Chapter IV: , the probability of the theory that the trigeminal group of (3)

+----------+----------+---------+---------+----------+---------+
| | | | Dorso- | | |
|V. Wijhe's|Eurypterid| Somatic | ventral |Coelomic| Coxal |
|segments. |segments. | motor |segmental| cavities.| glands. |
| | | nerves. | muscles.| | |
+----------+----------+---------+---------+----------+---------+
| | | | | | 1 |
| | 2} | 1 Oculo-| Sup. | 1 Pre- |Pituitary|
| 1 | 3} | motor |inf. int.|mandibular| body; |
| | 4} |supplying| rectus | fusion |fusion of|
| | 5} | 4 | and inf.| of 4 | 4 coxal |
| | | muscles | oblique | | glands |
+----------+----------+---------+---------+----------+---------+
| | | 1 Troch-| | | |
| 2 | 6 | learis | Sup. | 1 | |
| | |supplying| oblique |Mandibular| |
| | | 1 muscle| | | |
+----------+----------+---------+---------+----------+---------+

{309}THE TUBULAR MUSCLES.

The only musculature innervated by the trigeminal nerve which remains for further discussion, consists of those peculiar muscles found in the velum, known by the name of striated tubular muscles. This group of muscles has already been referred to in Chapter IV., dealing with respiration and the origin of the heart.

It is a muscular group of extraordinary interest in seeking an answer to the question of vertebrate ancestry, for, like the thyroid gland, it bears all the characteristics of a survival from a prevertebrate form, which is especially well marked in Ammocoetes. I have already suggested in this chapter that the homologues of these muscles are represented in Limulus by the veno-pericardial group of muscles. I will now proceed to deal with the evidence for this suggestion.

The structure of the muscle-fibres is peculiar and very characteristic, so that wherever they occur they are easily recognized. Each fibre consists of a core of granular protoplasm, in the centre of which the nuclei are arranged in a single row. This core is surrounded by a margin of striated fibrillæ, as is seen in Fig. 122. Such a structure is characteristic of various forms of striated muscle found in various invertebrates, such as the muscle-fibre of mollusca. It is, as far as I know, found nowhere in the vertebrate kingdom, except in Ammocoetes. At transformation these muscles entirely disappear, becoming fattily degenerated and then absorbed.

A, portion of fibre seen longitudinally; B, transverse section of fibre (osmic preparation); the black dots are fat-globules.]

For all these reasons they bear the stamp of a survival from a prevertebrate form. This alone would not make this tissue of any great importance, but when in addition these muscles are found to be arranged absolutely segmentally throughout the whole of the branchial region, then this tissue becomes a clue of the highest importance.

As mentioned in Chapter IV., the segmental muscles of respiration consist of the adductor muscle and the two constrictor muscles--the {310}striated constrictor and the tubular constrictor. Of these muscles, both the muscles possessing ordinary striation are attached to the branchial cartilaginous skeleton, whereas the tubular constrictors have nothing to do with the cartilaginous basket-work, but are attached ventrally in the neighbourhood of the ventral aorta.

These segmental tubular muscles are found also in the velar folds--the remains of the septum or velum which originally separated the oral from the respiratory chamber. In the branchial region they act with the other constrictors as expiratory muscles, forcing the water out of the respiratory chamber. In the living Ammocoetes, the velar folds on each side can be seen to move synchronously with the movements of respiration, contracting at each expiration; they thus close the slit by which the oral and respiratory chambers communicate, and therefore, in conjunction with the respiratory muscles, force the water of respiration to flow out through the gill-slits, as described by Schneider.

These tubular muscles thus form a dorso-ventral system of muscles essentially connected with respiration; they belong to each one of the respiratory segments, and are also found in the velum; anterior to this limit they are not to be found. What, then, are these tubular muscles in the velar folds? Miss Alcock has worked out their topography by means of serial sections, and, as already fully explained, has shown that they form exactly similar dorso-ventral groups, which belong to the two segments anterior to the purely branchial segments, _i.e._ to the facial or hyoid segments and the lower lip-segment of the trigeminal nerve. If the velar folds could be put back into their original position as a septum, then the hyoid or facial group of tubular muscles would take up exactly the same position as those belonging to each branchial segment.

The presence of these two so clearly segmental groups of muscles in the velum--the one belonging to the region of the trigeminal, the other to the region of the facial--is strong confirmation of my contention that this septum between the oral and respiratory chambers was caused by the fusion of the last prosomatic and the first mesosomatic appendages, represented in Limulus by the chilaria and the operculum.

Yet another clue to the meaning of these muscles is to be found in their innervation, which is very extraordinary and unexpected. Throughout the branchial region the striated muscles of each segment {311}are strictly supplied by the nerve of that segment, and, as already described, each segment is as carefully mapped out in its innervation as it is in any arthropod appendage. One exception occurs to this orderly, symmetrical arrangement: a nerve arises in connection with the facial nerve, and passes tailwards throughout the whole of the branchial region, giving off a branch to each segment as it passes. This nerve (_Br. prof._, Fig. 123) is known by the name of the _ramus branchialis profundus_ of the facial, and its extraordinary course has always aroused great curiosity in the minds of vertebrate anatomists. Miss Alcock, by the laborious method of following its course throughout a complete series of sections, finds that each of the segmental branches which is given off, passes into the tubular muscles of that segment (Fig. 124). The tubular muscles which belong to the velum, _i.e._ those belonging to the lower lip-segment and to the hyoid segments, receive their innervation from the velar or mandibular nerve, and belong, therefore, to the trigeminal, not to the facial, system.

Motor branches, _red_; sensory branches, blue.]

The evidence presented by these muscles is as follows:--

In the ancestor of the vertebrate there must have existed a segmentally arranged set of dorso-ventral muscles of peculiar structure, concerned with respiration, and confined to the mesosomatic segments and to the last prosomatic segment, yet differing from the other dorso-ventral muscles of respiration in their innervation and their attachment.

