Chapter XV: Final Remarks (9)

If, then, these cells were not retained for digestive purposes, what was their function? To answer this question we must first know the function of the corresponding gland-cells in the uterus of the scorpion, which undoubtedly secreted into the cavity of the uterus and took some part in connection with the generative act, and certainly not with digestion. What the function of these cells is or in what way they act I am unable at present to say. I can only suppose that the reason why the thyroid gland has persisted throughout the vertebrate kingdom, after the generative tissues had found a new outlet for their products in the body-cavity of the posterior region, is because it possessed some important function in addition to that connected with the exit of the products of the generative organs; a function which was essential to the well-being, or even to the life of the animal. We do not know its function in the scorpion, or the nature of its secretion in that animal. We know only that physiology at the present day has demonstrated clearly that the actual external secretion of a gland may be by no means its most important function; in addition, glands possess what is called an internal secretion, viz. a {215}secretion into the blood and lymph, and this latter secretion may be of the most vital importance. Now, the striking fact forces itself prominently forward, that the thyroid gland of the higher vertebrates is the most conspicuous example of the importance of such internal secretion. Here, although ductless, we have a gland which cannot be removed without fatal consequences. Here, in the importance of its internal secretion, we have a reason for the continued existence of this organ; an organ which remains much the same throughout the Vertebrata down to and including Petromyzon, but, as is seen at transformation, is all that remains of the more elaborate, more extensive organ of Ammocoetes. Surely we may argue that it is this second function which has led to the persistence of the thyroid, and that its original form, without its original function, is seen in Ammocoetes, because that is a larval form, and not a fully-developed animal. As soon as the generative organs of Petromyzon are developed at transformation, all trace of its connection with a genital duct vanishes, and presumably its internal secretory function alone remains.

Yet, strange to say, a mysterious connection continues to exist between the thyroid gland and the generative organs, even up to the highest vertebrate. That the thyroid gland, situated as it is in the neck, should have any sympathy with sexual functions if it was originally a gland concerned with digestion is, to say the least of it, extremely unlikely, but, on the contrary, likely enough if it originated from a glandular organ in connection with the sexual organs of the palæostracan ancestor of the vertebrate.

Freund has shown, and shown conclusively, that there is an intimate connection between the condition of the thyroid gland and the state of the sexual organs, not only in human beings, but also in numerous animals, such as dogs, sheep, goats, pigs, and deer. He points out that the swelling of the gland, which occurs in consequence of sexual excitement (a fact mentioned both in folk-lore tales and in poetical literature), and also the swelling at the time of puberty, may both lead to a true goitrous enlargement; that most of the permanent goitres commence during a menstrual period; that during pregnancy swelling of the thyroid is almost universal, and may become so extreme as to threaten suffocation, or even cause death; that the period of puberty and the climacteric period are the two maximal periods for the onset of goitre, and that exophthalmic goitre especially is associated with a special disease connected with the uterus.

{216}SUMMARY.

Step by step in the preceding chapters the evidence is accumulating in
favour of the origin of vertebrates from a member of the palæostracan
group. In a continuously complete and harmonious manner the evidence has
throughout been most convincing when the vertebrate chosen for the
purpose of my arguments has been Ammocoetes.

So many fixed points have been firmly established as to enable us to
proceed further with very great confidence, in the full expectation of
being able ultimately to homologize the Vertebrata with the Palæostraca
even to minute details.

Perhaps the most striking and unexpected result of such a comparison is
the discovery that the thyroid gland is derived from the uterus of the
palæostracan ancestor. Yet so clear is the evidence that it is difficult
to see how the homology can be denied.

In the one animal (Palæostraca) the foremost pair of mesosomatic
appendages forms the operculum, which always bears the terminal
generative organs and is fused in the middle line. In many forms,
essentially in Eurypterus and the ancient sea-scorpions, the operculum
was composed of two segments fused together: an anterior one which
carried the uterus, and a posterior one which carried the first pair of
branchiæ.

