Chapter VII: , is termed the endoskeleton. The latter consists (i.) of an (5)

Representatives of the great lateral veins of Elasmobranchs appear to be absent in the Teleostomi, the veins from the pectoral and pelvic limbs joining the Cuvierian duct and the posterior cardinal veins respectively.

The two large anterior cardinal veins, which collect the blood from the head and brain, occupy their usual position directly above the branchial apparatus, and are sometimes connected by transverse anastomoses as they pass backwards to join the Cuvierian ducts. The inferior jugular vein is either single (e.g. _Gadus_); or paired, as in _Perca_ (Fig. 190).

In the Dipnoi the venous system is distinguished by an interesting combination of characters, some of which are either primitive or peculiar to the group, while others exhibit a distinct transition to the embryonic or the adult condition of the lower air-breathing Vertebrates.

In _Neoceratodus_[366] (Fig. 191) the renal portal system is unusually complex, the veins distributing venous blood to the kidneys being derived from several sources, as follows: (1) from each of the two branches into which the caudal vein divides on its exit from the haemal canal (_af.r.v_); (2) from a common trunk (_pt.v_) which, on each side, is formed by the union of segmental veins from certain of the post-cloacal myotomes and is united with its fellow by a transverse anastomosis; (3) from more anteriorly situated intercostal or segmental veins (_i.c.v_) which enter each kidney directly; and (4) from a vein on each side corresponding to the renal portal vein of Amphibia. The latter vein (_rp.v_) is formed by one of the two branches of the iliac or femoral vein, and joins {324}the corresponding vein from the caudal myotomes; from the common trunk numerous branches enter the kidney.

In the derivation of renal portal veins from each of the two veins into which the caudal vein divides, _Neoceratodus_ approaches the Elasmobranchs. On the other hand, the utilisation of ordinary segmental veins from the caudal and pre-caudal myotomes, some of which directly enter the kidney, is a feature which has already been remarked in some Teleosts; while the formation of a renal portal affluent by a branch of the femoral vein is an even more striking Amphibian characteristic.

The efferent renal veins[367] join the root of the left posterior cardinal and the adjacent portion of the caudal vein.

Of the two great venous trunks into which the caudal vein divides, the right is much the larger and behaves somewhat differently to the left. The former (_i.v.c_) passes forwards in relation with the right kidney, receiving in its course the spermatic or ovarian veins from the gonad of its side, and then traverses the liver, finally opening into the median portion of the sinus venosus, between the orifices of the two hepatic veins. The left branch of the caudal vein (_l.p.c_) also passes forwards in relation with the left {325}kidney and receives veins from the corresponding gonad; but, instead of traversing the liver, it passes above that organ, and finally opens into the left Cuvierian duct. The course of the left vein, and the relations of the vessel to the caudal vein and the left Cuvierian duct, point to the conclusion that it represents the left posterior cardinal of other Fishes. From its continuity with the caudal vein it is also obvious that the hinder or renal portion of the right trunk is a remnant of the right posterior cardinal; but the more anterior section so closely resembles the postcaval vein, or inferior vena cava of the higher Vertebrates, in its relations to the liver, the hepatic veins, and the sinus venosus, that its identity as such seems beyond doubt, and this interpretation is supported by well-known observations[368] on the mode of origin of the inferior vena cava in Amphibia, and especially the union of the independently formed inferior vena cava with the posterior or inter-renal portion of the embryonic right posterior cardinal vein, combined with the atrophy of the anterior portion of the latter vein.[369] The singular connexions and relations of these two great veins afford an additional illustration of the significant transitional condition of the venous system in the Dipnoi. On the other hand, the direct continuity of the caudal vein with vessels which, wholly or in part, represent the two posterior cardinals, is a feature alike characteristic of the adult Cyclostome and the embryonic Elasmobranch, Teleost, and Amphibian.

As in the Cyclostomes and Elasmobranchs, the precaudal section of the embryonic subintestinal vein is represented in the adult by an intra-intestinal vein which traverses the spiral valve near its free edge and is a tributary of the hepatic portal vein.

The two veins from the undivided air-bladder unite to form a single vessel, which, instead of joining the hepatic portal or posterior cardinal veins as in other Fishes, opens into the left auricle, like the pulmonary veins of the Amphibia.

A further resemblance to the Amphibia is to be found in the presence of an anterior abdominal vein. After leaving the pelvic {326}limb each femoral vein divides into two branches; one of these forms a renal portal vein as previously described; the other, which may rightly be termed a pelvic vein (_pv.v_), unites with its fellow to form a median anterior abdominal vein (_a.ab_). Pursuing its course forwards in the ventral abdominal wall, the vein eventually reaches the heart and opens into the sinus venosus. The direct connexion of the anterior abdominal vein with the heart is yet another example of the retention in the adult _Neoceratodus_ of a transitory embryonic feature in the developing Amphibian.[370]

As in other Fishes, the blood from the head is conveyed to the Cuvierian ducts by an anterior cardinal and an inferior jugular on each side. There are no lateral veins, the blood from the pelvic fins flowing into the renal portal system or into the anterior abdominal vein, and that from the pectoral fin through subscapular and brachial veins into the Cuvierian ducts. Lateral cutaneous veins are, however, present; and, as in Elasmobranchs (e.g. _Mustelus antarcticus_), anastomose anteriorly with the subscapular vein and behind with the caudal vein.

{327}Less is known of the venous system of _Protopterus_,[371] but it is certain, nevertheless, that it presents a more advanced grade of evolution than in _Neoceratodus_, and, except for the doubt as to the existence of an anterior abdominal vein, it is essentially similar to that of a Urodele Amphibian in which the right posterior cardinal vein has aborted.

