Chapter IV: , the probability of the theory that the trigeminal group of (4)

I conclude, therefore, that originally the branchial or mesosomatic region was covered with a dorsal plate of muco-cartilage, which carried on its under surface the dorsal part of the branchial basket-work, and sprang from the central core of skeletogenous tissue around the notochord; this plate was separated from the plate which covered this region ventrally by the lateral grove in which the gill-slits are situated. The ventral plate carried on its under surface the ventral part of the branchial basket-work, and was originally continuous with the plate over the thyroid gland.

{335}

Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_. _sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na. cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._, notochord.]

{336}In Fig. 134, A, B, C, the cranial skeleton of Ammocoetes is represented from the dorsal, ventral, and lateral aspects. The muco-cartilage is coloured red, the branchial or soft cartilage blue, and the hard cartilage purple. The degenerated muco-cartilage of the branchial region is represented as an uncoloured plate, on which the branchial basket-work stands in relief. If it were restored to its original condition of muco-cartilage, it would represent a uniform plate, on the _under_ surface of which the basket-work would be situated; and if it were calcified and made solid, the branchial basket-work would not show at all in these figures.

Is it possible to find the reason why this skeletal covering has degenerated so early before transformation, and why the thyroid plate remains intact until transformation? We see that all that part which has degenerated is covered over by the somatic muscles,--by, in fact, muscles which, being innervated by the foremost spinal nerves, belong naturally to the region immediately following the branchial. I suggest, therefore, that the original skeletal covering of muco-cartilage has remained intact only where it has not been invaded and covered over by somatic muscles, but has been invaded by blood and undergone the same kind of degenerative change as overtakes the great mass of this tissue at transformation wherever the somatic muscles have overgrown it.

The covering somatic muscles in the branchial region form a dorsal and ventral group, of which the latter is formed in the embryo much later than the former, the line of separation between the two groups being the lateral groove, with its row of branchial openings. This groove ends at the first branchial opening, but the ventral and dorsal somatic muscles continue further headwards. It is instructive to see that, although the lateral groove terminates, the separation between the two groups of muscles is still marked out by a ridge of muco-cartilage, represented in Fig. 134, A, which terminates anteriorly in the opercular bar.

Passing now to the prosomatic region, we find that here, too, the muco-cartilaginous external covering is divisible into a dorsal and a ventral head-plate, the ventral head-plate being the plate of the lower lip, and the dorsal head-plate the plate of muco-cartilage over the front part of the head. The staining reaction with thionin maps out this dorsal head-plate in a most beautiful manner, and shows that the whole of the upper lip-region in front of the nasal orifice is one large plate of muco-cartilage, obscured largely by the invasion of the crossing muscles of the upper lip, but left pure and uninvaded all around the nasal orifice, and where the upper and lower lips come together. In addition to this foremost plate, a median tongue of muco-cartilage covers over the pineal eye and fills up the {337}median depression between the two median dorsal somatic muscles. Also, two lateral cornua pass caudalwards from the main frontal mass of muco-cartilage over the lateral eyes, forming the well-known wedge which separates the dorsal and lateral portions of the dorso-lateral somatic muscle. In fact, similarly to what we find in the branchial region, the muco-cartilaginous covering can be traced with greater or less completeness only in those parts which are not covered by somatic muscles.

In Fig. 134, A, B, C, this striking muco-cartilaginous head-shield, both dorsal and ventral, is shown. Seeing that the upper lip wraps round the lower one on each side, and that this most ventral edge of the upper lip contains muco-cartilage, as is seen in Fig. 117, the dorsal head-shield of muco-cartilage ought, strictly speaking, to extend more ventrally in the drawings. I have curtailed it in order not to interfere with the representation of the lower lip and tentacular muco-cartilages.

From what has been said, it follows that the past history of the skeletal covering of the whole head-region of Ammocoetes, both frontal and occipital, can be conjectured by means of the ontogenetic history of the foremost myomeres.

Dohrn and all other observers are agreed that during the development of this animal a striking forward growth of the foremost somatic myomeres takes place, so that, as Dohrn puts it, the body-musculature has extended forwards over the gill-region, and at the same time the gill-region has extended backwards. It is therefore probable that in the ancestral form the myotomes, innervated by the first spinal nerves, immediately succeeded the branchial region. Judging from Ammocoetes, the forward growth was at first confined to the dorsal region, and therefore invaded the dorsal head-plate, the ventral musculature being distinctly a later growth. With respect to this dorsal part of the myotomes, the first myotome is originally situated some distance behind the auditory capsule, and then grows forward towards the nasal opening; the lateral part, according to Hatschek, grows forward more quickly than the dorsal part, and splits itself above and below the eye into a dorso-lateral part, which extends up to the olfactory capsule, and a ventro-lateral part (_m. lateralis capitis_ anterior, superior, and inferior), thus giving rise to the characteristic appearance of the muco-cartilaginous head-shield of Ammocoetes.

According, then, to the extent of the growth of these somatic {338}muscles, the shape of the muco-cartilaginous head-shield will vary, and if it were calcified and then fossilized we should obtain fossil head-shields of widely differing configuration, although such fossils might be closely allied to each other. This is just what is found in this group. Let the muco-cartilage extend over the whole of the branchial region of Ammocoetes, the resulting head-shield would be as in Fig. 135, A; the branchial bars below the muco-cartilaginous shield might or might not be evident, and the line between the branchial and the trigeminal region might or might not be indicated. Such a head-shield would closely resemble those of Didymaspis and Tremataspis respectively. Now suppose the somatic musculature to encroach slightly on the branchial region and also laterally to the end of the anterior branchial region, then we should obtain a shape resembling that of Thyestes (Fig. 135, B). Continue the same process further, the lateral muscle always encroaching further than the median masses, until the whole or nearly the whole branchial region is invested, and we get the head-shield of Cephalaspis (Fig. 135, C); further still, that of Keraspis, and yet still further, that of Ammocoetes (Fig. 135, D).