In the other animal (Ammocoetes) the foremost segments of the mesosomatic
or respiratory region, immediately in front of the glossopharyngeal
segments, are supplied by the facial nerve, and are markedly different
from those supplied by the vagus and glossopharyngeal, for the facial
supplies two segments fused together; the anterior one, the thyroid
segment, carrying the thyroid gland, the posterior one, the hyoid
segment, carrying the first pair of branchiæ.

Just as in Eurypterus the fused segment, carrying the uterus on its
internal surface, forms a long median tongue which separates the most
anterior branchial segments on each side, so also the fused segment
carrying the thyroid forms in Ammocoetes a long median tongue, which
separates the most anterior branchial segments on each side.

Finally, and this is the most conclusive evidence of all, this thyroid
gland of Ammocoetes is totally unlike that of any of the higher
vertebrates, and, indeed, of the adult form Petromyzon itself, but it
forms an elaborate complicated organ, which is directly comparable with
the uterus and genital ducts of animals such as scorpions. Not only is
such a comparison valid with respect to its shape, but also with respect
to its structure, which is absolutely unique among vertebrates, and very
different to that of any other vertebrate gland, but resembles in a
striking manner a glandular structure found in the uterus, both of male
and female scorpions.

The generative glands in Limulus, together with the liver-glands, form a
large glandular mass, situated in the head-region closely surrounding the
central nervous system, so that the genital ducts pass from the
head-region tailwards to the operculum. In the scorpion they lie in the
abdominal region, so that their ducts pass headwards to the operculum.

Probably in the Palæostraca the generative mass was situated in the
cephalic region as in Limulus, and it is probable that the remnant of it
still exists in {217}Ammocoetes in the shape of the peculiar large cells
packed together, with pigment masses in between them, which form such a
characteristic feature of the glandular-looking material, which fills up
the space between the cranial walls and the central nervous system.

Finally, the relationship which has been known from time immemorial to
exist between the sexual organs and the thyroid in man and other animals,
and has hitherto been a mystery without any explanation, may possibly be
the last reminiscence of a time when the thyroid glands were the uterine
glands of the palæostracan ancestor.

The consideration of the facial nerve, and the segments it supplies,
still further points to the origin of the Vertebrata from the
Palæostraca.

{218}CHAPTER VI

_THE EVIDENCE OF THE OLFACTORY APPARATUS_

Fishes divided into Amphirhinæ and Monorhinæ.--Nasal tube of the
lamprey.--Its termination at the infundibulum.--The olfactory organs of
the scorpion group.--The camerostome.--Its formation as a tube.--Its
derivation from a pair of antennæ.--Its termination at the true
mouth.--Comparison with the olfactory tube of Ammocoetes.--Origin of the
nasal tube of Ammocoetes from the tube of the hypophysis.--Direct
comparison of the hypophysial tube with the olfactory tube of the
scorpion group--Summary.

In the last chapter I finished the evidence given by the consideration of the mesosomatic or opisthotic nerves, and the segments they supplied. The evidence is strongly in accordance with that of previous chapters, and not only confirms the conclusion that vertebrates arose from some member of the Palæostraca, but helps still further to delimit the nature of that member. It is almost startling to see how the hypothesis put forward in the second chapter, suggested by the consideration of the nature of the vertebrate central nervous system and of the geological record, has received stronger and stronger confirmation from the consideration of the vertebrate optic apparatus, the vertebrate skeleton, the respiratory apparatus, and, finally, the thyroid gland. All fit naturally into a harmonious whole, and give a feeling of confidence that a similar harmony will be found upon consideration of the rest of the vertebrate organs.

Following naturally upon the segments supplied by the opisthotic (mesosomatic) cranial nerves, we ought to consider now the segments supplied by the pro-otic (prosomatic) cranial nerves, i.e. the segments belonging to the trigeminal nerve-group in the vertebrate, and in the invertebrate the segments of the prosoma with their characteristic appendages. There are, however, in all vertebrates in this foremost cranial region, in addition to the optic nerves, two other well-marked nerves of special sense, the olfactory and the auditory. Of these, the former are in the same class as the optic nerves, for they arise {219}in the vertebrate from the supra-infundibular nerve-mass, and in the invertebrate from the supra-oesophageal ganglia. The latter arise in the vertebrate from the infra-infundibular nerve-mass, and, as the name implies, are situated in the region where the pro-otic nerves are contiguous to the opisthotic, _i.e._ at the junction of the prosomatic and mesosomatic nerve-regions.