The caudal vein (Fig. 192) divides into right and left renal portal branches, neither of which, however, is directly continuous with the inferior vena cava or the left posterior cardinal; on the contrary, each renal portal vein is joined by the corresponding iliac or femoral vein, and also by numerous segmental veins, and then distributes the whole of its venous blood to the kidney. The radicles of the inferior vena cava and the left posterior cardinal are formed by the renal veins from the two kidneys, and in their forward course to the heart both veins receive in addition genital and segmental veins. In its course through the liver the inferior vena cava receives several hepatic veins, and finally opens into the sinus venosus, while the left posterior cardinal vein joins the corresponding Cuvierian duct, which also receives anterior cardinal, inferior jugular, and subclavian veins. There is an intra-intestinal vein as in _Neoceratodus_, but an anterior abdominal vein has yet to be discovered. The two pulmonary veins from the double air-bladder form a single trunk before communicating with the left auricle.

With the exception of certain doubtful details which need further investigation, the venous system of _Lepidosiren_[372] seems to resemble that of _Protopterus_.

THE HEART.—The heart is more anteriorly placed than in other Vertebrates, being situated directly behind and beneath the last pair of branchial clefts and internal to the ventral portion of the pectoral girdle. The organ is enclosed in a pericardial cavity, which, in the adult, is separated from the abdominal portion of the coelom by a transverse pericardio-peritoneal septum, and in the Lamprey (_Petromyzon_) is partially enclosed within a cartilaginous, cup-like modification of the hinder part of the branchial basket. In the Ammocoetes-stage of the Lamprey the pericardium is in communication behind with the general coelom, but the connexion is lost in the adult. In Elasmobranchs the {328}two cavities are connected by a single pericardio-peritoneal canal, or by two such canals; and in _Chimaera_, and in the Sturgeon (_Acipenser_) and _Polyodon_, by a single canal.

The heart consists of at least three chambers, a sinus venosus which receives the venous blood from the body, an auricle and a ventricle, to which is added a conus arteriosus in the Elasmobranchs, certain Teleostomi (Crossopterygii, Chondrostei, and Holostei), and in the Dipnoi. Through these cardiac chambers the blood is forced in the order mentioned. In the Dipnoi the auricle is subdivided by a more or less complete interauricular septum into a right and left auricle,[373] the former receiving the venous blood from the sinus venosus, and the latter the aerated blood from the lung-like air-bladder.

The sinus venosus and the auricle have very thin walls; the ventricular walls, on the contrary, are very thick and in great measure are composed of a sponge-like network of muscular bundles which generally encroaches considerably on the ventricular cavity. Membranous valves, the sinu-auricular, and the auriculo-ventricular valves, are developed at the junctions of the sinus venosus with the auricle, and the auricle with the ventricle respectively. The conus arteriosus is muscular and contractile, and is interposed between the ventricle and the root of the ventral aorta. Internally, the conus is provided with several transverse rows of pocket-shaped or semilunar valves. In Teleosts the conus is non-muscular and vestigial, and has but a single row of valves, corresponding to the most anterior of the multiple rows of valves in the Elasmobranchs. In these {329}Fishes the vestigial conus is succeeded by a non-contractile, bulb-like dilatation, or bulbus aortae, of the root of the ventral aorta. In only a single Teleost, viz. _Albula_, one of the Albulidae, is the vestigial conus muscular, and at the same time provided with two rows of valves.[374] In the Cyclostomata there is a bulbus with a single row of two valves, but no true conus.

In the Dipnoi (e.g. _Protopterus_) the heart, like the rest of the vascular system, exhibits certain interesting resemblances to the Amphibian heart. In addition to a more or less complete interauricular septum separating right and left auricles, there is a median longitudinal ridge, partly muscular and partly fibrous, which incompletely subdivides the cavity of the ventricle. The spirally-twisted conus arteriosus is furnished with several transverse rows of valves, certain of which coalesce longitudinally to form a complete septum dividing the cavity of the conus into two distinct lateral channels: with this septum there coalesces another septum, which cuts off the origins of the anterior two pairs from the remaining afferent branchial arteries. The formation of these septa has the physiological effect of subdividing the series of cardiac cavities into two parallel channels, of which one has its origin behind in the sinus venosus and transmits venous blood to the posterior afferent branchial vessels; while the other, commencing with the left auricle, conveys arterial blood to the first two pairs of afferent branchial arteries.[375] In _Neoceratodus_, however, the longitudinal septum in the conus is incomplete, and hence the blood which is sent to the anterior afferent vessels is mixed.[376]

THE ARTERIAL SYSTEM.—The ventral aorta is a median artery situated beneath the floor of the pharynx, and having its origin, behind, either directly from the ventricle or from the conus arteriosus.

In the Cyclostomata[377] (e.g. _Petromyzon_) the ventral aorta (Fig. 194) is continued forwards from the heart as a single vessel to the fourth pair of gill-sacs, where it divides into right and left branches which extend as far as the anterior walls of the first pair of gill-sacs. Eight pairs of afferent branchial arteries arise from the ventral aorta and its two branches, of {330}which the first and last supply the anterior walls of the first pair of sacs and the posterior walls of the last pair respectively. Each of the remaining afferent vessels extends into an interbranchial septum, and supplies the gill-lamellae of the posterior wall of one sac and those of the anterior wall of the next sac behind. The corresponding efferent branchial vessels have a similar distribution, and unite dorsally to form a median dorsal aorta. Beneath the base of the skull the latter vessel divides into two branches which, after receiving the first pair of efferent branchial vessels, pursue a divergent course forwards, but subsequently converge and unite to form a "circulus cephalicus," as in Teleosts. From the cephalic circle are given off on each side (1) an "internal carotid" artery for the brain and eye; (2) an "external carotid" for the lateral and ventral walls of the head; and (3) a large ventral branch which supplies the lingual apparatus; while from the abdominal portion of the dorsal aorta are derived, first, a coeliaco-mesenteric artery for the liver and alimentary canal, and subsequently branches for the myotomes, kidneys, and the gonad. The terminal portion of the aorta then enters the tail and forms the caudal artery.