Interpreting these facts with the aid of my theory of the origin of vertebrates, and remembering that the homologue of the vertebrate ventral aorta in such a palæostracan as Limulus is the longitudinal {312}venous sinus, while the opercular and chilarial segments are respectively the foremost mesosomatic and the last prosomatic segments; they signify that the palæostracan ancestor must have possessed a separate set of segmental dorso-ventral muscles confined to the branchial, opercular and chilarial or metastomal segments, which, on the one hand, were respiratory in function, and on the other were attached to the longitudinal venous sinus. Further, these muscles must all have received a nerve-supply from the neuromeres belonging to the chilarial and opercular segments, an unsymmetrical arrangement of nerves, on the face of it, very unlikely to occur in an arthropod.

Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system. _Br. cart._, branchial cartilage; _M. con. str._, striated constrictor muscles; _M. con. tub._, tubular constrictor muscles; _M. add._, adductor muscle; _D.A._, dorsal aorta; _V.A._, ventral aorta; _S._, sense-organs on diaphragm; _n. Lat._, lateral line nerve; _X._, epibranchial ganglia of vagus; _R. br. prof. VII._, _ramus branchialis profundus_ of facial; _J.v._, jugular vein; _Ep. pit._, epithelial pit.]

{313}Is this prophecy borne out by the examination of Limulus? In the first place, these muscles were dorso-ventral and segmental, and, referring back to Chapter VII., Lankester arranges the segmental dorso-ventral muscles in three groups: (1) The dorso-ventral somatic muscles; (2) the dorso-ventral appendage muscles; and (3) the veno-pericardial muscles. Of these the first group is represented in the vertebrate by the muscles which move the eye, the second group by the striated constrictor and adductor muscles and the muscles for the lower lip. There is, then, the possibility of the third group for this system of tubular muscles.

Looking first at the structure of these muscles as previously described, so different are they in appearance from the ordinary muscles of Limulus, that Milne-Edwards, as already stated, called them "brides transparentes," and did not recognize their muscular character, while Blanchard called them in the scorpion, "ligaments contractils."

Consider their attachment and their function. They are attached to the longitudinal sinus, according to Lankester's observation, in such a way that the muscle-fibres form a hollow cone filled with blood; when they contract they force this blood towards the gills, and thus act as accessory or branchial hearts. According to Blanchard, in the scorpion they contract synchronously with the heart; according to Carlson, in Limulus they contract with the respiratory muscles. In Ammocoetes, where the respiration is effected after the fashion of Limulus, not of Scorpio, the tubular muscles are respiratory in function.

Look at their limits. The veno-pericardial muscles in Limulus are limited by the extent of the heart, they do not extend beyond the anterior limit of the heart. In Fig. 70 (p. 176) two of these muscles are seen in front of the branchial region also attached to the longitudinal venous sinus, although in front of the gill-region. In Ammocoetes the upper limit of the tubular muscles is the group found in the velum; this most anterior group belongs to a region in front of the branchial region--that of the trigeminal.

Moreover, the supposition that the segmental tubular muscles belong throughout to the veno-pericardial group gives an adequate reason why they do not occur in front of the velum; for, as their existence is dependent upon the longitudinal collecting sinus in Limulus and Scorpio, which is represented by the ventral aorta in {314}Ammocoetes, they cannot extend beyond its limits. Now, Dohrn asserts that the ventral aorta terminates in the spiracular artery, which exists only for a short time; and, in another place, speaking of this same termination of the ventral aorta, he states: "Dass je eine vorderste Arterie aus den beiden primären Aesten des Conus arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der Mesodermfalte des späteren Velums in die Höhe steigt um in die Aorta der betreffenden Seite einzumunden." These observations show that the vessel which in Ammocoetes represents the longitudinal collecting sinus in the Merostomata does not extend further forwards than the velum, and in consequence the representatives of the veno-pericardial muscles cannot extend into the segments anterior to the velum. One of the extraordinary characteristics of these tubular muscles which distinguishes them from other muscles, but brings them into close relationship with the veno-pericardial group, is the manner in which the bundles of muscle-fibres are always found lying freely in a blood-space; this is clearly seen in the branchial region, but most strikingly in the velum, the interior of which, apart from its muco-cartilage, is simply a large lacunar blood-space traversed by these tubular muscles.

All these reasons point to the same conclusion: the tubular muscles in Ammocoetes are the successors of the veno-pericardial system of muscles.

If this is so, then this homology ought to throw light on the extraordinary innervation of these tubular muscles by the _branchialis profundus_ branch of the facial nerve and the velar branch of the trigeminal. We ought, in fact, to find in Limulus a nerve arising exclusively from the ganglia belonging to the chilarial and opercular segments, which, instead of being confined to those segments, traverses the whole branchial region on each side, and gives off a branch to each branchial segment; this branch should supply the veno-pericardial muscle of that side.

Patten and Redenbaugh have traced out the distribution of the peripheral nerves in Limulus, and have found that from each mesosomatic ganglion a segmental cardiac nerve arises which passes to the heart and there joins the cardiac median nerve, or rather the median heart-ganglion, for this so-called nerve is really a mass of ganglion-cells. In all the branchial segments the same plan exists, each cardiac nerve belonging to that neuromere is strictly segmental. {315}Upon reaching the opercular and chilarial neuromeres an extraordinary exception is found; the cardiac nerves of these two neuromeres are fused together, run dorsally, and then form a single nerve called the pericardial nerve, which runs outside the pericardium along the whole length of the mesosomatic region, and gives off a branch to each of the cardiac nerves of the branchial neuromeres as it passes them.

This observation of Patten and Redenbaugh shows that the pericardial nerve of Limulus agrees with the very nerve postulated by the theory, as far as concerns its origin from the chilarial and opercular neuromeres, its remarkable course along the whole branchial region, and its segmental branches to each branchial segment.