The chapter dealing with the evidence given by the olfactory nerves and the olfactory apparatus ought logically to have followed immediately upon the one dealing with the optic apparatus, seeing that both these special sense-nerves belong to the supra-infundibular segments in the vertebrate and to the supra-oesophageal in the invertebrate.

I did not deal with them in that logical sequence because it was necessary for their understanding to introduce first the conception of modified appendages as important factors in any consideration of vertebrate segments; a conception which followed naturally after the evidence afforded by the skeleton in Chapter III., and by the branchial segments in Chapter IV. So, too, now, although the discussion of the prosomatic segmentation ought logically to follow immediately on that of the mesosomatic segmentation, I have determined to devote this chapter to the evidence of the olfactory organs, because the arguments as to the segments belonging to the trigeminal nerve-group are so much easier to understand if the position of the olfactory apparatus is first made clear.

In all vertebrates the nose is double and opens into the pharynx, until we descend to the fishes, where the whole group Pisces has been divided into two subsidiary groups, Monorhinæ and Amphirhinæ, according as they possess a median unpaired olfactory opening, or a paired opening. The Monorhinæ include only the Cyclostomata--the lampreys and hag-fishes.

In the lampreys the single olfactory tube ends blindly, while in the hag-fishes it opens into the pharynx. In the lamprey, both in Petromyzon and Ammocoetes, the opening of this nasal tube is a conspicuous object on the dorsal surface of the head in front of the transparent spot which indicates the position of the right median eye. It is especially significant, as showing the primitive nature of this median olfactory passage, that a perfectly similar opening in the {220}same position is always found in the dorsal head-shields of all the Cephalaspidæ and Tremataspidæ, as will be explained more fully in Chapter X.

All the evidence points to the conclusion that the olfactory apparatus of the vertebrate originated as a single median tube, containing the special olfactory sense-epithelium, which, although median and single, was innervated by the olfactory nerve of each side. The external opening of this tube in the lamprey is dorsal. How does it terminate ventrally?

The ventral termination of this tube is most instructive and suggestive. It terminates blindly at the very spot where the infundibular tube terminates blindly and the notochord ends. After transformation, when the Ammocoete becomes the Petromyzon, the tube still ends blindly, and does not open into the pharynx as in Myxine; it, however, no longer terminates at the infundibulum, but extends beyond it towards the pharynx.

This position of the nasal tube suggests that it may originally have opened into the tube of the central nervous system by way of the infundibular tube. This suggestion is greatly enhanced in value by the fact that in the larval Amphioxus the tube of the central nervous system is open to the exterior, its opening being known as the anterior neuropore, and this anterior neuropore is situated at the base of a pit, known as the olfactory pit because it is supposed to represent the olfactory organ of other fishes.

Following the same lines of argument as in previous chapters, this suggestion indicates that the special olfactory organs of the invertebrate ancestor of the vertebrates consisted of a single median olfactory tube or passage, which led directly into the oesophagus and was innervated, though single and median, by a pair of olfactory nerves which arose from the supra-oesophageal ganglia. Let us see what is the nature of the olfactory organs among arthropods, and whether such a suggestion possesses any probability.

THE OLFACTORY ORGANS OF THE SCORPION GROUP.

At first sight the answer appears to be distinctly adverse, for it is well known that in all the Insecta, Crustacea, and the large majority of Arthropoda, the first pair of antennæ, often called the antennules, are olfactory in function, and these are free-moving, bilaterally {221}situated, independent appendages. Still, even here there is the striking fact that the nerves of these olfactory organs always arise from the supra-oesophageal ganglia, although those to the second pair of antennæ arise from the infra-oesophageal ganglia, just as the olfactory nerves of the vertebrate arise from the supra-infundibular brain-mass. Not only is there this similarity of position, but also a similarity of structure in the olfactive lobes of the brain itself of so striking a character as to cause Bellonci to sum up his investigations as follows:--

"The structure and connections of the olfactive lobes present the same fundamental plan in the higher arthropods and in the vertebrates. In the one, as in the other, the olfactory fibres form, with the connecting fibres of the olfactory lobes, a fine meshwork, which, consisting as it does of separate groups, may each one be called an olfactory glomerulus."