In Elasmobranchs[378] (e.g. _Mustelus antarcticus_) the undivided ventral aorta gives off five pairs of afferent branchial arteries which, on each side, ascend in succession the outer convex sides of the hyoid and first four branchial arches (Fig. 195).

{331}

The first or most anterior of these arteries supplies the hyoidean hemibranch, while the succeeding four supply the holobranchs of the four branchial arches. The blood is collected from the capillaries of the branchial lamellae by a series of efferent branchial vessels, a pair for the two hemibranchs of each branchial arch and a single vessel for the hyoidean hemibranch, which unite with one another in a somewhat singular fashion. The efferent arteries from the anterior and posterior hemibranchs of each branchial cleft unite above and below each cleft in such a way as to form a series of complete vascular loops round the hyoidean cleft and the three succeeding branchial clefts, which are connected by short longitudinal trunks in each arch and also by a longitudinal commissural vessel between their ventral extremities. As the fifth arch is gill-less, there is no complete loop round the fifth cleft, the blood collected by the efferent vessel of the posterior hemibranch of the fourth arch being conveyed to the corresponding vessel of the anterior hemibranch of the same arch by one of the short longitudinal vessels above mentioned. Dorsally, each arterial loop is continuous with an epibranchial artery; and by the dorsal union of the four {332}epibranchial arteries of the two sides the dorsal aorta is formed. It may be pointed out that the anterior efferent vessel of each arch, which is usually larger than the posterior one, is to be regarded as the primary efferent artery of the corresponding holobranch, and as such is directly continuous with an epibranchial artery, the posterior efferent artery being a secondary vessel which opens not into the primary trunk of its own branchial arch, but into that of the succeeding arch.[379] The principal arteries which supply the various parts of the head with blood are derived from the first efferent branchial vessel. From the ventral end of this artery a mandibular artery is given off, which subdivides into branches for the muscles of the lower jaw as well as into nutrient vessels for the hyoidean hemibranch. At about the middle of its length the same artery gives off an afferent pseudobranchial or hyoidean artery, to the spiracular or mandibular pseudobranch. From the latter organ the blood is collected by an anterior carotid artery which, after giving off an ophthalmic branch to the eye, perforates the orbital wall and enters the cranial cavity, where it is joined by an anastomotic trunk from the posterior carotid of the opposite side; finally, the anterior carotid divides into anterior and posterior cerebral arteries for the brain. The third and last of the cephalic arteries is the posterior carotid; this artery arises from the dorsal extremity of the first efferent branchial vessel, and, on entering the orbit, gives off the anastomotic trunk previously mentioned. The latter vessel enters the cranial cavity, and, after crossing its fellow, joins the anterior carotid of the opposite side, as described above. The main trunk is then continued forwards in the orbit, and its various branches eventually supply the eye-muscles, the mandibular adductor muscle, and some other parts of the head.

It is worthy of note that the median dorsal aorta is prolonged forwards in front of the first pair of epibranchial arteries as a slender median vessel (_a.d.a_), which ultimately divides into two branches, each branch uniting with the posterior carotid of its side.

A remarkable system of arteries for the supply of nutrient blood to the gills and heart has its origin in the following {333}manner. On each side, the longitudinal commissural vessel, which connects the ventral ends of the arterial loops surrounding the different gill-clefts, gives origin to a series of pairs of short transverse vessels, and by their union these combine to form a median longitudinal hypobranchial artery which lies beneath the ventral aorta. From the hypobranchial artery are derived the coronary arteries for the heart; and from the same artery, or from its lateral connexions with the longitudinal commissural artery, and, in the case of the hyoidean hemibranch, from the mandibular artery, are derived the various nutrient vessels for the gills.

The arteries for the trunk, and for the pectoral and pelvic limbs, arise in succession from the dorsal aorta. The first of the series is the subclavian artery, which has its origin from the aorta close to the dorsal extremities of the fourth pair of epibranchial arteries. Each subclavian artery gives off a brachial artery to the pectoral fin, and is then continued forwards as a lateral hypobranchial artery, which, with its fellow of the opposite side, eventually becomes continuous with the hinder end of the median hypobranchial artery. Behind the subclavian artery there is a median coeliaco-mesenteric artery, the various branches of which are distributed to the liver, stomach, and intestine. A lieno-gastric artery supplies the pancreas and spleen, and also sends branches to the stomach. In addition, there are also arteries for the gonads, numerous segmental arteries for the myotomes, and renal arteries for the kidneys. Finally, the aorta gives off a pair of iliac arteries for the pelvic fins, and then enters the haemal canal as the caudal artery.

The more important differences in the arterial system of the Holocephali and the Teleostomi relate to (1) the absence of the posterior efferent branchial artery in each branchial arch; (2) modifications dependent on the condition of the spiracular and hyoidean hemibranchs, and the mode of origin and the course of their afferent and efferent vessels; and (3) the source from whence the air-bladder derives its blood when that organ is present.

(1) The branchial arterial system is somewhat more primitive than in the generality of Elasmobranchs.[380] There are no complete vascular loops round the gill-clefts, and the blood from the two {334}hemibranchs of each branchial arch is conveyed to the dorsal aorta by a single efferent vessel which corresponds to the more anterior of the two in _Mustelus antarcticus_.[381]

(2) In _Callorhynchus_[382] among the Holocephali, where the spiracle is absent but the hyoidean hemibranchi is still a true gill, the latter organ is supplied with venous blood by a branch from the ventral aorta, the corresponding efferent vessel joining the dorsal aorta (Fig. 196). In the absence of a spiracular pseudobranch the anterior carotid may be regarded as continuous with the hyoidean artery,[383] and as having its origin directly from the efferent artery of the hyoidean hemibranch (Fig. 196). At its origin the anterior carotid anastomoses with the mandibular artery.