At present the comparison goes no further; there is no evidence available to show what is the destination of these segmental branches of the pericardial nerve, and so far all evidence of their having any connection with the veno-pericardial muscles is wanting. Carlson, at my request, endeavoured in the living Limulus to see whether stimulation of the pericardial nerve caused contraction of the veno-pericardial muscles, but was unable to find any such effect. On the contrary, his experimental work indicated that each veno-pericardial muscle received its motor supply from the corresponding mesosomatic ganglion. This is not absolutely conclusive, for if, as Blanchard asserts in the case of the scorpion, a close connection exists between the action of these muscles and of the heart, it is highly probable that their innervation conforms to that of the heart. Now Carlson has shown that this cardiac nerve from the opercular and chilarial neuromeres is an inhibitory nerve to the heart, while the segmental cardiac nerves belonging to the branchial ganglia are the augmentor nerves of the heart.

His experiments, then, show that the motor nerves of the heart and of the veno-pericardial muscles run together in the same nerves, but he says nothing of the inhibitory nerves to the latter muscles. If they exist and if they are in accordance with those to the heart, then they ought to run in the pericardial nerve, and would naturally reach the veno-pericardial muscles by the segmental branches of the pericardial nerve.

Moreover, inhibitory nerves are, in certain cases, curiously associated with sensory fibres; so that the nerve which corresponds {316}to the pericardial nerve, viz. the _branchialis profundus_ of the facial, may be an inhibitory and sensory nerve, and not motor at all. Miss Alcock's observations are purely histological; no physiological experiments have been made.

At present, then, it does not seem to me possible to say that Carlson's experiments have disproved _any_ connection of the pericardial nerve with the veno-pericardial muscles. We do not know what is the destination of its segmental branches; they may still supply the veno-pericardial muscles even if they do not cause them to contract; they certainly do not appear to pass directly into them, for they pass into the segmental cardiac nerves, and can only reach the muscles in conjunction with their motor nerves. Such a course would not be improbable when it is borne in mind how, in the frog, the augmentor nerves run with the inhibitory along the whole length of the vagus nerve.

Until further evidence is given both as to the function of the segmental branches of the pericardial nerve in the Limulus, and of the _branchialis profundus_ in Ammocoetes, it is impossible, I think, to consider that the phylogenetic origin of these tubular muscles is as firmly established as is that of most of the other organs already considered. I must say, my own bias is strongly in favour of looking upon them as the last trace of the veno-pericardial system of muscles, a view which is distinctly strengthened by Carlson's statement that the latter system contracts synchronously with the respiratory movements, for undoubtedly in Ammocoetes their function is entirely respiratory. Then again, although at present there is no evidence to connect the pericardial nerve in Limulus with this veno-pericardial system of muscles, yet it is extraordinarily significant that in such animals as Limulus and Ammocoetes, in both of which the mesosomatic or respiratory region is so markedly segmental, an intrusive nerve should, in each case, extend through the whole region, giving off branches to each segment. Still more striking is it that this nerve should arise from the foremost mesosomatic and the last prosomatic neuromeres in Limulus--the opercular and chilarial segments--precisely the same neuromeres which give origin to the corresponding nerve in Ammocoetes, for according to my theory of the origin of vertebrates, the nerves which supplied the opercular and metastomal appendages have become the facial nerve and the lower lip-branch of the trigeminal nerve.

{317}With the formation of the vertebrate heart from the two longitudinal venous sinuses and the abolition of the dorsal invertebrate heart, the function of these tubular muscles as branchial hearts was no longer needed, and their respiratory function alone remained. The last remnant of this is seen in Ammocoetes, for the ordinary striated muscles were always more efficient for the respiratory act, and so at transformation the inferior tubular musculature was got rid of, there being no longer any need for its continued existence.

THE PALÆOSTOMA, OR OLD MOUTH.

The arrangement of the oral chamber in Ammocoetes is peculiar among vertebrates, and, upon my theory, is explicable by its comparison with the accessory oral chamber which apparently existed in Eurypterus. According to this explanation, the lower lip of the original vertebrate mouth was formed by the coalescence of the most posterior pair of the prosomatic appendages--the chilaria; from which it follows that the vertebrate mouth was not the original mouth, but a new structure due to such a formation of the lower lip.

It is very suggestive that the direct following out of the original working hypothesis should lead to this conclusion, for it is universally agreed by all morphologists that the present mouth is a new formation, and Dohrn has argued strongly in favour of the mouth being formed by the coalescence of a pair of gill-slits. Interpret this in the language of my theory, and immediately we see, as already explained, gill-slits must mean in this region the spaces between appendages which did not carry gills; the mouth, therefore, was formed by the coalescence of a pair of appendages to form a lower lip just as I have pointed out.

Where, then, must we look for the palæostoma, or original mouth? Clearly, as already suggested, it was situated at the base of the olfactory passage, and the olfactory passage or nasal tube of Ammocoetes was originally the tube of the hypophysis, so that the following out of the theory points directly to the tube of the hypophysis as the place where the palæostoma must be looked for.

This conclusion is not only not at variance with the opinions of morphologists, but gives a straightforward, simple explanation why the palæostoma was situated in the very place where they are most inclined to locate it. Thus, if we trace the history of the question, {318}we see that Dohrn's original view of the comparison of the vertebrate and the annelid led him to the conception that the vertebrate mouth was formed by the coalescence of a pair of gill-slits, and that the original mouth was situated somewhere on the dorsal surface and opened into the gut by way of the infundibulum and the tube of the hypophysis. This, also, was Cunningham's view as far as the tube of the hypophysis was concerned. Beard, in 1888, holding the view that the vertebrates were derived from annelids which had lost their supra-oesophageal ganglia, and that, therefore, there was no question of an oesophageal tube piercing the central nervous system of the vertebrate, explained the close connection of the infundibulum with the hypophysis by the comparison of the tube of the hypophysis with the annelidan mouth, so that the infundibular or so-called nervous portion was a special nervous innervation for the original throat, just as Kleinenberg had shown to be the case in many annelids. Beard therefore called this opening of the hypophysial tube the old mouth, or palæostoma. Recently, in 1893, Kupffer has also put forward the view that the hypophysial opening is the palæostoma. basing this view largely upon his observations on Ammocoetes and Acipenser.