He attributes this remarkable resemblance to a physiological necessity that similarity of function necessitates similarity of structure, for he considers it out of the question to suppose any near relationship between arthropods and vertebrates.

Truly an interesting remark, with the one fallacy that relationship is out of the question.

The evidence so far has consistently pointed to some member of the palæostracan group as the ancestor of the vertebrates--a group which had affinities both to the crustaceans and the arachnids; indeed, many of its members resembled scorpions much more than they resemble crustaceans. The olfactory organs of the scorpions and their allies are, therefore, more likely than any others to give a clue to the position of the desired olfactory organs. In these animals and their allies paired olfactory antennæ are not present, either in the living land-forms or the extinct sea-scorpions, for all the antennæ-like, frequently chelate, appendages seen in Pterygotus, etc. (Fig. 8), represent the cheliceræ, and correspond, therefore, to the second pair of antennæ in the crustaceans.

What, then, represents the olfactory antennæ in the scorpions? The answer to this question has been given by Croneberg, and very striking it is. The two olfactory antennæ of the crustacean have combined together to form a hollow tube at the base of which the mouth of the animal is situated, so that the food passes along this olfactory passage before it reaches the mouth. This organ is often called after Latreille, the camerostome, sometimes the rostrum; it is naturally median in position and appears, therefore, to be an unpaired organ; its paired {222}character is, of course, evident enough, for it is innervated by a pair of nerves, and these nerves, as ought to be the case, arise from the supra-oesophageal ganglia. In Galeodes it is a conspicuously paired antennæ-like organ (Fig. 94).

Croneberg has also shown that this rostrum, or camerostome, arises embryologically as a pair of appendages similar to the other appendages. This last observation of Croneberg has been confirmed by Brauer in 1894, who describes the origin of the upper lip, as he calls it, in very similar terms, without, however, referring to Croneberg's paper. Croneberg further shows that this foremost pair of antennæ not only forms the so-called upper lip or camerostome, but also a lower lip, for from the basal part of the camerostome there projects on each side of the pharynx a dependent accessory portion, which in some cases fuses in the middle line, and forms, as it were, a lower lip. The entosclerite belonging to this dependent portion is apparently the post-oral entosclerite of Lankester and Miss Beck.

_cam._, camerostome; _pr. ent._, pre-oral entosclerite; _l.l._, dependent portion of camerostome; _ph._, pharynx; _al._, alimentary canal; _n. op._, median optic nerves; _pl._, plastron; _v.c._, ventral nerve chain; 2, 3, second and third appendages.]

At the base of the tubular passage formed by this modified first pair of antennæ the true mouth is found opening directly into the dilated pharynx, the muscles of which enable the act of suction to be carried out. The narrow oesophagus leads out from the pharynx and is completely surrounded by the supra- and infra-oesophageal nerve masses.

Huxley also describes the mouth of the scorpion in precisely the same position (_cf. o_, Fig. 96).

{223}In order to convey to my readers the antennæ-like character of the camerostome in Galeodes (Fig. 101), and its position, I give a figure (Fig. 94) of the organ from its dorsal aspect, after removal of the cheliceræ and their muscles. A side view of the same organ is given in Fig. 95 to show the feathered termination of the camerostome, and the position of the dependent accessory portion (_l.l._) (Croneberg's 'untere Anhang') with its single long antenna-like feather. In both figures the alimentary canal (_al._) is seen issuing from the conjoined supra- and infra-oesophageal mass.

As is seen in the figures, the bilateral character of the rostrum, as Croneberg calls it, is apparent not only in its feathered extremity but also in its chitinous covering, the softer median dorsal part (left white in figure) being bounded by two lateral plates of hard chitin, which meet in the middle line near the extremity of the organ. In all the members of the scorpion group, as is clearly shown in Croneberg's figures, the rostrum or camerostome is built up on the same plan as in Galeodes, though the antenna-like character may not be so evident.