A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammocoetes.]

So close is this similarity, from the comparative point of view, between the dorsal head-shield of the Osteostraci and the dorsal cephalic region of Ammocoetes that it justifies us in taking Ammocoetes as the nearest living representative of such types; it is justifiable, therefore, to interpret by means of Ammocoetes the position of other organs in these forms. First and foremost is the hard plate {339}known as the post-orbital plate, so invariably found. In Fig. 134, C, I have inserted (_cr._) the position of the membranous cranium of Ammocoetes, and it is immediately evident that the primordial cranium of the Osteostraci must occupy the exact position indicated by this median hard plate. For this very reason this median plate would be harder than the rest in order to afford a better protection to the brain underneath. This plate, because of its position, may well receive the same name as the similar plate in the trilobite and various palæostracans and be called the glabellum.

EVIDENCE OF SEGMENTATION IN THE HEAD-SHIELD--FORMATION OF CRANIUM.

We may thus conceive the position of the nose, lateral eyes, median eyes, and cranium in these old fishes. In addition, other indications of a segmentation in this head-region have been found. The most striking of all the specimens hitherto discovered are some of _Thyestes verrucosus_, discovered by Rohon, in which the dorsal shield has been removed, and so we are able to see what that dorsal shield covered.

In Fig. 136, I reproduce his drawing of one of his specimens from the dorsal and lateral aspects. These drawings show that the frontal part of the shield covered a markedly segmented part of the animal; five distinct segments are visible apart from the median most anterior region. This segmented region is entirely confined to the prosomatic region, _i.e._ to the region innervated by the trigeminal nerve. An indication of similar markings is given in Lankester's figure of _Eukeraspis pustuliferus_ (see Fig. 127, B), and, indeed, evidence of a segmentation under the antero-lateral border of the head-shield is recognized at the present time, not only in the Cephalaspidæ, but also in the Pteraspidæ, as was pointed out to me by Smith Woodward in the specimens at the British Museum. Also, in _Cyathaspis_, Jaekel has drawn attention to markings of a similar segmental nature (Fig. 137).

There seems, then, little doubt but that these primitive fishes possessed something in this region which was of a segmental character, and indicated at least five segments, probably more.

Rohon entitles his discovery 'the segmentation of the primordial cranium.' It would, I think, be better to call it the segmentation of {340}the anterior region of the head, for that is in reality what his figures show, not the segmentation of the primordial cranium, which, to judge from Ammocoetes, was confined to the region of the glabellum.

What is the interpretation of this appearance?

_A._, lateral eyes; _Ep._, median eyes.]

Any segmentation in the head-region must be indicative of segments belonging to the trigeminal or prosomatic region, or of segments belonging to the vagus or mesosomatic region. Many palæontologists, looking upon segmentation as indicative of gills and gill-slits, have attempted to interpret such markings as branchial segments, regardless of their position. As the figures show, they extend in front of the eyes and reach round to the front middle line, a position which is simply impossible for gills, but points directly to a segmentation connected with the trigeminal nerve. Comparison with Ammocoetes makes it plain enough that the markings in question are prosomatic in position, and that the gill-region must be sought for in the place {341}where Schmidt and Rohon located it in Thyestes, viz. the so-called occipital region.

This discovery of Rohon's is, in my opinion, of immense importance, for it indicates that, in these early fishes, the prosomatic segmentation, associated with the trigeminal nerve, was much more well-marked than in any fishes living in the present day. Why should it be more well-marked? Turning to the palæostracan, it is very suggestive to compare the markings on their prosomatic carapace with these markings. Again and again we find indications of segmentation in these fossils similar to those seen in the ancient fishes. Thus in Fig. 138 I have put side by side the palæostracan _Bunodes_ and the fish _Thyestes_, both life size. In the latter I have indicated Rohon's segments; in the former the markings usually seen.

From the evidence of Phrynus, Mygale, etc., as already pointed out, such markings in the palæostracan fossils would indicate the position of the tergo-coxal muscles of the prosomatic appendages, even though such appendages have not yet been discovered, and it is significant that in all these cases there is a distinct indication of a median plate or glabellum in addition to the segmental markings. Especially instructive is the evidence of Phrynus, as is seen by a comparison of Figs. 107 and 108, which shows clearly that this median plate (_glab._) covered the brain-region, a brain-region which is isolated and protected from the tergo-coxal muscles by the growth dorsalwards of the flanges of the plastron. In this way an incipient cranium of a membranous character is formed, which helps to give attachment to these tergo-coxal muscles. As such cranium is derived directly from the plastron, it is natural that it should ultimately become cartilaginous, just as occurs when Ammocoetes becomes Petromyzon and the cartilaginous cranium of the latter arises from the membranous cranium of the former. In Galeodes also the growth dorsalwards of the lateral flanges of the plastron to form an incipient cranium in which the brain lies is very apparent.

Both figures natural size.]

{342}I venture, then, to suggest that in the Osteostraci the median hard plate or glabellum protected a brain which was enclosed in a membranous cranium, very probably not yet complete in the dorsal region--certainly not complete if the median pineal eyes so universally found in these ancient fishes were functional--a cranium derived from the basal trabeculæ, in precisely the same manner as we see it already in its commencement in Phrynus and other scorpions. With the completion of this cranium and its conversion into cartilage, and subsequently into bone, an efficient protection was afforded to the most vital part of the animal, and thus the hard head-shield of the Palæostraca and of the earliest fishes was gradually supplanted by the protecting bony cranium of the higher vertebrates.