_Nc._, neural canal with its infundibular termination; _Nch._, notochord; _Al._, alimentary canal with its anterior diverticulum; _Hy._, hypophysial or nasal tube; _Or._, oral chamber closed by septum.]

As is seen in Fig. 125, the position of this palæostoma is a very suggestive one. At this single point in Ammocoetes, four separate tubes terminate; here is the end of the notochordal tube, the termination of the infundibulum, the blind end of the nasal tube or tube {319}of the hypophysis, and the pre-oral elongation of the alimentary canal.

It is perfectly simple and easy for the olfactory tube to open into any one of the other three. By opening into the infundibulum it reproduces the condition of affairs seen in the scorpion; by opening into the gut it produces the actual condition of things seen in Myxine and other vertebrates; by opening into the notochordal tube it would produce a transitional condition between the other two.

The view held by Kupffer is that this nasal tube (tube of the hypophysis) opened into the anterior diverticulum of the vertebrate gut, and was for this reason the original mouth-tube; then a new mouth was formed, and this connection was closed, being subsequently reopened as in Myxine. My view is that this tube originally opened into the infundibulum, in other words, into the original gut of the palæostracan ancestor, and was for this reason the original mouth-tube, in the same sense as the olfactory passage of the scorpion may be, and often is, called the mouth-tube. When, with the breaking through of the septum between the oral and respiratory chambers, the external opening of the oral chamber became a new mouth, the old mouth was closed but the olfactory tube still remained, owing to the importance of the sense of smell. Subsequently, as in Myxine and the higher vertebrates, it opened into the pharynx, and so formed the nose of the higher vertebrates.

It is not, to my mind, at all improbable that during the transition stage, between its connection with the old alimentary canal, as in Eurypterus or the scorpions, and its blind ending, as in Ammocoetes, the nasal tube opened into the tube of the notochord. This question will be discussed later on when the probable significance of the notochord is considered.

THE PITUITARY GLAND.

Turning back to the comparison of Fig. 106, B, and Fig. 106, C, which represent respectively an imaginary sagittal section through an Eurypterus-like animal and through Ammocoetes at a larval stage, all the points for comparison mentioned on p. 244 have now been discussed with the exception of the suggested homology between the coxal glands of the one animal and the pituitary body of the other.

{320}This latter gland undoubtedly arises posteriorly to the hypophysial tube, or Rathke's pouch (as it is sometimes called), and, as already mentioned, is supposed by Kupffer to be formed from the posterior wall of this pouch. More recently, as pointed out in Haller's paper, Nusbaum, who has investigated this matter, finds that the glandular hypophysis is not formed from the walls of Rathke's pouch, but from the tissue of the rudimentary connection or stalk between the two premandibular cavities, which becomes closely connected with the posterior wall of Rathke's pouch, and becoming cut off from the rest of the premandibular cavity on each side, becomes permanently a part of the 'Hypophysis Anlage.'

The importance of Nusbaum's investigation consists in this, that he derives the glandular hypophysis from the connecting stalk between the two premandibular cavities, and therefore from the walls of the ventral continuation of this cavity on each side.

This may be expressed as follows:--

The coelomic cavity, known as the premandibular cavity, divides into a dorsal and a ventral part; the walls of the dorsal part give origin to the somatic muscles belonging to the oculomotor nerve, while the walls of the ventral part on each side form the connecting stalk between the two cavities, and give origin to the glandular hypophysis.

Now, as already pointed out, the premandibular cavity is homologous with the 2nd prosomatic coelomic cavity of Limulus, and this 2nd prosomatic coelomic cavity divides, according to Kishinouye, into a dorsal and a ventral part; and, further, the walls of this ventral part form the coxal gland. Both in the vertebrate, then, and in Limulus, we find a marked glandular tissue in a corresponding position, and the conclusion is forced upon us that the glandular hypophysis was originally the coxal gland of the invertebrate ancestor. As in all other cases already considered, when the facts of topographical anatomy, of morphology and of embryology, all combine to the same conclusion as to the derivation of the vertebrate organ from that of the invertebrate, then there must be also a structural similarity between the two. What, then, is the nature of the coxal gland in the scorpions and Limulus? Lankester's paper gives us full information on this point as far as the scorpion and Limulus are concerned, and he shows that the coxal gland of Limulus differs markedly from that of Scorpio in the size of the cells and in the {321}arrangement of the tubes. In Fig. 126, A, I give a picture of a piece of the coxal gland of Limulus taken from Lankester's paper.

Turning now to the vertebrate, Bela Haller's paper gives us a number of pictures of the glandular hypophysis from various vertebrates, and he especially points out the tubular nature of the gland and its solidification in the course of development in some cases. In Fig. 126, B, I give his picture of the gland in Ammocoetes.

The striking likeness between Haller's picture and Lankester's picture is apparent on the face of it, and shows clearly that the histological structure of the glands in the two cases confirms the deductions drawn from their anatomical and morphological positions.

_n.a._, termination of nasal passage.]

The sequence of events which gave rise to the pituitary body of the vertebrate was in all probability somewhat as follows:--

Starting with the excretory glands of the Phyllopoda, known as shell-glands, which existed almost certainly in the phyllopod Trilobite, we pass to the coxal gland of the Merostomata. Judging from Limulus, these were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor appendages. When these appendages became reduced in size and purely tactile they were compressed and concentrated round the mouth region, forming the endognaths of the Merostomata; as a necessary consequence of the concentration of the coxæ of the endognaths, the coxal gland also became concentrated, {322}and took up a situation close against the pharynx, as represented in Fig. 106, B. When, then, the old mouth closed, and the pharynx became the _saccus vasculosus_, the coxal gland remained in close contact with the _saccus vasculosus_, and became the pituitary body, thus giving the reason why there is always so close a connection between the pituitary body and the infundibular region.