_gl. supr. oes._, supra-oesophageal ganglion; _gl. infr. oes._, infra-oesophageal ganglion. The rest of the lettering same as in Fig. 94.]

When we consider that the first pair of antennæ in the crustaceans are olfactory in function, Croneberg's observations amount to this--

In the arachnids and their allies the first pair of antennæ form a pre-oral passage or tube, olfactory in function; the small mouth, which opens directly into the pharynx, being situated at the end of this olfactory passage.

{224}Croneberg's observations and conclusions are distinctly of very great importance in bringing the arachnids into line with the crustaceans, and it is therefore most surprising that they are absolutely ignored by Lankester and Miss Beck in their paper published in 1883, in which Latreille only is mentioned with respect to this organ, and his term "camerostome," or upper lip, is used throughout, in accordance with the terminology in Lankester's previous paper. That this organ is not only a movable lip or tongue, but essentially a sense-organ, almost certainly of smell and taste, as follows from Croneberg's conclusions, is shown by the series of sections which I have made through a number of young Thelyphonus (Fig. 102).

I give in Fig. 96 a sagittal median section through the head-end of the animal, which shows clearly the nature of Croneberg's conception. At the front end of the body is seen the median eye (_ce._), _o_ is the mouth, _Ph._ the pharynx, _oes._ the narrow oesophagus, compressed between the supra-oesophageal (_supr. oes._) and infra-oesophageal (_infr. oes._) brain mass, which opens into the large alimentary canal (_Al._); _Olf. pass._ is the olfactory passage to the mouth, lined with thick-set, very fine hairs, which spring from the hypostome (_Hyp._) as well as from the large conspicuous camerostome (_Cam._), which limits this tube anteriorly. The space between the camerostome and the median eye is filled up by the massive cheliceræ, which are not shown in this section, as they begin to appear in the {225}sections on each side of the median one. The muscles of the pharynx and the muscles of the camerostome are attached to the pre-oral entosclerite (_pr. ent._). The post-oral entosclerite is shown in section as _post. ent._ The dorsal blood-vessel, or heart, is indicated at _H._

In Fig. 97 I give a transverse section through another specimen of the same litter, to show the nature of this olfactory tube when cut across. Both sections show most clearly that we are dealing here with an elaborate sense-organ, the surface of which is partly covered with very fine long hairs, partly, as is seen in the figure, is composed of long, separate, closely-set sense-rods (_bat._), well protected by the long hairs which project on every side in front of them, which recall to mind Bellonci's figure of the 'batonnets olfactives' on the antennæ of Sphæroma. Finally, we have the observation of Blanchard quoted by Huxley, to the effect that this camerostome is innervated by nerves from the supra-oesophageal ganglia which are clearly bilateral, seeing that they arise from the ganglion on each side and then unite to pass into the camerostome; in other words, paired olfactory nerves from the supra-oesophageal ganglia.

These facts demonstrate with wonderful clearness that in one group of the Arthropoda the olfactory antennæ have been so modified as to form an olfactory tube or passage, which leads directly into the mouth and so to the oesophagus of the animal, and, strikingly enough, this group, the Arachnida, is the very one to which the scorpions belong.

If for any cause the mouth _o_ in Fig. 96 were to be closed, then the olfactory tube (_olf. pass._) might still remain, owing to its importance as the organ of smell, and the olfactory tube would terminate blindly at the very spot where the corresponding tube does terminate in the vertebrate, according to the theory put forward in this book.

THE OLFACTORY TUBE OF AMMOCOETES.

In all cases where there is similarity of topographical position in the organs of the vertebrate and arthropod we may expect also to find similarity of structure. At first sight it would appear as though such similarity fails us here, for a cross-section of the olfactory tube in Petromyzon represents an elaborate organ such as is shown in Fig. 98, very different in appearance to the section across the olfactory passage of a young Thelyphonus given in Fig. 97.

{226}

1 and 2, sections of first and second appendages.]

_cart._, nasal cartilage.]