The Sturgeon more closely resembles the Elasmobranchs. The hyoidean gill is supplied by an afferent branchial artery from the ventral aorta, and its efferent vessel joins the corresponding trunk from the holobranch of the first branchial arch. A hyoidean artery supplies the spiracular pseudobranch, the efferent vessel of which contributes to the blood-supply of the brain and the eye, and probably represents an anterior carotid.

_Lepidosteus_[384] offers a singularly interesting transition from the {335}Elasmobranch to the Teleost. As indicated in the preceding chapter, this Fish possesses both a hyoidean gill and a spiracular pseudobranch (Figs. 197 and 198). The hyoidean gill is supplied by an afferent artery direct from the ventral aorta, but the proper efferent vessel of the gill, which primitively joined the dorsal aorta, is suppressed, and the blood is collected into a vessel, which, like the hyoidean artery in Elasmobranchs, becomes the afferent artery of the spiracular pseudobranch. The latter artery unites, however, with a second hyoidean artery derived from the efferent branchial vessel of the first branchial arch, and represents the artery termed "hyoidean" in Teleosts. The efferent vessel from the spiracular pseudobranch joins an internal branch from the carotid artery, and then distributes its blood both to the eye and the brain.

In Teleosts, as already mentioned in a preceding chapter, it is probable that the hyoidean hemibranch is suppressed, the so-called hyoidean pseudobranch being a spiracular pseudobranch. The latter is now supplied by a "hyoidean" artery, which has its origin from the ventral end of the efferent {336}branchial artery of the first branchial arch, the corresponding efferent trunk forming an ophthalmic artery, and passing to the choroid gland of the eye (Fig. 199). Both the proper afferent and efferent arteries of the hyoidean hemibranch either disappear or, as in the Cod (_Gadus morrhua_), the efferent artery may be represented on each side by an anastomosis between the hyoidean artery and the cephalic circle. Hence, the "hyoidean" artery of Teleosts corresponds to the one which has a similar origin in _Lepidosteus_.

A brief description of the remaining efferent branchial arteries and their derivatives in the Cod (_Gadus morrhua_) will illustrate the condition of these structures in a well-known Teleost.

In this Fish the efferent branchial vessels open dorsally into right and left suprabranchial arteries,[385] which unite behind to form a median dorsal aorta (Fig. 199). Anteriorly, the paired suprabranchial arteries extend towards the base of the skull as the so-called "carotid" arteries. The two carotids enter the cranial cavity, and there unite in the median line, as in the Cyclostomes. By the union of these arteries in front, and of the {337}right and left suprabranchial arteries behind, the characteristic "circulus cephalicus" of Teleosts is completed.[386] From the anterior part of the cephalic circle are derived two internal carotid arteries[387] for the brain, and also a pair of orbito-nasal arteries for the eye-muscles and the nasal sacs, while more posteriorly an external carotid has its origin from each suprabranchial artery.

_l.d.a_, left suprabranchial artery; _m.a_, mesenteric artery; _on_, orbito-nasal artery; _oph.a_, ophthalmic artery; _r.d.a_, right suprabranchial artery; _sb.a_, subclavian artery; _v.a_, ventral aorta; 1-5, hyobranchial and succeeding gill-clefts. (Altered from T. Jeffery Parker.)]

(3) In most Teleostomi the air-bladder is supplied with blood by branches of the coeliac artery, with the addition of small branches arising directly from the dorsal aorta. _Polypterus_ and _Amia_[388] are, however, exceptional, inasmuch as the arteries for the air-bladder are derived from the last or fourth pair of efferent branchial vessels, and in this respect, but not in the destination {338}of the corresponding veins, the two genera exhibit a significant resemblance to the Dipnoi.

In the Dipnoi the ventral aorta is so short that the afferent branchial arteries arise almost directly from the conus arteriosus with their roots in close contiguity to one another (Fig. 200).

In _Neoceratodus_ (Fig. 200),[389] there are two efferent vessels to each gill-bearing branchial arch, which unite above to form an epibranchial artery, and by the successive union of the four epibranchial arteries a short common trunk is formed on each side. Posteriorly, the two trunks unite to form a median dorsal aorta. Immediately above the gill-clefts each efferent vessel gives off a branch which, passing either forwards or backwards, unites with the corresponding branch of the efferent vessel in front or behind as the case may be. A hyoidean artery arises from the ventral extremity of the anterior efferent artery of the first branchial arch, and, after giving off a lingual artery, ascends the hyoid arch and supplies the hyoidean pseudobranch. The efferent vessel of the pseudobranch (_a.c.a_) or anterior {339}carotid artery, eventually enters the cranial cavity and subdivides into anterior and posterior cerebral arteries for the brain, also giving off a branch which unites with its fellow of the opposite side directly behind the infundibulum. A posterior carotid springs from the epibranchial of the first branchial arch and divides into palatine, orbital, and ocular branches; and from the ventral end of the anterior efferent vessel of the second branchial arch is derived a hypobranchial artery for the heart and pericardium. The pulmonary arteries for the lung-like air-bladder have their origin from the fourth pair of epibranchial arteries.

As in so many other details of its anatomy, _Neoceratodus_ exhibits in its arterial system abundant evidence of the wide-spreading affinities of the group to which it belongs. In its branchial arterial system _Neoceratodus_ presents a singular combination of features which, individually, are characteristic of Amphibia and Elasmobranchs. Special Amphibian features may be noted in the origin of the afferent branchial arteries almost simultaneously from the anterior end of the conus arteriosus; in the mode of union of the epibranchial arteries to form the dorsal aortae; in the origin of a lingual artery from the efferent vessel of the first branchial arch; and in the derivation on either side of a pulmonary artery from the fourth epibranchial artery. Agreement with Elasmobranchs is to be found in the presence of two efferent branchial vessels in each branchial arch, although the relations of these arteries are more primitive than in most adult Elasmobranchs, inasmuch as the two efferent vessels of the _same_ arch unite to form an epibranchial artery; and also in the origin and distribution of the anterior and posterior carotids. Lastly may be mentioned the fact that _Neoceratodus_ agrees not only with the Amphibia but also with those generalised Teleostomi, _Polypterus_ and _Amia_, in the mode of origin of the great arteries for the air-bladder.