Whatever was the condition of the digestive tracts at the transition stage between the arthropod and the vertebrate, the original mouth-opening at the base of the olfactory tube was ultimately closed. The method of its closure was exceedingly simple and evident. The membranous cranium was in process of formation by the extension of the plastron laterally and dorsally; a slight growth of the same tissue in the region of the mouth would suffice to close it and thus separate the infundibulum from the olfactory tube. As evidence that such was the method of closure, it is instructive to see how in Ammocoetes the glandular tissue of the pituitary body is embedded in and mixed up with the tissue of this cranial wall; how the termination of the nasal tube is embedded in this same thickened mass of the cranial wall--how, in fact, both coxal gland and olfactory tube have become involved in the growth of the tissue of the plastron, by means of which the mouth was closed.

I have now passed in review the nature of the evidence which justifies a comparison between the segments supplied by the cranial nerves of the vertebrate and the prosomatic and mesosomatic segments of the palæostracan. For the convenience of my readers I have put these conclusions into tabular form (see p. 323), for all the segments as far as that supplied by the glossopharyngeal nerves. In both vertebrate and invertebrate this is a fixed position, for in the former, however variable may be the number of branchial segments which the vagus supplies, the second branchial segment is always supplied by a separate nerve, the glossopharyngeal, and in the latter, though the number of segments bearing branchiæ varies, the minimum number of such segments (as seen in the Pedipalpi) is never less than two.

{323}TABLE OF COMPARISON OF CORRESPONDING SEGMENTS IN THE EURYPTERIDS AND IN AMMOCOETES (_i.e._ IN CEPHALASPIDS).

Key:
So. = Supra-oesophageal.
Si. = Supra-infundibular.
Io. = Infra-oesophageal.
Ii. = Infra-infundibular.
Ps. = Prosomatic.
Ms. = Mesosomatic.

+---+--------------------------------------------------------+
| | Median Eyes. |
| +--------------------------------------------------------+
|So.| Lateral Eyes. |
| +--------------------------------------------------------+
| | Camerostome. |
+---+--------------------------------------------------------+
| | Invertebrate (Limulus or Eurypterid). |
+---+---+---------+-----------------------------+------------+
| | | | Appendages. | Coelomic |
| | |Segments.+-------------+---------------+ Cavities. |
| | | | Limulus. | Eurypterid. | |
| | +---------+-------------+---------------+------------+
| | | 1 | Cheliceræ | Cheliceræ | 1 |
| | +---------+-------------+---------------+------------+
| | | 2 |1st Locomotor|} | |
| | +---------+-------------+} | 2 |
| | | 3 | 2nd " |} |Ventral part|
| |Ps.+---------+-------------+}4 Endognaths | forms coxal|
| | | 4 | 3rd " |} | gland. |
| | +---------+-------------+} | |
| | | 5 | 4th " |} | |
| | +---------+-------------+---------------+------------+
|Io.| | 6 | 5th " | Ectognath | 3 |
| | | | | | |
| | +---------+-------------+---------------+------------+
| | | 7 | Chilaria | Metastoma | 4 |
| | | | | | |
| +---+---------+-------------+---------------+------------+
| | | 8 | Operculum |Genital } | 5 |
| | |---------+-------------+ }Oper-+------------+
| | | 9 |1st Branchial| }culum| 6 |
| | | | |Branchial} | |
| |Ms.+---------+-------------+---------------+------------+
| | | 10 | 2nd " | 2nd Branchial | 7 |
+---+---+---------+-------------+---------------+------------+

+-----------------------------------------------------+------+
| Pineal Eyes. | |
+-----------------------------------------------------+ |
| Lateral Eyes. | Si. |
+-----------------------------------------------------+ |
| Olfactory Organ. | |
+-----------------------------------------------------+------+
| Vertebrate (Ammocoetes or Cephalaspid). | |
+-------------+---------------+-------------+---------+------+
| | Splanchnic | Somatic | Somatic | |
| Appendages. | Nerves. | Segmental | Nerves. | |
| | | Muscles. | | |
+-------------+---------------+-------------+---------+ |
| ... | ... | ... | ... | |
+-------------+---------------+-------------+---------+ |
| | | | | |
| | V | Muscles | | |
|4 tentacles | Tentacular | supplied by | III | |
| and upper | and upper | oculomotor | | |
| lip. | lip nerve. | nerve. | | |
| | | | | |
| | | | | |
+-------------+---------------+-------------+---------+ |
| Tongue | V | Sup.oblique | IV | Ii. |
| | Tongue nerve | | | |
+-------------+---------------+-------------+---------+ |
| Lower lip | V | | | |
| |Lower lip nerve| | | |
+-------------+---------------+-------------+---------+ |
| Thyroid |} | | | |
+-------------+} VII | Ext. rectus | VI | |
| Hyoid or |} |Retract oculi| | |
|1st Branchial|} | | | |
+-------------+---------------+-------------+---------+ |
|2nd Branchial| IX | ... | ... | |
+-------------+---------------+-------------+---------+------+

+-------------------------+------+
| Pineal Nerve. | |
+-------------------------+ |
| II | Si. |
+-------------------------+ |
| I | |
+-------------------------+------+
| Vertebrate. | |
+-------------+-----------+------+
| Coelomic |V. Wijhe's | |
| Cavities. | Segments. | |
| | | |
+-------------+-----------+ |
| Anterior. | ... | |
+-------------+-----------+ |
| Premandi- | | |
| bular | | |
| Ventral | | |
| part forms | 1 | |
| pituitary | | |
| body. | | |
| pituitary | | |
+-------------+-----------+ |
| Mandibular |} | Ii. |
| |} | |
+-------------+} 2 | |
| Mandibular |} | |
| |} | |
+-------------+-----------+ |
| Hyoid_1 | 3 | |
+-------------+-----------+ |
| Hyoid_2 | 4 | |
| | | |
+-------------+-----------+ |
|2nd Branchial| 5 | |
+-------------+-----------+------+

{324}SUMMARY.