{227}As is seen, it is difficult to see any connection between these folds of olfactory epithelium and the simple tube of the scorpion. But in the nose, as in all other parts of the head-region of the lamprey, remarkable changes take place at transformation, and examination of the same tube in Ammocoetes demonstrates that the elaborate structure of the adult olfactory organ is actually derived from a much simpler form of organ, represented in Fig. 99. Here, in Ammocoetes, the section is no longer strikingly different from that of the Thelyphonus organ, but, instead, most strikingly similar to it. Thus, again, it is shown that this larval form of the lamprey gives more valuable information as to vertebrate ancestry than all the rest of the vertebrates put together.

_cart._, nasal cartilage.]

Still, even now the similarity between the two organs is not complete, for the tube in the lamprey opens on to the exterior on the dorsal surface of the head, while in the scorpion tribe it is situated ventrally, being the passage to the mouth and alimentary canal. In accordance with this there is no sign of any opening on the dorsal carapace of any of the extinct sea-scorpions or of the living land-scorpions, such as is so universally found in the cephalaspids, tremataspids, and lampreys. Here is a discrepancy of an apparently serious character, yet so wonderfully does the development of the individual recapitulate the development of the race, that this very discrepancy becomes converted into a triumphant vindication of the {228}correctness of the theory advocated in this book, as soon as we turn our attention to the development of this nasal tube in the lamprey.

We must always remember not only the great importance of a larval stage for the unriddling of problems of ancestry, but also the great advantage of being able to follow more favourably any clues as to past history afforded by the development of the larva itself, owing to the greater slowness in the development of the larva than of the embryo. Such a clue is especially well marked in the course of development of Ammocoetes according to Kupffer's researches, for he finds that when the young Ammocoetes is from 5 to 7 mm. in length, some time after it has left the egg, when it is living a free larval life, a remarkable series of changes takes place with considerable rapidity, so that we may regard the transformation which takes place at this stage, as in some degree comparable with the great transformation which occurs when the Ammocoetes becomes a Petromyzon.

All the evidence emphasizes the fact that the latter transformation indicates the passage from a lower into a higher form of vertebrate, and is to be interpreted phylogenetically as an indication of the passage from the Cephalaspidian towards the Dipnoan style of fish. If, then, the former transformation is of the same character, it would indicate the passage from the Palæostracan to the Cephalaspid.

What is the nature of this transformation process as described by Kupffer?

It is characterized by two most important events. In the first place, up to this time the oral chamber has been cut off from the respiratory chamber by a septum--the velum--so that no food could pass from the mouth to the alimentary canal. At this stage the septum is broken through, the oral chamber communicates with the respiratory chamber, and the velar folds of the more adult Ammocoetes are left as the remains of the original septum. The other striking change is the growth of the upper lip, by which the orifice of the nasal tube is transferred from a ventral to a dorsal position. Fig. 100, taken from Kupffer's paper, represents a sagittal section through an Ammocoetes 4 mm. long; _l.l._ is the lower lip, _u.l._ the upper lip, and, as is seen, the short oral chamber is closed by the septum, _vel._ Opening ventrally is a tube called the tube of the hypophysis, _Hy._, which extends close up to the termination of the infundibulum. On the anterior surface of this tube is the projection called by Kupffer the olfactory plakode. At this stage the upper lip grows with great {229}rapidity and thickens considerably, thus forcing the opening of the hypophysial tube more and more dorsalwards, until at last, in the full-grown Ammocoetes, it becomes the dorsal opening of the nasal tube, as already described. Here, then, in the hypophysial tube we have the original position of the olfactory tube of the vertebrate ancestor, and it is significant, as showing the importance of this organ, to find that such a hypophysial tube is characteristic of the embryological development of every vertebrate, whatever may be the ultimate form of the external nasal orifices.

The single median position of the olfactory organ in the Cyclostomata, in contradistinction to its paired character in the rest of the vertebrates, has always been a stumbling-block in the way of those who desired to consider the Cyclostomata as degenerated Selachians, for the origin of the olfactory protuberance, as a single median plakode, seemed to indicate that the nose arose as a single organ and not as a paired organ.