Step by step it is easy to follow in the mind's eye the evolution of the vertebrate cranium, and because it was evolved direct from the plastron, the impossibility of resolving it into segments is at once manifest; for although the plastron was probably originally segmented, as Schimkéwitsch thinks, all sign of such segmentation had in all probability ceased, before ever the vertebrates first made their appearance on the earth.

It follows further, from the comparison here made, that those antero-lateral markings indicative of segments, found so frequently in these primitive fishes, must be interpreted as due not to gills but to aponeuroses, due to the presence of muscles which moved prosomatic appendages, muscles which arose from the dorsal region in very much the same position as do the muscles of the lower lip in Ammocoetes; the latter, as already argued, represent the tergo-coxal muscles of the last pair of prosomatic appendages--the chilaria or metastoma. Such an interpretation of these markings signifies that the first-formed fishes must have possessed prosomatic appendages of a more definite character than the tentacles of Ammocoetes, something intermediate between those of the palæostracan and Ammocoetes.

For my part I should not be in the least surprised were I to hear that something of the nature of appendages in this region had been found, especially in view of the well-known existence of the pair of appendages in the members of the Asterolepidæ--large, oar-like appendages which may well represent the ectognaths.

{343}THE RELATIONSHIP OF THE OSTRACODERMS.

Of the three groups of fishes--the Heterostraci, the Osteostraci, and the Antiarcha--the last is Devonian, and therefore the latest in time of the three, while the earliest is the first group, as both Pteraspis and Cyathaspis have been found in lower levels of the Silurian age than any of the Osteostraci, and, indeed, Cyathaspis has been discovered in Sweden in the lower Silurian. This, the earliest of all groups of fishes, is confined to two forms only--Pteraspis and Cyathaspis,--for Scaphaspis is now recognized to be the ventral shield of Pteraspis.

Hitherto a strong tendency has existed in the minds both of the comparative anatomist and the palæontologist to look on the elasmobranchs as the earliest fishes, and to force, therefore, these strange forms of fish into the elasmobranch ranks. For this purpose the same device is often used as has been utilized in order to account for the existence of the Cyclostomata, viz. that of degeneration. The evidence I have put forward is very strongly in favour of a connection between the cyclostomes and the cephalaspids, and agrees therefore with all the rest of the evidence that the jawless fishes are more ancient than those which bore jaws--the Gnathostomata.

This is no new view. It was urged by Cope, who classified the Heterostraci, Osteostraci, and Antiarcha under one big group--the Agnatha--from which subsequently the Gnathostomata arose. Cope's arguments have not prevailed up to the present time, as is seen in the writings of Traquair, one of the chief authorities on the subject in Great Britain. He is still an advocate of the elasmobranch origin of all these earliest fishes, and claims that the latest discoveries of the Silurian deposits (_Thelodus Pagei_) and other members of the Coelolepidæ confirm this view of the question.

This view may be summed up somewhat as follows:--

Cartilaginous jaws would not fossilize, and the Ostracoderms may have possessed them.

They may have degenerated from elasmobranchs just as the cyclostomes are supposed to have degenerated.

Seeing that bone succeeds cartilage, the presence of bony shields in Cephalaspis, etc., indicates that their precursors were cartilaginous, presumably elasmobranch fishes.

Of these arguments the strongest is based on the supposed bony {344}covering of the Osteostraci, with the consequent supposition that their ancestors possessed a cartilaginous covering. This argument is entirely upset, if, as I have pointed out, the structure of the cephalaspid shield is that of muco-cartilage and not of bone. If these plates are a calcified muco-cartilage, then the whole argument for their ancestry from animals with a cartilaginous skeleton falls to the ground, for muco-cartilage is the precursor not only of bone, but also of cartilage itself.

The evidence, then, points strongly in favour of Cope's view that the most primitive fishes were Agnatha, after the fashion of cyclostomes, as is also believed by Smith Woodward, Bashford Dean, and Jaekel.

Among living animals, as I have shown, the Limulus is the sole survivor of the palæostracan type, and Ammocoetes alone gives a clue to the nature of the cephalaspid, _i.e._ the osteostracan fish. Older than the latter is the heterostracan, Pteraspis, and Cyathaspis. Is it possible from their structure to obtain any clue as to the actual passage from the palæostracan to the vertebrate?

Here again, as in the case of the Osteostraci, a relationship to the elasmobranch has been supposed, for the following reasons:--

The latest discoveries in the Silurian and Devonian deposits have brought to light strange forms such as Thelodus and Drepanaspis, of which the latter from the Devonian must, according to Traquair, be included in the Heterostraci. It possessed, as seen in Fig. 139, large plates, after the fashion of Pteraspis, and also many smaller ones.

The former, from the upper Silurian, belongs to the Coelolepidæ, and was covered over with shagreen composed of small scutes, after the fashion of an elasmobranch. Traquair suggests that Thelodus arose from the original elasmobranch stock; that by the fusion of scutes such a form as Drepanaspis occurred, and, with still further fusion, Pteraspis.

There are always two ways of looking at a question, and it seems to me possible and more probable to turn the matter round and to argue that the original condition of the surface-covering was that of large plates, as in Pteraspis. By the subsequent splitting up of such plates, Drepanaspis was formed, and later on, by further splitting, the elasmobranch, Thelodus being a stage on the way to the formation of an elasmobranch, and not a backward stage from the elasmobranch towards Pteraspis.