The general consideration of the evidence of the number of segments, and
their nature in the pro-otic region of the vertebrate, as given in the
last chapter, is not incompatible with the view that the trigeminal nerve
originally supplied seven appendages, which appendages did not carry
branchiæ, but were originally used for purposes of locomotion as well as
of mastication.

Such appendages clearly no longer exist in the higher vertebrates, the
muscles of mastication only remaining; but in the earliest fish-forms
they must have existed, as, indeed, is seen in Pterichthys and
Bothriolepis. Judging from all the previous evidence some signs of their
existence may reasonably be expected still to remain in Ammocoetes. Such
is indeed the case.

In the adult Petromyzon the trigeminal nerve innervates specially a
massive suctorial apparatus, by means of which it holds on to other
fishes, or to stones in the bottom of the stream. There is here no
apparent sign of appendages. Very great, however, is the difference in
the oral chamber of Ammocoetes; here there is no sign of any suctorial
apparatus, but instead, a system of tentacles, together with the remains
of the septum or velum, which originally closed off the oral from the
respiratory chamber. These tentacles are the last remnants of the
original foremost prosomatic appendages of the palæostracan ancestor.
Like the lateral eyes they do not develop until the transformation comes,
but during the whole larval condition their musculature remains in an
embryonic condition, and then from these embryonic muscles the whole
massive musculature of the suctorial apparatus develops; a sucking
apparatus derived from the modification of appendages, as so frequently
occurs in the arthropods.

The study of Ammocoetes indicates that the velum and lower lip correspond
to the metastoma of the Eurypterid, _i.e._ the chilaria of Limulus, while
the large ventral pair of tentacles, called the tongue, correspond to the
ectognaths of the Eurypterids, and probably to the oar-like appendages of
Pterichthys and Bothriolepis. From these two splanchnic segments the
suctorial apparatus in the main arises; the motor supply of these two
segments forms the mass of the trigeminal nerve-supply, and the nerves
supplying them, the velar nerve and the tongue-nerve, are markedly
separate from the rest of the trigeminal nerve.

The rest of the tentacles present much less the sign of independent
segments. In their nerves, their muco-cartilaginous skeleton, and their
rudimentary muscles, they indicate a concentration and amalgamation, such
as might be expected from the concentrated endognaths. The continuation
of the dwindling process, already initiated in the Eurypterid, would
easily result in the tentacles of Ammocoetes.

The nasal tube of Ammocoetes, which originates in the hypophysial tube,
corresponds absolutely in position and in its original structure, to the
olfactory tube of a scorpion-like animal. From this homology two
conclusions of importance follow: (1) the old mouth, or palæostoma, of
the vertebrate was situated at the end of this tube, therefore, at the
termination of the infundibulum; (2) the upper lip, which by its growth,
brings the olfactory tube from a ventral to a dorsal position, was
originally formed by the foremost sternites or endostoma, or else by the
sterno-coxal processes of the second pair of prosomatic appendages of the
palæostracan ancestor.

In strict accordance with the rest of the comparisons made in this
region, the pituitary body shows by similarity of structure, as well as
of position, that it arose from the coxal glands, which were situated at
the base of the four endognaths.

{325}One after another, when once the clue has been found, all these
mysterious organs of the vertebrate, such as the pituitary and thyroid
glands, fall harmoniously into their place as the remnants of
corresponding important organs in the palæostraca.

Yet another clue is afforded by the tubular muscles of Ammocoetes, that
strange set of non-vertebrate striated muscles, which are so markedly
arranged in a segmental manner, which disappear at transformation, and
are never found in any of the higher vertebrates, for the limits of their
distribution correspond to the veno-pericardial muscles of Limulus.

Their nerve-supply in Ammocoetes is most extraordinary; for, although
they are segmentally arranged throughout the whole respiratory region,
which is segmentally supplied by the VIIth, IXth, and Xth nerves, and are
found in front of this region only in one segment, that of the lower lip,
which is supplied by the velar branch of the Vth nerve, yet they are not
supplied segmentally, but only by the velar nerve and a branch of the
VIIth, the _ramus branchialis profundus_. This latter nerve extends
throughout the respiratory region, and gives off segmental branches to
supply these muscles.

It is also a curious coincidence that in such a markedly segmented animal
as Limulus, a nerve--the pericardial nerve--which arises from the nerves
of the chilarial and opercular segments, should pass along the whole
respiratory region and give off branches to each mesosomatic segment. It
is strange, to say the least of it, that the chilarial or metastomal and
the opercular segments of Limulus should, on the theory advocated in this
book, correspond to the lower lip and hyoid segments of the vertebrate.
At present the homology suggested is not complete, for there is no
evidence as yet that the veno-pericardial muscles have anything to do
with the pericardial nerve.

{326}CHAPTER X

_THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE OSTRACODERMATA_

The nose of the Osteostraci.--Comparison of head-shield of Ammocoetes and
of Cephalaspis.--Ammocoetes the only living representative of these
ancient fishes.--Formation of cranium.--Closure of old mouth.--Rohon's
primordial cranium.--Primordial cranium of Phrynus and
Galeodes.--Summary.

The shifting of the orifice of the olfactory passage, which led to the old mouth, from the ventral to the dorsal side, as seen in the transformation of the ventrally situated hypophysial tube of the young Ammocoetes, to the dorsally situated nasal tube of the full-grown Ammocoetes, affords one of the most important clues in the whole of this story of the origin of vertebrates; for, if Ammocoetes is the nearest living representative of the first-formed fishes, then we ought to expect to find that the dorsal head-shield of such fishes is differentiated from that of the contemporary Palæostraca by the presence of a median frontal opening anterior to the eyes. Conversely, if such median nasal orifice is found to be a marked characteristic of the group, in combination with lateral and median eyes, as in Ammocoetes, then we have strong reasons for interpreting these head-shields by reference to the head of Ammocoetes.