_A-B_, the line of epibranchial ganglia; _au._, auditory capsule; _nc._, notochord; _Hy._, tube of hypophysis; _Or._, oral cavity; _u.l._, upper lip; _l.l._, lower lip; _vel._, septum between oral and respiratory cavities; _V._, _VII._, _IX._, _X._, cranial nerves; _x._, nerve with four epibranchial ganglia.]

On the other hand, the two olfactory nerves of Ammocoetes compare absolutely with the olfactory nerves of other vertebrates, and force one to the conclusion that this median organ of Ammocoetes arose from a pair of bilateral organs, which have fused in the middle line.

{230}

{231}The comparison of this olfactory organ with the camerostome gives a satisfactory reason for its appearance in the lowest vertebrates as an unpaired median organ; equally so, the history of the camerostome itself supplies the reason why the olfactory nerves are double, why the organ is in reality a paired organ and not a single median one. Thus, in a sense, the grouping of the fishes into Monorhinæ and Amphirhinæ has not much meaning, seeing that the olfactory organ is in all cases double.

The evidence of the olfactory organs in the vertebrate not only confirms, in a most striking manner, the theory of the origin of the {232}vertebrate from the Palæostracan, but points indubitably to an origin from a scorpion-like rather than a crustacean-like stock. To complete the evidence, it ought to be shown that the ancient sea-scorpions did possess an olfactory passage similar to the modern land-scorpions. The evidence on this question will come best in the next chapter, where I propose to deal with the prosomatic appendages of the Palæostracan group.

SUMMARY.

The vertebrate olfactory apparatus commences as a single median tube
which terminates dorsally in the lamprey, and is supplied by the two
olfactory nerves which arise from the supra-infundibular portion of the
brain. It is a long, tapering tube which passes ventrally and terminates
blindly at the infundibulum in Ammocoetes. The dorsal position of the
nasal opening is not the original one, but is brought about by the growth
of the upper lip. The nasal tube originally opened ventrally, and was at
that period of development known as the tube of the hypophysis.

The evidence of Ammocoetes thus goes to show that the olfactory apparatus
started as an olfactory tube on the ventral side of the animal, which led
directly up to, and probably into, the oesophagus of the original
alimentary canal of the palæostracan ancestor.

Strikingly enough, although in the crustaceans the first pair of antennæ
form the olfactory organs, no such free antennæ are found in the
arachnids, but they have amalgamated to form a tube or olfactory passage,
which leads directly into the mouth and oesophagus of the animal.

This olfactory passage is very conspicuous in all members of the scorpion
group, and, like the olfactory tube of the vertebrate, is innervated by a
pair of nerves, which resemble those supplying the first pair of antennæ
in crustaceans as to their origin from the supra-oesophageal ganglia.

This nasal passage, or tube of the hypophysis, corresponds in structure
and in position most closely with the olfactory tube of the scorpion
group, the only difference being that in the latter case it opens
directly into the oesophagus, while in the former, owing to the closure
of the old mouth, it cannot open into the infundibulum.

The evidence of the olfactory apparatus, combined with that of the optic
apparatus, is most interesting, for, whereas the former points
indubitably to an ancestor having scorpion-like affinities, the structure
of the lateral eyes points distinctly to crustacean, as well as arachnid,
affinities.

Taking the two together the evidence is extraordinarily strong that the
vertebrate arose from a member of the palæostracan group with marked
scorpion-like affinities.

{233}CHAPTER VII

_THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES_

Comparison of the trigeminal with the prosomatic region.--The prosomatic
appendages of the Gigantostraca.--Their number and nature.--Endognaths
and ectognath.--The metastoma.--The coxal glands.--Prosomatic region of
Eurypterus compared with that of Ammocoetes.--Prosomatic segmentation
shown by muscular markings on carapace.--Evidence of coelomic cavities in
Limulus.--Summary.

The derivation of the olfactory organs of the vertebrate from the olfactory antennæ of the arthropod in the last chapter is confirmatory proof of the soundness of the proposition put forward in Chapter IV., that the segmentation in the cranial region of the vertebrate was derived from that of the prosomatic and mesosomatic regions of the palæostracan ancestor. Such a segmentation implies a definite series of body-segments, corresponding to the mesomeric segmentation of the vertebrate, and a definite series of appendages corresponding to the splanchnic segmentation of the vertebrate.