{345}This method of looking at the problem seems to me to be more in consonance with the facts than the reverse; for, as pointed out by Jaekel, the fishes with large plates are the oldest, and in Cyathaspis, the very oldest of all, the size of the plates is most conspicuous; he considers, therefore, this preconceived view that large plates are formed by the fusion of small ones must give way to the opposite belief.

_A._, anus; _E._, lateral eyes.]

So also Rohon, as quoted by Traquair, who, in his first paper accepted Lankester's view that the ridges of the pteraspidian shield were formed by the fusion of a linear arrangement of numbers of placoid scales, suggests in his second paper that these ridges may have been the most primitive condition of the dermal skeleton of the vertebrate, out of which, by differentiation, the dermal denticles (placoid scales) of the selachian, as well as their modifications in the ganoids, teleosteans, and amphibians, have arisen.

One thing is agreed upon on all sides; no sign of bone-corpuscles is to be found in this dermal covering of Pteraspis. In the deeper layers are large spaces, the so-called pulp-cavities leading into narrow canaliculi, the so-called dentine canals. The structure is {346}looked upon as similar to that of the pulp and dentine canals of many fish-scales.

On the other hand, this dermal covering of Pteraspis has been compared by Patten with the arrangement of the chitinous structure of certain parts of the external covering of Limulus, a comparison which to my mind presents a great difficulty. The chitin-layers in Limulus are _external_ to the epidermal cells, being formed by them; the layers in Pteraspis which look like chitin must have been _internal_ to the epidermal layer; for each vascular canal which passes from a pulp-cavity on its way to be distributed into the dentine canals of the ridge gives off short side branches, which open directly into the groove between the ridges. If these canals were filled with blood they could not possibly open directly into the open grooves between the ridges; these openings must, therefore, have been covered over with an epithelial layer which covered over the surface of the animal, and consequently the chitin-like structure must have been internal to the epidermis, and not external, as on Patten's view. The comparison of this structure with the dentine of fish-scales signifies the same thing, for in the latter the epidermis is external to the dentine-plates, the hard skeletal structure is in the position of the cutis, not of the cuticle.

The position appears to me to be this: the dermal cranial skeleton of vertebrates, whether it takes the form of a bony skull or of the dorsal plates of a cephalaspid or a pteraspid is, in all cases, not cuticular, _i.e._ is not an external formation of the epidermal cells, but is formed in tissue of the nature of connective tissue underlying the epidermis. On the contrary, the hard part of the head-carapace of the palæostracan is an external formation of the epidermal cells.

If, then, this tissue of Pteraspis is not to be looked upon as chitin, how can we imagine its formation? It is certainly not bone, for there are no bone-corpuscles; it is a very regular laminated structure resembling in appearance chitin rather than anything else.

As in all cases of difficulty, turn to Ammocoetes and let us see what clue there is to be found there. The skin of Ammocoetes is peculiar among vertebrates in many respects. It consists of a number of epidermal cells, as in Fig. 140, the varying function of which need not be considered here, covered over with a cuticular layer which is extraordinarily thick for the cuticle of a vertebrate skin; this cuticular layer is perforated with fine canaliculi, through which the {347}secretion of the underlying cells passes, as is seen in Fig. 140, A and B. This cuticle corresponds to the chitinous covering of the arthropod, and like it is perforated with canaliculi, and, according to Lwoff, possibly contains chitin. The epidermal cells rest on a thick layer of most striking appearance (Fig. 141), for it resembles, in an extraordinary degree, when examined superficially, a layer of chitin; it is called the laminated layer, and is characterized by the extreme regularity of the laminæ. This appearance is due, as the observations of Miss Alcock show, to alternate layers of connective tissue fibres arranged at right angles to each other, each fibre running a straight course and possessing its own nucleus. Although the fibres in each layer are packed close together, they are sufficiently apart to form with the fibres of the alternate layers a meshwork rather than a homogeneous structure, and thus the surface view of this layer shows a regular network of very fine spaces through which nerve-fibres and fluid pass. This layer is easily dissolved in a solution of hypochlorite of soda, a fluid which dissolves chitin. Any one looking at Ammocoetes would say that the only part of its skin which resembles chitin is this laminated layer, and therefore the only part of its skin which would afford an indication of the nature of the skeleton of Pteraspis is this laminated layer, which belongs to the cutis, and not to the cuticle. Yet another significant peculiarity of this layer is its entire disappearance at transformation. Miss Alcock, in a research not yet published, has shown that this layer is completely broken up and absorbed at transformation; the cutis of Petromyzon is formed entirely anew, and no longer presents any regular laminated character, but resembles rather the sub-epidermal connective tissue layer of the skin of higher vertebrates. This laminated layer, then, just like the muco-cartilage, shows, by its complete disappearance at transformation, its ancestral character.

Very suggestive is the arrangement of the different skeletal {348}tissues in the head-region of Ammocoetes. Fig. 141 represents a section through the head near the pineal eye. Most internally is _a_, a section of the membranous cranium, then comes _b_, the muco-cartilaginous skeleton, then _c_, the laminated layer, and finally _d_, the external cuticle. If in Ammocoetes we possess an epitome of the history of the vertebrate, how would these layers be represented in the past ages, supposing they could be fossilized?

_a_, cranial wall; _b_, muco-cartilage; _c_, laminated layer; _d_, external cuticular layer.]

The most internal layer _a_, by the formation of cartilage and then bone, represents the great mass of vertebrate fossils; the next layer _b_, by a process of calcification, as previously argued, represents the head-shield of the Osteostracan fishes; while the cuticular layer _d_, no longer thin, is the remnant of the Palæostracan head-carapace. Between these two layers, _b_ and _d_, lies the laminated layer _c_. Intermediate to the Palæostracan and the Osteostracan comes the Heterostracan, with its peculiar head-shield--a head-shield whose origin is more easily conceivable as arising from something of the nature of the laminated layer than from any other structure represented in Ammocoetes.

