushing outwards except as regards the small olfactory lobes. On the contrary, it remains as a great thickening of the lateral wall of the thalamencephalon (the so-called basal ganglia), additional space for which, however, may be obtained by a considerable increase in length of the fore-brain region (cf. fig. 30, A) or by actual involution into the third ventricle (_Polypterus_).[46] The great nervous thickenings of the thalamencephalic wall bulge into its cavity and are covered over by the thin epithelial roof of the thalamencephalon which is as a consequence liable to be confused with the pallium or roof of the hemispheres with which it has nothing to do: the homologue of the pallium as of other parts of the hemisphere is contained within the lateral thickening of the thelamencephalic wall, not in its membranous roof.[47]

Associated with the parts of the fore-brain devoted to the sense of smell (especially the corpora striata) is the important system of bridging fibres forming the anterior commissure which lies near the anterior end of the floor, or in the front wall, of the primitive fore-brain. It is of great interest to note the appearance in the _Dipnoans_ (_Lepidosiren_ and _Protopterus_) of a corpus callosum (cf. fig. 30 B) lying dorsal to the anterior commissure and composed of fibres connected with the pallial region of the two hemispheres.

_Sense Organs._--The olfactory organs are of special interest in the Selachians, where each remains through life as a widely-open, saccular involution of the ectoderm which may be prolonged backwards to the margin of the buccal cavity by an open oronasal groove, thus retaining a condition familiar in the embryo of the higher vertebrates. In Dipnoans the olfactory organ communicates with the roof of the buccal cavity by definite posterior nares as in the higher forms--the communicating passage being doubtless the morphological equivalent of the oronasal groove, although there is no direct embryological evidence for this. In the Teleostomes the olfactory organ varies from a condition of great complexity in the Crossopterygians down to a condition of almost complete atrophy in certain Teleosts (Plectognathi).[48]

The _eyes_ are usually of large size. The lens is large and spherical and in the case of most Teleostomes accommodation for distant vision is effected by the lens being pulled bodily nearer the retina. This movement is brought about by the contraction of smooth muscle fibres contained in the _processus falciformis_, a projection from the choroid which terminates in contact with the lens in a swelling, the _campanula Halleri_. In _Amia_ and in Teleosts a network of capillaries forming the so-called choroid gland surrounds the optic nerve just outside the retina. As a rule the eyes of fishes have a silvery, shining appearance due to the deposition of shining flakes of guanin in the outer layer of the choroid (_Argentea_) or, in the case of Selachians, in the inner layers (_tapetum_). Fishes which inhabit dark recesses, e.g. of caves or of the deep sea, show an enlargement, or, more frequently, a reduction, of the eyes. Certain deep-sea Teleosts possess remarkable telescopic eyes with a curious asymmetrical development of the retina.[49]

The otocyst or auditory organ agrees in its main features with that of other vertebrates. In Selachians the otocyst remains in the adult open to the exterior by the _ductus endolymphaticus_. In _Squatina_[50] this is unusually wide and correlated; with this the calcareous otoconia are replaced by sand-grains from the exterior. In Dipnoans (_Lepidosiren_ and _Protopterus_) curious outgrowths arise from the ductus endolymphaticus and come to overlie the roof of the fourth ventricle, recalling the somewhat similar condition met with in certain Amphibians.

In various Teleosts the swimbladder enters into intimate relations
with the otocyst. In the simplest condition these relations consist in
the prolongation forwards of the swimbladder as a blindly ending tube
on either side, the blind end coming into direct contact either with
the wall of the otocyst itself or with the fluid surrounding it
(perilymph) through a gap in the rigid periotic capsule. A wave of
compression causing a slight inward movement of the swimbladder wall
will bring about a greatly magnified movement of that part of the wall
which is not in relation with the external medium, viz. the part in
relation with the interior of the auditory capsule. In this way the
perception of delicate sound waves may be rendered much more perfect.
In the Ostariophysi (Sagemehl), including the _Cyprinidae_, the
_Siluridae_, the _Characinidae_ and the _Gymnotidae_, a
physiologically similar connexion between swimbladder and otocyst is
brought about by the intervention of a chain of auditory ossicles
(Weberian ossicles) formed by modification of the anterior
vertebrae.[51]

_Lateral Line Organs._[52]--Epidermal sense buds are scattered about in the ectoderm of fishes. A special arrangement of these in lines along the sides of the body and on the head region form the highly characteristic sense organs of the lateral line system. In _Lepidosiren_ these organs retain their superficial position; in other fishes they become sunk beneath the surface into a groove, which may remain open (some Selachians), but as a rule becomes closed into a tubular channel with openings at intervals. It has been suggested that the function of this system of sense organs is connected with the perception of vibratory disturbances of comparatively large wave length in the surrounding medium.

_Peripheral Nerves._--In the Cyclostomes the dorsal afferent and ventral efferent nerves are still, as in _Amphioxus_, independent, but in the gnathostomatous fishes they are, as in the higher vertebrates, combined together into typical spinal nerves.