The oldest known fishes belong to a large group of strange forms which inhabited the Silurian and Devonian seas, classed together by Smith Woodward under the name of Ostracodermi. These are divided into three orders: (1) the Heterostraci, including one family, the Pteraspidæ, to which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into two families, the Cephalaspidæ and Tremataspidæ, which include Cephalaspis, Eukeraspis, Auchenaspis or Thyestes, and Tremataspis; and (3) the Antiarcha, with one family, the Astrolepidæ, including Astrolepis, Pterichthys, and Bothriolepis. {327}Of these, the first two orders belong to the Upper Silurian, while the third is Devonian.

THE DORSAL HEAD-SHIELD OF THE OSTEOSTRACI.

Of the three orders above-named, the Heterostraci and Osteostraci are the oldest, and among them the Cephalaspidæ have afforded the most numerous and best worked-out specimens. At Rootziküll, in the island of Oesel, the form known as _Thyestes (Auchenaspis) verrucosus_ is especially plentiful, being found thickly present in among the masses of Eurypterid remains, which give the name to the deposit. Of late years this species has been especially worked at by Rohon, and many beautiful specimens have been figured by him, so that a considerable advance has been made in our knowledge since Pander, Eichwald, Huxley, Lankester, and Schmidt studied these most interesting primitive forms.

All observers agree that the head-region of these fishes was covered by a dorsal and ventral head-shield, while the body-region was in most cases unknown, or, as in Eichwald's specimens, and in the specimens figured in Lankester and Smith Woodward's memoirs, was made up of segments which were not vertebral in character, but formed an aponeurotic skeleton, being the hardened aponeuroses between the body-muscles. This body-skeleton, which possesses its exact counterpart in Ammocoetes, will be considered more fully when I discuss the origin of the spinal region of the vertebrate.

Of the two head-shields, ventral and dorsal, the latter is best known and characterizes the group. It consists of a dorsal plate, with characteristic horns, which in _Thyestes verrucosus_ (Fig. 128), as described by Rohon, is composed of two parts, a frontal part and an occipital part (_occ._), the occipital part being composed of segments, and possessing a median ridge--the _crista occipitalis_. In Lankester's memoir and in Smith Woodward's catalogue, a large number of known forms are described and delineated, and we may perhaps say that in some of the forms, such as _Eukeraspis pustuliferus_ (Fig. 127, B), the frontal part of the shield only is capable of preservation as a fossil, while in Cephalaspis (Fig. 127, A) not only the frontal part but a portion of the occipital region is preserved, the latter being small in extent when compared with the occipital region of Auchenaspis (Thyestes). Finally, in Tremataspis and Didymaspis, the whole of both frontal {328}and occipital region is capable of preservation, the line of demarcation between these two regions being well marked in the latter species.

In the best preserved specimens of all this group of fishes a frontal median orifice is always present; it appears in some specimens obscurely partially divided into two parts. Perhaps the best specimen of all was obtained by Rohon at Rootziküll, and is thus described by him:--

The frontal part of the dorsal head-plate carried (Fig. 128) the two orbits for the lateral eyes (_l.e._), a marked frontal organ (_fro._), and a median depression (_gl._), to which he gives the name parietal organ. The occipital part (_occ._) was clearly segmented, and carried, he thinks, the branchiæ. I reproduce Rohon's figure of the frontal organ in Thyestes (Fig. 129); he describes it as a deeply sunk pit, divided in the middle by a slit, which leads deeper in, he supposes, towards the central nervous system.

_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate over brain; _Occ._, occipital region.]

{329}A similar organ was described by Schmidt in Tremataspis, and considered by him to be a median nose. Such also is the view of Jaekel, who points out that a median pineal eye exists between the two lateral eyes in this animal, as in all other of these ancient fishes, so that this frontal organ does not, as Patten thinks, represent the pineal eye. The whole of this group of fishes, then, is characterized by the following striking characteristics:--

1. Two well-marked lateral eyes near the middle line.

2. Between the lateral eyes, well-marked median eyes, very small.

3. In front of the eye-region a median orifice, single.

In addition, behind the eye-region a median plate is always found, frequently different in structure to the rest of the head-shield, being harder in texture--the so-called post-orbital plate.

STRUCTURE OF HEAD-SHIELD OF CEPHALASPIS COMPARED WITH THAT OF AMMOCOETES.

What is the structure of this head-shield? It has been spoken of as formed of bone because it possesses cells, being thus unlike the layers of chitin, which are formed by underlying cells but are not themselves cellular. At the same time, it is recognized on all sides that it has no resemblance to bone-structure as seen in fossil remains of higher vertebrates. The latest and best figure of the structure of this so-called bone is given in Rohon's paper already referred to. It is, so he describes, clearly composed of fibrillæ and star-shaped cells, arranged more or less in regular layers, with other sets of similar cells and fibrillæ arranged at right angles to the first set, or at varying angles. The groundwork of this tissue, in which these cells and fibrils are embedded, contained calcium salts, and so the whole tissue was preserved. In places, spaces are found in it, in the deepest layer large medullary spaces; more superficially, ramifying spaces which he considers to be vascular, and calls Haversian canals; the {330}star-like cells, however, are not arranged concentrically around these spaces, as in true Haversian canals.

This structure is therefore a calcareous infiltration of a tissue with cells in it. Where is there anything like it?

As soon as I saw Rohon's picture (Fig. 130), I was astounded at its startling resemblance to the structure of muco-cartilage as is seen in Fig. 131, taken from Ammocoetes. If such muco-cartilage were infiltrated with lime salts, then the muco-cartilaginous skeleton of Ammocoetes would be preserved in the fossil condition, and be comparable with that of Cephalaspis, etc.

The whole structure is clearly remarkably like Rohon's picture of a section of the head-plate of a Cephalaspid (Fig. 130). In the latter case the matrix contains calcium salts, in the former it is composed of the peculiar homogeneous mucoid tissue which stains so characteristically with thionin. With respect to this calcification, it is instructive to recall the calcification in the interior of the branchial cartilages of Limulus, as described in Chapter III., for this example shows how easy it is to obtain a calcification in this chondro-mucoid material. With respect to the medullary spaces and smaller spaces in this tissue, as described by Rohon, I would venture to suggest that they need not all necessarily indicate blood-vessels, for similar spaces would appear in the head-shield of Ammocoetes if its muco-cartilage alone {331}were preserved. Of these, some would indicate the position of blood-vessels, such, for instance, as of the external carotid which traverses this structure; but the largest and most internal spaces, resembling Rohon's medullary spaces, would represent muscles, being filled up with bundles of the upper lip-muscles.