My present suggestion, then, is this: the transition from the skeletal covering of the Palæostracan to that of the highest vertebrates was brought about by the calcification of successive layers from without inwards, all of which still remain in Ammocoetes and show how the external chitinous covering of the arthropod was gradually replaced by the deep-lying internal bony cranium of the higher vertebrates.

In Ammocoetes the layer which represents the covering of the {349}Palæostracan has already almost disappeared. At transformation the layers representing the stage arrived at by the Heterostracan and the Osteostracan disappear; but the stage representing the higher vertebrates, far from disappearing, by the formation of cartilage reaches a higher stage and prepares the way for the ultimate stage of all--the formation of the bony cranium.

So much for the evidence as to the nature of the structure of the head-shield of the Pteraspidæ.

It suggests that these fishes were covered anteriorly with armoured plates derived from the cutis layer of the skin, a layer which was specially thickened and very vascular, apparently, to enable respiration to be very largely, if not entirely, effected by the surface of the body. It is difficult to understand how the sea-scorpions breathed, and it is easy to see how the formation of ventral and dorsal plates enclosing the mesosomatic appendages may at the outset have hindered the action of the branchiæ. The respiratory chamber, according to my view, had at first the double function of respiration and digestion. A new digestive apparatus was the pressing need at the time; it would, therefore, be of distinct advantage to remove, as much as possible, the burden of respiration from this incipient alimentary canal.

What can be said as to the shape of these ancient forms of fishes? Certain parts of them are absolutely known, other parts are guesswork. They are known to have possessed a dorsal shield, a ventral shield formerly looked upon as belonging to a separate species, called Scaphaspis, and a spine attached to the dorsal shield. The rest of their configuration, as given in Smith Woodward's restoration (Fig. 142) is guesswork; the fish-like body with its scales, the heterocercal tail, is based on the most insufficient evidence of something of the nature of scales having being found near the head-plates.

The dorsal shield is characterized by a pair of lateral eyes situated on the edge of the shield, not as in Cephalaspis near the middle line. In the middle line, where the rostrum meets the large dorsal plate, median eyes were situated. But the slightest sign of any median single nasal opening, such as is so characteristic of the head-shield of the Osteostraci and of Ammocoetes has never been discovered. The olfactory organ must have been situated on the ventral side as in the larval stage of Ammocoetes, or in the Palæostraca. Many of these head-shields are remarkably well preserved, {350}and it is difficult to believe that an olfactory opening would not be seen if any such had existed, as it does in Thyestes.

The difficulty of interpreting these types is the difficulty of understanding their method of locomotion; that is largely the reason why the spine has been placed as if projecting from the back, and a fish-like body with a heterocercal tail-fin added. If, on the contrary, the spine is a terminal tail-spine, then, as far as the fossilized remains indicate, the animal consisted of a dorsal shield, a ventral shield, and a tail-spine, to which must be added two apparently lateral pieces and a few scales. If the animal did not possess a flexible body with a tail-fin, but terminated in a rigid spike after the fashion of a Limulus-like animal, then it must have moved by means of {351}appendages. At present we have not sufficient evidence to decide this question.

That the animal crawled about in the mud by means of free appendages is by no means an impossible view, seeing how difficult it is to find the remains of appendages in the fossils of this far-back time, even when we are sure that they existed. Thus, for many generations, the appendages of trilobites, which occur in such countless numbers, and in such great variety of form, were absolutely unknown, until at last, in consequence of a fortunate infiltration by pyrites, they were found by Beecher preserved down to the minutest detail. Even to this day no trace of appendages has been found in such forms as Hemiaspis, Bunodes, Belinurus, Prestwichia.

The whole question of the evidence of any prosomatic appendages in these ancient fishes is one of very great interest, and of late years has been investigated by Patten. It has long been known that forms such as Pterichthys and Bothriolepis possessed two large, jointed locomotor appendages, and Patten has lately obtained better specimens of Bothriolepis than have ever been found before, which show not only the general configuration of the fish, but also the presence of mandibles or gnathites in the mouth-region resembling those of an arthropod. These mandibles had been seen before (Smith Woodward), but Patten's specimens are more perfect than any previously described, and cause him to conclude that these ancient fish were of the nature of arthropods rather than of vertebrates.

Patten has also been able to obtain some excellent specimens of the under surface of the head of Tremataspis, which, as evident in Fig. 143, show the presence of a series of holes, ranging on each side from the mouth-opening, in a semicircular fashion towards the middle line. He considers that these openings indicate the attachments of appendages, in opposition to other observers, such as Jaekel, who look upon them as gill-slits. To my mind, they are not in the right position for gill-slits; they are certainly in a prosomatic rather than in a mesosomatic position, and I should not be at all surprised if further research justified Patten's position. So convinced is he of the presence of appendages in all these old forms, that he considers them to be arthropods rather than vertebrates, although, at the same time, he looks upon them as indicating the origin of vertebrates from arthropods. Here, perhaps, it is advisable to say a few words on Patten's attitude towards this question.

{352}Two years after I had put forward my theory of the derivation of vertebrates from arthropods, Patten published, in the _Quarterly Journal of Microscopical Science_, simultaneously with my paper in that journal, a paper entitled "The Origin of Vertebrates from Arachnids." In this paper he made no reference to my former publications, but he made it clear that there was an absolutely fundamental difference between our treatment of the problem; for he took the old view that of necessity there must be a reversal of surfaces in order that the internal organs should be in the same relative positions in the vertebrate and in the invertebrate. He simply, therefore, substituted Arachnid for Annelid in the old theory. Because of this necessity for the reversal of surfaces he discarded the terms dorsal and ventral as indicative of the surfaces of an animal, and substituted hæmal and neural, thereby hopelessly confusing the issue and making it often very difficult to understand his meaning.