As regards the cranial nerves the chief peculiarities of fishes relate to (1) the persistence of the branchial clefts and (2) the presence of an elaborate system of cutaneous sense organs supplied by a group of nerves (_lateralis_) connected with a centre in the brain which develops in continuity with that which receives the auditory nerve. These points may be exemplified by the arrangements in Selachians (see fig. 31). I., II., III., IV. and VI. call for no special remark.

FIG. 31.--Diagram of Cranial nerves of a Fish. Cranial nerves and branchial clefts are numbered with Roman figures. Trigeminus black; Facialis dotted; Lateralis oblique shading; Glossopharyngeal cross-hatched; Vagus white.

bucc, Buccal.
c, Commissure between pre- and postauditory parts of lateralis
system.
d.r, Dorsal roots of spinal nerves.
g.g, Gasserian ganglion.
gn.g, (Geniculate) ganglion of VII.
hy, Hyomandibular.
l.n.X, Lateralis vagi.
m, Motor branches of hy.
md, Mandibular.
md.ex, External mandibular.
mk.c, Meckel's cartilage.
mx, Maxillary.
oc, Occipitospinal.
ol.o, Olfactory organ.
op.p, Ophthalmicus profundus.
op.s, Ophthalmicus superficialis.
pn, Palatine.
pq., Palatopterygo-quadrate cartilage.
s, Spiracle.
st, Supra-temporal branch of lateralis system.
t.a, Lateralis centre in brain.
v.n, Visceral nerve.
v.r, Ventral roots.]

_Trigeminus_ (V.).--The _ophthalmicus profundus_ branch (op.p.)--which probably is morphologically a distinct cranial nerve--passes forwards along the roof of the orbit to the skin of the snout. As it passes through the orbit it gives off the long ciliary nerves to the eyeball, and is connected with the small ciliary ganglion (also connected with III.) which in turn gives off the short ciliary nerves to the eyeball. The _ophthalmicus superficialis_ (cut short in the figure) branch passes from the root ganglion of V. (Gasserian ganglion), and passes also over the orbit to the skin of the snout. It lies close to, or completely fused with, the corresponding branch of the lateralis system.

The main trunk of V. branches over the edge of the mouth into the _maxillary_ (mx.) and _mandibular_ (md.) divisions, the former, like the two branches already mentioned, purely sensory, the latter mixed--supplying the muscles of mastication as well as the teeth of the lower jaw and the lining of the buccal floor.

The main trunk of the _Facialis_ (VII.) bifurcates over the spiracle into a pre-spiracular portion--the main portion of which passes to the mucous membrane of the palate as the palatine (pnVII.)--and a postspiracular portion, the hyomandibular (hy.) trunk which supplies the muscles of the hyoid arch and also sends a few sensory fibres to the lining of the spiracle, the floor of mouth and pharynx and the skin of the lower jaw. Combined with the main trunk of the facial are branches belonging to the _lateralis_ system.

_Lateralis Group of Nerves._--The _lateralis_ group of nerves are charged with the innervation of the system of cutaneous sense organs and are all connected with the same central region in the medulla. A special sensory area of the ectoderm becomes involuted below the surface to form the otocyst, and the nerve fibres belonging to this form the auditory nerve (VIII.). Other portions of the _lateralis_ group become mixed up with various other cranial nerves as follows:

(a) Facial portion.

(1) _Ophthalmicus superficialis_ (op.s.VII.): passes to lining of nose or to the lateral line organs of the dorsal part of snout.

(2) _Buccal_ (bucc.VII): lies close to maxillary division of V. and passes to the sensory canals of the lower side of the snout.

(3) _External mandibular_ (md.ex.): lies in close association with the mandibular division of V., supplies the sensory canals of the lower jaw and hyoid region.

_Lateralis vagi_ (l.n.X.) becomes closely associated with the vagus. It supplies the lateral line organs of the trunk.

In the lamprey and in Dipnoans the _lateralis vagi_ loses its superficial position in the adult and comes into close relation with the notochord.

In Actinopterygians and at least some Selachians a _lateralis_ set of fibres is associated with IX., and in the former fishes a conspicuous trunk of _lateralis_ fibres passes to some or all (_Gadus_) of the fins. This has been called the _lateralis accessorius_ and is apparently connected with V., VII., IX., X. and certain spinal nerves.[53]

_Vagus Group_ (IX., X., XI.).--The _glossopharyngeus_ (IX.) forks over the first branchial cleft (pretrematic and post-trematic branches) and also gives off a palatine branch (pn.IX.). In some cases (various Selachians, Ganoids and Teleosts) it would seem that IX. includes a few fibres of the _lateralis_ group.

Vagus (X.) is shown by its multiple roots arising from the medulla and also by the character of its peripheral distribution to be a compound structure formed by the fusion of a number of originally distinct nerves. It consists of (1) a number of branchial branches (X.¹ X.² &c.), one of which forks over each gill cleft behind the hyobranchial and which may (Selachians) arise by separate roots from the medulla; (2) an intestinal branch (v.n.X.) arising behind the last branchial and innervating the wall of the oesophagus and stomach and it may be even the intestine throughout the greater part of its length (_Myxine_).