Of the two remaining Dipnoi, the arterial system of _Protopterus_[390] is better known than that of _Lepidosiren_, but in both cases further research is needed before a satisfactory comparison can be made with _Neoceratodus_ and other Vertebrates. It is evident, nevertheless, that both genera differ from _Neoceratodus_ in approximating more closely to the Amphibia than to the {340}lower Fishes, in so far as the branchial part of the arterial system is concerned.

In their origin from the conus the four afferent branchial arteries of _Protopterus_ resemble those of _Neoceratodus_, but their relations to the branchial clefts are somewhat different (Fig. 201). The first or hyoidean cleft is closed, and the first afferent vessel lies between the second cleft and the third, and is therefore in relation with the second branchial arch. The remaining afferent arteries are disposed between the succeeding clefts and are related to the corresponding arches. As the second and third arches, like the vestigial first arch, bear no gill-lamellae, their afferent arteries are directly continuous with the corresponding efferent vessels, as in those Teleosts in which certain arches are gill-less, as well as in the Tadpole-stage of the tailless Amphibia when the internal gills begin to degenerate; and they apparently transmit arterial blood directly to the dorsal aorta.[391] The third and fourth afferent arteries, on the contrary, supply venous blood to the two hemibranchs which are borne by each of the two corresponding arches, viz.: the fourth and fifth, and from each pair of hemibranchs the blood is collected into two efferent vessels which unite dorsally {341}to form an epibranchial artery. From the dorsal end of the fourth afferent artery there arises a recurrent branch which curves round the upper margin of the sixth cleft and supplies the gill-lamellae on the posterior margin of that cleft, a fact which lends support to the view that these lamellae are "emigrants" from the anterior margin of the cleft; the efferent vessel from the "emigrant" lamellae joins the fourth epibranchial artery. The blood-supply of the external or cutaneous gills is derived from the dorsal extremities of the second, third, and fourth afferent arteries, while the efferent vessels from these organs join the corresponding epibranchial arteries; in this respect there is a close resemblance between _Protopterus_ and those larval Amphibians which possess similar cutaneous gills. All four epibranchial arteries unite together at about the same point to form a short common trunk, the right or left dorsal aorta, which subsequently unites with its fellow to form the median dorsal aorta.

There is a so-called "hyoidean" artery, which, however, has its origin, not from an anterior efferent branchial vessel as in _Neoceratodus_, but from the first _afferent_ branchial artery. After giving off a submaxillary or lingual artery, the "hyoidean" artery (_af_) becomes the afferent vessel for the "opercular gill" or "hyoidean pseudobranch,"[392] and supplies the latter with arterial blood. The efferent vessel (_ef_) from the pseudobranch unites with the four epibranchial arteries in forming the right or left dorsal aorta. A "carotid" artery arises from the efferent vessel of the "hyoidean pseudobranch," and a pulmonary artery has its origin from the root of the dorsal aorta of its side, and not from the fourth epibranchial artery as in _Neoceratodus_.

THE BLOOD.—The blood consists of a nutritive fluid plasma in which float red corpuscles and leucocytes. In the Cyclostomata (e.g. _Petromyzon_) the red corpuscles are circular, but in _Myxine_ they have the usual oval shape. In Fishes the red corpuscles are almost invariably flat, oval, biconvex, and nucleated, and owe their colour to the presence of the characteristic oxygen-absorbing, iron-containing pigment, haemoglobin. They are unusually large in the Dipnoi and are only exceeded in size by those of certain Urodele Amphibians. The leucocytes are much less numerous than the red corpuscles, although their relative proportions are very variable, even in the same species under different {342}conditions. They appear to be more numerous in the Dipnoi (e.g. _Protopterus_) than in any other Vertebrates, except under pathological conditions.[393]

THE LYMPHATIC SYSTEM.—In addition to blood-vessels, Fishes possess a lymphatic system, consisting of smaller vessels, lymph-capillaries or lymph-spaces, distributed in the connective tissue of different parts of the body, and by their union ultimately forming larger lymph-vessels or sinuses which communicate with certain of the principal veins, the whole forming a series of channels for the collection of the blood-plasma which has exuded from the blood-capillaries for the nutrition of the tissues, and for its conveyance to the general venous system. The fluid in the lymphatics, or lymph, consists of dilute blood-plasma containing leucocytes but devoid of red corpuscles. At the points where the larger lymphatics open into the veins, lymph-hearts may be developed. In the Eel (_Anguilla vulgaris_) there is a lymph-heart in the tail, which communicates by a valvular orifice with the smaller of the two caudal veins, and by its rhythmical pulsations propels the lymph into the vein. In _Silurus_ there are two caudal lymph-hearts. Apart from the lymphoid tissue, which is so abundantly present in certain parts of the body, Fishes appear to be devoid of the special "lymphatic glands" of the higher Vertebrates.

THE DUCTLESS OR BLOOD-GLANDS.—All the important blood-glands of other Vertebrates have their representatives in Fishes. Nothing is certainly known of the function of these organs in Fishes, but from the general structural resemblance which they present to their equivalents in the higher Vertebrates, it is perhaps not unreasonable to infer that they are similar in function. If this be so, the blood-glands of Fishes are organs for leucocyte-formation and phagocytosis, involving the destruction and removal of effete red blood-corpuscles; in addition, they may also be concerned with certain obscure chemical changes in the composition of the blood, which have an important relation to general or local nutrition.