He still holds to his original opinion, and I am still waiting to find out when the reversal of surfaces took place, for his investigations lead him, as must naturally be the case, to compare the dorsal (or, as he would call it, the hæmal) surface of Bothriolepis, of the Cephalaspidæ, and of the Pteraspidæ with the dorsal surface of the Palæostraca.

All these ancient fishes are, according to him, still in the arthropod stage, have not yet turned over, though in a peculiarly unscientific manner he argues elaborately that they must have swum on their back rather than on their front, and so indicated the coming reversal. Because they were arthropods they cannot have had a {353}frontal nose-organ; therefore, Patten looks upon the nose and the two lateral eyes of the Osteostraci as a complex median eye, regardless of the fact that the median eyes already existed.

Every atom of evidence Patten has brought forward, every new fact he has discovered, confirms my position and makes his still more hopelessly confused. Keep the animal the right side uppermost, and the evidence of the rocks confirms the transition from the Palæostracan to the Cyclostome; reverse the surfaces, and the attempt to derive the vertebrate from the palæostracan becomes so confused and hopelessly muddled as to throw discredit on any theory of the origin of vertebrates from arthropods. For my own part, I fully expect that appendages will be found not only in the Cephalaspidæ but also in the Pteraspidæ, and I hope Patten will continue his researches with increasing success. I feel sure, however, his task will be much simplified if he abandons his present position and views the question from my standpoint.

SUMMARY.

The shifting of the nasal tube from a ventral to a dorsal position, as
seen in Ammocoetes, is, perhaps, the most important of all clues in
connection with the comparison of Ammocoetes to the Palæostracan on the
one hand, and to the Cephalaspid on the other; for, whereas the exact
counterpart of the opening of such a tube is always found on the dorsal
head-shield in all members of the latter group, nothing of the kind is
ever found on the dorsal carapace of the former group.

The reason for this difference is made immediately evident in the
development of Ammocoetes itself, for the olfactory tube originates as a
ventral tube--the tube of the hypophysis--in exactly the same position as
the olfactory tube of the Palæostracan, and later on in its development
takes up a dorsal position.

In fact, Ammocoetes in its development indicates how the Palæostracan
head-shield became transformed into that of the Cephalaspid.

In another most important character Ammocoetes indicates its relationship
to the Cephalaspidæ, for it possesses an external skeleton or head-shield
composed of muco-cartilage, which is the exact counterpart of the
so-called bony head-shield of the latter group; and still more strikingly
the structure of the cephalaspidian head-shield is remarkably like that
of muco-cartilage. In the one case, by the deposition of calcium salts, a
hard external skeleton, capable of being preserved as a fossil, has been
formed; in the other, by the absence of the calcium salts, a soft
chondro-mucoid matrix, in which the characteristic cells and fibrils are
embedded, distinguishes the tissue.

The recognition that the head-shields of these most primitive fishes were
not composed of bone, but of muco-cartilage, the precursor of both
cartilage and bone, immediately clears up in the most satisfactory manner
the whole {354}question of their derivation from elasmobranch fishes; for
the main argument in favour of the latter derivation is the exceedingly
strong one that bone succeeds cartilage--not _vice versâ_--therefore,
these forms, since their head-shield is bony, must have arisen from some
other fishes with a cartilaginous skeleton, most probably of an
elasmobranch nature. Seeing, however, that the structure of their shields
resembles muco-cartilage much more closely than bone, and that Ammocoetes
forms a head-shield of muco-cartilage closely resembling theirs, there is
no longer any necessity to derive the jawless fishes from the
gnathostomatous; but, on the contrary, we may look with certainty upon
the Agnatha as the most primitive group from which the others have been
derived.

The history of the rocks shows that the group of fishes, Pteraspis and
Cyathaspis, are older than the Cephalaspidæ--come, therefore,
phylogenetically between the Palæostraca and the latter group. In this
group the head-shields are of a very different character, without any
sign of any structure comparable with that of bone, and although they
possessed both lateral and median eyes, there is never in any case any
trace of a dorsal nasal orifice. Their olfactory passage, like that of
the Palæostraca, must have been ventral.

The remarkable comparison which exists between the head-shields of
Ammocoetes and Cephalaspis, enables us to locate the position of the
brain and cranium of the latter with considerable accuracy, and so to
compare the segmental markings found in many of these fossils with the
corresponding markings, found either in fossil Palæostraca or on the
head-carapaces of living scorpions and spiders, such as Phrynus and
Mygale. In all cases the cranial region was covered with a median plate,
often especially hard, which corresponded to the glabellum of the
trilobite; the growth of the cranium can be traced from its beginnings as
the upturned lateral flanges of the plastron to the membranous cranium of
Ammocoetes.

From such a comparison it follows that the segments, found in the
antero-lateral region of the head-shield, were not segments of the
cranium, but of parts beyond the region of the cranium, and from their
position must have been segments supplied by the trigeminal nerve, and
not by the vagus group; segments, therefore, which did not indicate gills
and gill-slits, but muscles, innervated by the trigeminal nerve; muscles
which, as indicated by the corresponding markings on the carapace of
Phrynus, Mygale, etc., were the tergo-coxal muscles of the prosomatic
appendages.