THE MUCO-CARTILAGINOUS HEAD-SHIELD OF AMMOCOETES.

The resemblance between the structure of the head-shield of Thyestes and the muco-cartilage of Ammocoetes, is most valuable, for muco-cartilage is unique, occurs in no other vertebrate, and every trace of it vanishes at transformation; it is essentially a characteristic of the larval form, and must, therefore, in accordance with all that has gone before, be the remnant of an ancestral skeletal tissue. The whole story deduced from the study of Ammocoetes would be incomplete without some idea of the meaning of this tissue. So also, as already mentioned, the skeleton of Ammocoetes is incomplete without taking this tissue into account. It is confined entirely to the head-region; no trace of it exists posteriorly to the branchial basket-work. It consists essentially of dorsal and ventral head-shields, connected together by the tentacular, metastomal, and thyroid bars, as already described. The ventral shield forms the muco-cartilaginous plate of the lower lip and the plate over the thyroid gland, so that the skeleton ventrally is represented by Fig. 118, B, which shows how the cartilaginous bars of the branchial basket-work are separated from each other by this thyroid plate. At transformation, with the disappearance of this muco-cartilaginous plate, the bars come together in the middle line, as in the more posterior portion of the branchial basket-work.

The dorsal head-shield of muco-cartilage covers over the upper lip, sends a median prolongation over the median pineal eyes and a lateral prolongation on each side as far as the auditory capsules, giving the shape of the head-shield of muco-cartilage, as in Fig. 118, C.

Not only then is the structure of the head-shield of a Cephalaspid remarkably like the muco-cartilage of Ammocoetes, but also its general distribution strangely resembles that of the Ammocoetes muco-cartilage.

Now, these head-shields in the Cephalaspidæ and Tremataspidæ {332}vary very much in shape, as is seen by the comparison of Tremataspis and Auchenaspis with Cephalaspis and Eukeraspis, and yet, undoubtedly, all these forms belong to a single group, the Osteostraci.

The conception that Ammocoetes is the solitary living form allied to this group affords a clue to the meaning of this variation of shape, which appears to me to be possible, if not indeed probable. There is a certain amount of evidence given in the development of Ammocoetes which indicates that the branchial region of its ancestors was covered with plates of muco-cartilage as well as the prosomatic region.

The evidence is as follows:--

The somatic muscles of Ammocoetes form a continuous longitudinal sheet of muscles along the length of the body, which are divided up by connective tissue bands into a series of imperfect segments or myotomes. This simple muscular sheet can be dissected off along the whole of the head-region of the animal, with the exception of the most anterior part, without interfering with the attachments or arrangements of the splanchnic muscular system in the least. The reason why this separation can be so easily effected is to be found in the fact that the two sets of muscles are not attached to the same fascia. The sheet of fascia to which the somatic muscles are attached is separated from the fascia which encloses the branchial cavity by a space (_cf._ Figs. 63 and 64) filled with blood-spaces and cells containing fat, in which space is also situated the cartilaginous branchial basket-work. These branchial bars are closely connected with the branchial sheet of fascia, and have no connection with the somatic fascia, their perichondrium forming part of the former sheet. Upon examination, this space is seen to be mainly vascular, the blood-spaces being large and frequently marked with pigment; but it also possesses a tissue of its own, recognized as fat-tissue by all observers. The peculiarity of the cells of this tissue is their arrangement; they are elongated cells arranged at right angles to the plates of fascia, just as the fibres of the muco-cartilage are largely arranged at right angles to their limiting plates of perichondrium. These cells do not necessarily contain fat; and when they do, the fat is found in the centre of each cell, and does not push the protoplasm of the cell to the periphery, as in ordinary fat cells.

{333}In Fig. 132, B, I give a specimen of this tissue stained by osmic acid; in Fig. 132, A, I give a drawing of ordinary muco-cartilage taken from the plate of the lower lip; and in Fig. 133, A, a modification of the muco-cartilage taken from the velum, which shows the formation of a tissue intermediate between ordinary muco-cartilage and this branchial fat-tissue.

Further, in fully-grown specimens of Ammocoetes, in the region of undoubted muco-cartilage, a fatty degeneration of the cells frequently appears, together with an increase in the blood spaces,--the precursor, in fact, of the great change which overtakes this tissue soon afterwards, at the time of transformation, when it is invaded by blood, and swept away, except in those places where new cartilage is formed. I conclude, then, that the tissue of this vascular space was originally muco-cartilage, which has degenerated during the life of the Ammocoetes. The fact that in most cases undoubted muco-cartilage is to be found here and there in this space, is strong confirmation of the truth of this conclusion.

If this conclusion is correct, we may expect that it would be confirmed by the embryological history of the tissue, and we ought to find that in much younger stages a homogeneous tissue of the same nature as muco-cartilage fills up the spaces in the branchial {334}region, where in the Ammocoetes only blood and fat-containing cells are present. For this purpose Shipley kindly allowed me to examine his series of sections through the embryo at various ages. These specimens are very instructive, especially those stained by osmic acid, which preserves the natural thickness of this space better than other staining methods. At an age when the branchial cartilages are seen to be formed, when no fat-cells are present, a distinctive tissue (Fig. 133, B) is plainly visible in the velum and at the base of the tentacles, in the very position where in the more advanced Ammocoetes muco-cartilage exists. Taking, then, this tissue as our guide, the specimens show that the space between the skin and the visceral muscles in which the cartilaginous basket-work lies is filled with a similar material. At this stage a sheet of embryonic tissue occupies the position where, later on, blood-spaces and fat-cells are found, and this tissue resembles that seen in the velum and other places where muco-cartilage is afterwards found.