THE SPLEEN.—This lymphoid organ is the largest of all the blood-glands, and, in the form of a compact or more or less lobulated body, is present in all Fishes, and possibly in Cyclostomes. In position the spleen is usually in close proximity to the stomach, to which it is attached by an extension round it {343}of the peritoneal investment of that organ. Thus, in the Dog-Fish (_Scyllium_), the spleen is a large reddish body attached to the convexity of the U-shaped stomach, and, in addition, sends a long narrow lobe between the distal limb and the valvate portion of the intestine (Fig. 153, _spl_). In the Sturgeon (_Acipenser_), the organ is also large, but is attached to the left side of the commencement of the intestine. In the Cod (_Gadus_) among Teleosts the spleen is much elongated and is situated on the dorsal side of the stomach. In the Dipnoi (e.g. _Protopterus_)[394] the organ is probably represented by a large compact lymphoid mass, closely connected with the dorsal and lateral walls of the stomach (Fig. 154, A, _s_).

THE THYROID GLAND.—This organ[395] usually arises in the form of a small median evagination of the hypoblastic epithelium of the ventral wall of the pharynx, in the region of the second visceral arch. Later it becomes detached from the place of origin and converted into a solid spherical body. Eventually the component cells form the limiting epithelium of a series of follicles or vesicles embedded in a matrix of connective tissue and blood-vessels, and the characteristic adult structure is attained.

Among the Cyclostomata the evagination is relatively large in the young Lamprey (_Petromyzon fluviatilis_), as also is the orifice of communication with the pharynx (Fig. 202, _th_).[396] The aperture soon becomes reduced to a mere pore, and finally disappears. During the larval or Ammocoetes-stage the organ consists of a median ciliated portion, communicating with a pair of laterally placed glandular sacs, but in the adult it is much smaller, and acquires the usual follicular structure. In adult Elasmobranchs the thyroid is represented by a moderately large compact organ, situated near the anterior end of the ventral aorta. In Teleostomi the organ may be paired, or, as in the Perch (_Perca_), more diffuse, consisting of masses of reddish lobules lying beneath the aorta, and also scattered for a variable distance along the course of the afferent branchial arteries.

In the Dipnoi (e.g. _Protopterus_)[397] the thyroid is small, {344}consisting of two lateral lobes connected by a constricted median portion, and situated beneath the epithelium of the tongue, immediately above the hyoidean symphysis. A similar structure has been described by Bischoff[398] in _Lepidosiren_, and was regarded by him as a salivary gland.

As in Reptiles, Birds, and Mammals, paired or accessory thyroid bodies ("supra-pericardial organs")[399] are present in many Fishes, and appear to be similar in structure to the median thyroid. In Elasmobranchs these bodies originate as a pair of outgrowths from the epithelium of the pharynx behind the last pair of branchial arches (Fig. 202, B, _a.th_). Subsequently they become detached from the pharynx, and in the adult are situated on the dorsal side of the pericardium, remote from the median thyroid.

According to Dohrn the median thyroid is to be regarded as the vestige of a gill-cleft which primitively existed between the hyomandibular cartilage and the hyoidean arch. This conclusion seems, however, to be less in harmony with the facts of development than the view[400] that the organ is derived from the characteristic hypobranchial groove or "endostyle" of Ascidians {345}and Amphioxus, which has undergone a change of function from a mucus-conveying groove to a blood-gland. On the other hand, the mode of origin of the paired thyroids certainly favours the suggestion that they represent a posterior pair of vestigial gill-clefts, a view which derives some support from the fact that in _Notidanus_, where additional branchial arches and clefts are present, the paired thyroids are absent.

THE THYMUS.—In the embryo Elasmobranch and Teleost[401] the thymus has a multiple origin, being derived from a series of distinct epithelial thickenings, one of which is developed at the dorsal extremity of each of the gill-clefts except of the spiracle. These rudiments subsequently detach themselves from the epithelial surface and sink inwards, eventually fusing together on each side to form a single independent structure. Later, the epithelial mass thus formed becomes invaded by connective tissue, and by leucocytes which form lymph follicles, and the thymus gradually assumes the structure of a lymphoid organ. From its mode of development it has been suggested that the thymus owes its evolution to the metamorphosis and ingrowth of branchial filaments,[402] but it is also noteworthy that each embryonic rudiment of the organ closely resembles, both in position and origin, one of the developing branchial tongue-bars of Amphioxus.[403] The abundance of leucocytes which it contains has also prompted the further suggestion that the origin of the thymus may be due to the necessity of providing for the phagocytic protection of the gills themselves from the ravages of harmful micro-organisms, fungoid spores, etc., as well as to aid in the removal of such portions of the gills as may have been injured.[404]

A thymus is probably present in all Fishes, if not in the adult at all events in the embryo, but is always relatively small in size. In Elasmobranchs the organ lies on each side above the branchial arches and beneath the dorsal musculature; and in Teleostomi at the dorsal extremity of the last branchial arch, in close proximity to the mucous membrane of the branchial cavity. In a similar position in the Dipnoi (e.g. _Protopterus_)[405] there are, on each side, {346}two contiguous lobes of lymphoid tissue which apparently represent a thymus.

THE SUPRA-RENAL BODIES.—The supra-renal bodies are organs of problematic function, which are present in the Cyclostomata, and probably in all Fishes, and situated in close proximity to the kidneys.