The discovery of the nature of these appendages in the Pteraspidæ and
Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and
Bothriolepis), is a problem of the future, though in the latter, not only
have the well-known oar-like appendages been long since discovered, but
Patten has recently found specimens of Bothriolepis which throw light on
the anterior masticating gnathite-like appendages which these ancient
forms possessed.

{355}CHAPTER XI

_THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE_

Lateral line organs.--Function of this group of organs.--Poriferous
sense-organs on the appendages in Limulus.--Branchial
sense-organs.--Prosomatic sense organs.--Flabellum.--Its structure and
position.--Sense-organs of mandibles.--Auditory organs of insects and
arachnids.--Poriferous chordotonal organs.--Balancers of
Diptera.--Resemblance to organs of flabellum.--Racquet-organs of
Galeodes.--Pectens of scorpions.--Large size of nerve to all these
special sense-organs.--Origin of parachordals and auditory
capsule.--Reason why VIIth nerve passes in and out of capsule.--Evidence
of Ammocoetes.--Intrusion of glandular mass round brain into auditory
capsule.--Intrusion of generative and hepatic mass round brain into base
of flabellum.--Summary.

When speaking of the tripartite arrangement of the cranial nerves, an arrangement which gave the clue to the meaning of the cranial segments, I spoke of the trigeminal as supplying the sensory nerves to the skin in the head-region, and I compared this dorsal system of afferent nerves to the system of epimeral nerves in Limulus which supply the prosomatic and mesosomatic carapaces of Limulus with sensory fibres. I compared the ventral system of eye-muscle nerves with the system of nerves supplying the segmental dorso-ventral somatic muscles of the prosomatic region, and I compared the lateral system of mixed nerves with the nerves supplying the prosomatic and mesosomatic appendages of Limulus. I compared, also, the optic nerves and the olfactory nerves with the corresponding nerves in the same invertebrate group. My readers will see at once that one well-marked group of nerves--the auditory and lateral line system--has been entirely omitted up to the present, it has not even been mentioned in the scheme of the cranial segments; I have purposely reserved its consideration until now, because the organs these nerves supply, though situated in the skin, are of such a special character {356}as to form a category by themselves. These nerves cannot be classed among the afferent nerves of the skin any more than the nerves of the optic and olfactory apparatus; they require separate consideration. A very extensive literature has grown up on the subject of this system of lateral line sense-organs and their innervation, the outcome of which is decisively in favour of this system being classed with the sense-organs supplied by the auditory nerve, so that in endeavouring to understand the position of the auditory nerve, we must always bear in mind that any theory as to its origin must apply to the system of lateral line nerves as well.

Now, although the auditory apparatus is common to all vertebrates, the lateral line system is not found in any land-dwelling animals; it belongs essentially to the fishes, and is, therefore, an old system so far as concerns the vertebrate group. Its sense-organs are arranged along the lateral line of the fish, and, in addition, on the head-region in three well-marked lines known as the supra-orbital, infra-orbital, and mandibular line systems. These sense-organs lie in the skin in a system of canals, and are innervated by a special nervous system different to that innervating adjacent skin-areas. The great peculiarity of their innervation consists in the fact that their nerves all belong to the branchial system of nerves; no fibres arise in connection with the trigeminal, but all of them in connection with the facial, glossopharyngeal and vagus nerves. In other words, although organs in the skin, their nerve-supply belongs to the lateral nervous system which supplies splanchnic and not somatic segments, a system which, according to the theory advanced in this book, originated in the nerves supplying appendages. The conclusion, therefore, is that in order to obtain some clue as to the origin of the sense-organs of this system in the assumed palæostracan ancestor, we must examine the mesosomatic appendages and see whether they possess any special sense-organs of similar function.

Further, considering that the auditory organ is to be regarded as a specially developed member of this system, we must especially look for an exceptionally developed organ in the region supplied by the auditory nerve.

The question of the origin of this system of lateral line sense-organs possesses a special interest for all those who attempt to obtain a solution of the origin of vertebrates, for the upholders of the view that the vertebrates have descended from annelids have always {357}found its strongest support in the similarity of two sets of segmental organs found in annelids and vertebrates. On the one hand, great stress was laid upon the similarity of the segmental excretory organs in the two groups of animals, as will be discussed later; on the other, of the similarity of the segmentally arranged lateral sense-organs.

These lateral sense-organs of the annelids have been specially described by Eisig in the Capitellidæ, and, according to Lang, "there are many reasons for considering these lateral organs to be homologous with the dorsal cirri of the ventral parapodia of other Polychæta, and in the family of the Glyceridæ we can follow, almost step by step, the transformation of the cirri into lateral organs." Eisig describes them in the thoracic prebranchial region as slightly different from those in the abdominal branchial region; in the latter region, the ventral parapodia are gill-bearing, so that these lateral organs are in the branchial region closely connected with the branchiæ, just as is also the case in the vertebrates. It is but a small step from the gill-bearing ventral parapodia of the annelid to the gill-bearing appendages of the phyllopod-like protostracan; so that if we assume that this is the correct line along which to search for the origin of the vertebrate auditory apparatus, then, on my theory of the origin of the vertebrates from a group resembling the Protostraca, it follows that special sense-organs must have existed either on or in close connection with the branchial and prebranchial appendages of the protostracan ancestor of the vertebrates, which would form an intermediate link between the lateral organs of the annelids and the lateral and auditory organs of the vertebrates.

Further, these special sense-organs could not have been mere tactile hairs, but must have possessed some special function, and their structure must have been compatible with that function. Can we obtain any clear conception of the original function of this whole system of sense-organs?