In the Cyclostomata (_Petromyzon_) these bodies are represented by lobules of cells along the posterior cardinal veins, and also by masses of peculiar cells ("chromaffin cells") along the sides of the aorta and segmental arteries.[406] In Elasmobranchs there are two distinct structures, the paired supra-renals and the inter-renals (Fig. 203, A). The former are a series of pairs of segmentally arranged bodies, situated on the successive pairs of segmental arteries given off from the dorsal aorta. The two bodies which form the first pair are much larger than any of the others, and were formerly spoken of as "axillary hearts." The inter-renal is usually a thin elongated "ochre-yellow" body, from which one or two lobes may be detached in front, and extends for a variable distance in {347}the median line between the two kidneys, or is unsymmetrically placed on the ventral surface of either kidney.[407] Sometimes (e.g. in _Raia_) the inter-renals are paired, in which case they are applied to the inner and hinder margins of the kidneys. In the Sturgeon (_Acipenser sturio_) the "supra-renals" appear as numerous "ochre-yellow" bodies, variable in size and distribution (Fig. 203, B). Some of them are visible on the surface of the kidneys, while others are scattered about in their substance, but on the whole are more anteriorly placed than in Teleosts. In the latter group the "supra-renals" are usually two in number (Fig. 203, C), but may be as many as five or reduced to one. They are disposed either on the ventral or the dorsal surface of the kidneys, generally near their hinder extremities, or more or less deeply embedded in their substance. Besides these bodies there are also chromaffin cells in the walls of the anterior cardinal veins.[408]

Histologically, the paired segmentally arranged bodies of Elasmobranchs differ considerably in structure from the inter-renal bodies, the former resembling the "medulla," while the inter-renals, as well as the so-called supra-renals of _Acipenser_, exhibit a striking resemblance to the alveolar "cortical" substance of the Mammalian supra-renals.[409] In Cyclostomes the cortex is apparently represented by the lobules of cells along the posterior cardinal veins and the medulla by the "chromaffin" cells, while in Teleosts the cortex and the medulla have their respective counterparts in the supra-renals and the "chromaffin" cells in the walls of the anterior cardinal veins. It may be concluded, therefore, that Elasmobranchs, Cyclostomes, and Teleosts possess anatomically distinct representatives of both the "medulla" and "cortex" of Mammalia, although the Sturgeon is at present only known to possess the equivalent of the "cortex." In Amphibia, Reptilia, and Aves both "cortex" and "medulla" are present, and in the varying intimacy of their relations offer a transition to the Mammalian arrangement of a central medulla closely invested by a sheath of cortical substance. A more or less intimate connexion exists between the paired supra-renals of Elasmobranchs and the sympathetic nervous {348}system. The former are usually well supplied with sympathetic nerve fibres, and contain ganglion-cells in their substance.

The primitive origin of these organs is very obscure, and as regards their development there is much diversity of opinion. It seems certain, however, that the cortex and medulla of the higher Vertebrates, including their equivalents in the Elasmobranchs, have independent origins, and the balance of opinion seems to point to the derivation of the cortex from some portion of the germinal coelomic epithelium, while the medulla is derived from the embryonic nerve cells of the sympathetic ganglia.

LYMPHOID TISSUE.—In addition to certain of the ductless glands, and the local or diffused masses of their characteristic tissue already mentioned in connexion with the alimentary canal, lymphoid tissue is often abundantly present in other parts of the body. There is, for example, a mass of this tissue on the heart of the Sturgeon (_Acipenser_). The anterior enlarged portion of the mesonephros, commonly termed the "head-kidney" of the Teleostomi (Fig. 203, B, C), is almost entirely composed of lymphoid tissue,[410] which has replaced, wholly or partially, the proper renal structure; and from the presence of free red blood-corpuscles and of crystals of oxy-haemoglobin and other derivatives of haemoglobin, it may be inferred that the "head-kidney," in common with the more orthodox blood-glands, performs a blood-destroying function.[411] On the other hand, the example of the spleen, which is alike the seat of leucocyte-formation and of blood-destruction, renders it unnecessary to reject the view that the "head-kidney" is an organ in which leucocytes or blood-corpuscles are formed. In but few Teleostomi is a purely lymphoid "head-kidney" entirely wanting, as, for example, in the Sun-Fish (_Orthagoriscus mola_).[412] As previously mentioned the Dipnoi are remarkable for the extraordinary development of lymphoid tissue, inasmuch as it forms a thick investing mass round the kidneys and gonads in addition to its exceptional abundance in the walls of the alimentary canal.

The absence of ordinary lymphatic glands in Fishes is well known, and it is at least probable that, functionally, the want of these lymphoid organs may be compensated for by the superabundance of lymphoid tissue in other parts of the body.[413]

{349}CHAPTER XIII

MUSCULAR SYSTEM—LOCOMOTION—SOUND-PRODUCING ORGANS—ELECTRIC ORGANS

MUSCULAR SYSTEM.—The various muscles of the body may be arranged in two systems: (i.) the _somatic_ or _parietal_, composed of striated or voluntary muscle-fibres; and (ii.) the _splanchnic_ or _visceral_, consisting for the most part of unstriated or involuntary fibres. Somatic muscles form the great lateral longitudinal muscles of the trunk and tail, which retain the primitive embryonic metamerism to a greater extent in Fishes than in any other Vertebrates, and are the principal muscles associated with locomotion. The lateral muscles are composed of a series of transverse muscle-segments or myotomes, which are >-shaped, or S-shaped, or they even take a zigzag course from above downward. The myotomes are disposed in pairs, and they are separated from one another by fibrous septa or myocommata. Each myotome is divided into a dorsal or epiaxial portion, and a ventral or hypaxial portion, by a longitudinal, horizontal, fibrous septum extending outwards from the vertebral centra to the skin. The muscles of the pectoral and pelvic fins are derivatives from more or fewer of the adjacent myotomes. The splanchnic muscles include the musculature of the walls of the alimentary canal, as well as those specialised portions of the visceral system which are represented by the muscles of the branchial arches and the jaws, and are composed of striated fibres.

LOCOMOTION.—A Fish and a Bird are equally remarkable for the many and various ways in which they are adapted for locomotion in the particular medium in which they live. In its shape the Fish is admirably adapted for cleaving the water. Spindle-like in shape, but thicker in front than behind, a Fish resembles {350}a double wedge, the thick part of which is represented by the head and one of the thin edges by the free hinder margin of the caudal fin. The body is bounded by smooth flowing contour lines, unbroken by any sharp separation of the body regions from one another, and with no points of resistance to its forward motion through the water. The body being thicker in front than behind, and, as seen in transverse section, broader above than below, it follows that its centre of gravity will be nearer the head than the tail, and nearer the dorsal than the ventral surface. The dorsal position of the centre of gravity necessarily renders the equilibrium of the body unstable, and were it not for the balancing action of the paired fins the Fish would float belly upwards, as is always the case after death. Most Fishes are provided with a membranous gas-containing sac, the air-bladder, the principal function of which is to render the Fish, bulk for bulk, of the same weight as the water, so that in this position of equilibrium, or plane of least effort, the animal can execute its various locomotor movements with a minimum expenditure of muscular effort—an advantage which no other animal possesses.[414] To give stability to the body, and to steady its course when swimming, the Fish has a dorsal and a ventral keel, formed by the anal and dorsal fins, which, like the sliding keel of a yacht, can be raised or lowered as occasion requires. When these fins are removed the course of the Fish becomes zigzag, and the animal wobbles.

The organs more directly concerned with swimming are the tail and the caudal fin, and the pectoral and pelvic fins, but the relative share which these structures take in the actual propulsion of the Fish differs greatly. The principal organ of locomotion in the typical Fish is the powerful muscular tail, which, in swimming, is lashed from side to side by the alternating contraction of the great longitudinal muscles on opposite sides of the vertebral column.[415] In such movements the tail is first flexed or bent, say to the right side: this stroke has been termed the non-effective or back stroke. By a stroke in the reverse direction the tail is then extended and straightened, that is to say, the Fish makes the forward or effective stroke. By a rapid succession of such strokes to the right and left sides alternately the Fish is {351}forced through the water. It is obvious, however, that the extension or effective stroke must have a considerable surplus of power over the flexion or non-effective stroke, and how this result is achieved will now be briefly considered. Experiment, and the observation of Fishes like the Sturgeon, which habitually move with sufficient slowness to allow the phases of their swimming movements to be followed without much difficulty, show that in swimming a Fish throws its body into two opposite and complementary curves, a cephalic curve formed by the anterior half of the body and a caudal curve by the tail. The double curve enables the Fish always to present a convex, less resisting or non-biting surface to the water during the flexion of the tail to the right or left as the case may be, and a concave or biting surface during extension, that is when the tail is straightening itself during the effective stroke.

Fig. 204, which represents a Fish in two successive positions while swimming, will serve to illustrate these conclusions. A Fish in the position A has its body thrown into a cephalic concavity directed to the right and a caudal concave surface facing the left. The tail is bent to the right of the line _a b_, which corresponds to the axis of the Fish when at rest and to the course pursued by the animal when swimming, and is in the position which it assumes during a flexion stroke, with its convex non-biting surface directed outwards and its concave biting surface inwards. The tail is now ready for an extension stroke, and while this is in progress it is clear that the concave biting surface of the tail will meet the water, while at the conclusion of the stroke the tail will be in a line with _a b_. At the same time the cephalic curve has so far diminished that the long axis of the body for a momentary period will also coincide with _a b_, and the Fish is free to advance without impediment. The tail, {352}however, continues its movement to the left, but now as a flexion stroke, and assumes the curvature and position indicated in B, with a reversal in the direction of both the cephalic and caudal curves, but in the meantime the force of the preceding extension stroke has forced the Fish along the line _a b_ to the new position indicated by B. By a rapid succession of alternating flexions and extensions, during which the tail describes figure-of-8 curves, the Fish travels in an undulating forward course with a maximum of propelling power and a minimum of "slip." In short, the action of the tail precisely resembles the action of the stern-oar in the operation of sculling a boat.

There are also other considerations which add to the surplus power of the extension stroke by lessening the resistance of the water to the flexion or non-effective stroke. During the flexion stroke the tail fin is less expanded and its area diminished, and by the rotation of the Fish on its long axis the surface of the tail strikes the water obliquely, and further, the tail moves with less rapidity. On the contrary, when the extension stroke is made these conditions are reversed. The caudal fin is expanded, the stroke is more rapid, and by the reverse rotation of the Fish the tail now strikes the water with its flat surface. In other words, the action of the tail during the two strokes may be compared to the "feathering" of an oar in rowing. Nor is this all. A Fish in motion through the water produces a suction current behind it. The current offers but little resistance to the flexion stroke, inasmuch as the direction of the two coincide, but during the extension stroke the tail meets the full force of the current, and consequently its grip and propelling power are greatly enhanced. There is a striking analogy between the movements of a Fish's tail in swimming and the action of the screw of a steamer, but as a propelling organ the former is far superior to the latter. As we have seen, the tail of a living Fish can so adjust its shape and surface that it alternately eludes and grips the water in accordance with the needs of particular strokes.

The curves into which the body of a Fish is thrown when swimming are never less than two, but in long-bodied Fishes, such as the Eels, the number may be increased, and in every case the curves occur in pairs and are complementary to one another.

{353}Many Fishes can jump out of the water, either in pursuit of insect food, like the Trout, or to enable them to escape the pursuit of their foes, like the Flying-Fish (_Exocoetus_), by means of a single forcible stroke of the tail, when the Fish is in a nearly vertical position close to the surface of the water. It is thus that the Salmon executes its remarkable leaps over weirs or up salmon-ladders when ascending rivers for spawning.

The tail is also used for steering. If kept bent to one side when the Fish is moving the tail acts like a rudder, and the course of the Fish is deflected to that side; or the direction may be altered by single strokes of the tail to the right or left, according to the course which the Fish desires to pursue.