A large amount of experimental work has been done to determine the function of the lateral line organs in fishes, and they have been thought at one time or another to be supplementary organs for equilibration, organs for estimating pressure, etc. The latest experimental work done by Parker points directly to their being organs for estimating slow vibrations in water in contradistinction to the quicker vibrations constituting sound. He concludes that surface wave-movements, whether produced by air moving on the water or {358}solid bodies falling into the water, are accompanied by disturbances which are stimuli for the lateral line organs.

One of these segmental organs has become especially important and exists throughout the whole vertebrate group, whether the animal lives on land or in water--this is the auditory organ. Throughout, the auditory organ has a double function--the function of hearing and the function of equilibration. If, then, this is, as is generally supposed, a specialized member of the group, it follows that the less specialized members must possess the commencement of both these functions, just as the experimental evidence suggests.

In our search, then, for the origin of the auditory organ of vertebrates, we must look for special organs for the estimation of vibrations and for the maintenance of the equilibrium of the animal, situated on the appendages, especially the branchial or mesosomatic appendages; and, further, we must specially look for an exceptional development of such segmental organs at the junction of the prosomatic and mesosomatic regions.

Throughout this book the evidence which I have put forward has in all cases pointed to the same conclusion, viz. that the vertebrate arose by way of the Cephalaspidæ from some arthropod, either belonging to, or closely allied to, the group called Palæostraca, of which the only living representative is Limulus. If, then, my argument so far is sound, the appendages of Limulus, both prosomatic and mesosomatic, ought to possess special sense-organs which are concerned in equilibration or the appreciation of the depth of the water, or in some modification of such function, and among these we might expect to find that somewhere at the junction of the prosoma and mesosoma such sense-organs were specially developed to form the beginning of the auditory organ.

Now, it is a striking fact that the appendages of Limulus do possess special sense-organs of a remarkable character, which are clearly not simply tactile. Thus Gegenbaur, as already stated, has drawn attention to the remarkable branchial sense-organs of Limulus; and Patten has pointed out that special organs, which he considers to be gustatory in function, are present on the mandibles of the prosomatic appendages. I myself, as mentioned in my address to the British Association at Liverpool in 1896, searched for some special sense-organ at the junction of the prosoma and mesosoma, and was rewarded by finding that that extraordinary adjunct to the {359}last locomotor appendage, known as the flabellum, was an elaborate sense-organ. I now propose to show that all these special sense-organs are constructed on a somewhat similar plan; that the structure of the branchial sense-organs suggests that they are organs for the estimation of water pressures; that among air-breathing arthropods sense-organs, built up on a somewhat similar plan, are universally found, and are considered to be of the nature of auditory and equilibration organs; and, what is especially of importance, in view of the fact that the most prominent members of the Palæostraca were the sea-scorpions, that the remarkable sense-organs of the scorpions known as the pectens belong apparently to the same group.

THE PORIFEROUS SENSE-ORGANS OF THE APPENDAGES IN LIMULUS.

On all the branchial appendages in Limulus, special sense-organs are found of a most conspicuous character. They form in the living animal bluish convex circular patches, the situation of which on the appendages is shown in Fig. 58. These organs are not found on the non-branchial operculum. Gegenbaur, who was the first to describe them, has pointed out how the surface of the organ is closely set with chitinous goblets shaped as seen in Fig. 144, A, which do not necessarily project free on the surface, but are extruded on the slightest pressure. Each goblet fits into a socket in the chitinous covering, and is apparently easily protruded by variations of pressure from within. The whole surface of the organ on the appendage is slightly bulged in the living condition, and the chitin is markedly softer here than in the surrounding part of the limb. Each of these organs is surrounded by a thick protection of strongly branching spines. On the surface of the organ itself no spines are found, only these goblets, so that the surface-view presents an appearance as in Fig. 144, B. Each goblet possesses a central pore, which is the termination of a very fine, very tortuous, very brittle chitinous tubule (_ch.t._), which passes from the goblet through the layers of the chitin into the subjacent tissue. The goblets vary considerably in size, a few very large ones being scattered here and there. The fine chitinous tubule is especially conspicuous in connection with these largest goblets. In the smaller ones there is the same appearance of a pore and a commencing tube, but I have not been able to trace the tube through the chitinous layers, as in the case of the larger goblets.

{360}

Gegenbaur, in his picture, draws a straight tubule passing from every goblet among the fine canaliculi of the chitin. He says they are difficult to see, except in the case of the larger goblets. The tubule from the larger goblets is most conspicuous, and is in my sections always tortuous, never straight, as represented by Gegenbaur. A special branch of the appendage-nerve passes to these organs, and upon the fine branches of this nerve groups of ganglion-cells are seen, very similar in appearance to the groups described by Patten on the terminal branches of the nerves which supply the mandibular organs. At present I can see no mechanism by which the goblets are extruded or returned into place. In the case of the Capitellidæ, Eisig describes retractor muscles by means of which the lateral sense-organs are {361}brought below the level of the surface, and he imagines that the protrusion is effected by hydraulic means, by the aid of the vascular system. In the branchial sense-organs of Limulus there are no retractor muscles, and it seems to me that both retraction and protrusion must be brought about by alterations of pressure in the vascular fluids. Certainly the cavity of the organ is very vascular. If this be so, it seems likely enough that such an organ should be a very delicate organ for estimating changes in the pressure of the external medium, for the position of the goblets would depend on the relation between the pressure of the fluid inside the organ and that on the surface of the appendage. Whether the chitinous tubule contains a nerve-terminal or not I am unable to decide from my specimens, but, judging from Patten's description of the similar chitinous tubules belonging to the mandibular organs, it is most highly probable that these tubules also contain a fine terminal nerve-fibre.

These organs, then, represent segmental branchial sense-organs, of which it can be said their structure suggests that they may be pressure-organs; but the experimental evidence is at present wanting.