Of the foremost segments belonging to the supra-oesophageal region characterized by the presence of the median eyes, of the lateral eyes, and of the olfactory organs, a wonderfully exact replica has been shown to exist in the pineal eyes, the lateral eyes, and the olfactory organ of the vertebrate, belonging, as they all do, to the supra-infundibular region.

Of the infra-oesophageal segments belonging to the prosoma and mesosoma respectively, the correspondence between the mesosomatic segments carrying the branchial appendages and the uterus, with those in the vertebrate carrying the branchiæ and the thyroid gland respectively, has been fully proved in previous chapters.

There remain, then, only the segments of the prosomatic region to be considered, a region which, both in the vertebrate and invertebrate, is never respiratory in function but always masticatory, such {234}mastication being performed in Limulus and its allies by the muscles which move the foot-jaws or gnathites, which are portions of the prosomatic appendages specially modified for that purpose, and in the vertebrates by the masticatory muscles, which are always innervated by the trigeminal or Vth cranial nerve. This comparison implies that the motor part of the trigeminal nerve originally supplied the prosomatic appendages.

The investigations of van Wijhe and of all observers since the publication of his paper prove that in this trigeminal region, as in the vagus region, a double segmentation exists, of which the ventral or splanchnic segments, corresponding to the appendages in the invertebrate, are supplied by the trigeminal nerves, while the dorsal or somatic segments, corresponding to the somatic segments in the invertebrate, are supplied by the IIIrd or oculomotor and the IVth or trochlear nerves--nerves which supply muscles moving the lateral eyes.

In accordance, then, with the evidence afforded by the nerves of the branchial segments, it follows that the muscles supplied by the motor part of the trigeminal ought originally to have moved the appendages belonging to a series of prosomatic segments. On the other hand, the eye-muscles ought to have belonged to the body-part of the prosomatic segments, and must therefore have been grouped originally in a segmental series corresponding to the prosomatic appendages.

The evidence for and against this conclusion will be the subject of consideration in this and the succeeding chapters. At the outset it is evident that any such comparison necessitates an accurate knowledge of the number of the prosomatic segments in the Gigantostraca and of the nature of the corresponding appendages.

In all this group of animals, the evidence as to the number of segments in either the prosomatic or mesosomatic regions is given by--

1. The number of appendages.

2. The segmental arrangement of the muscles of the prosoma or mesosoma respectively.

3. The segmental arrangement of the coelomic or head-cavities.

4. The divisions of the central nervous system, or neuromeres, together with their outgoing segmental nerves.

It follows, therefore, that if from any cause the appendages are not apparent, as is the case in many fossil remains, or have dwindled {235}away and become insignificant, we still have the muscular, coelomic, and nervous arrangements left to us as evidence of segmentation in these animals, just as in vertebrates.

In this prosomatic region, we find in Limulus the same tripartite division of the nerves as in the mesosomatic region, so that the nerves to each segment may be classed as (1) appendage-nerve; (2) sensory or dorsal somatic nerve, supplying the prosomatic carapace; (3) motor or ventral somatic nerve, supplying the muscles of the prosoma, and containing possibly some sensory fibres. The main difference between these two regions in Limulus consists in the closer aggregation of the prosomatic nerves, corresponding to the concentration of the separate ganglia of origin in the prosomatic region of the brain.

The number of prosomatic segments in Limulus is not evident by examination of the prosomatic carapace, so that the most reliable guide to the segmentation of this region is given by the appendages, of which one pair corresponds to each prosomatic segment.

The number of such segments, according to present opinion, is seven, viz.:--

(1) The foremost segment, which bears the cheliceræ.

(2, 3, 4, 5, 6) The next five segments, which carry the paired locomotor appendages; and

(7) The last segment, to which belongs a small abortive pair of appendages, known by the name of the chilaria, situated between the last pair of locomotor appendages and the operculum or first pair of mesosomatic appendages. These appendages are numbered from 1-7 in the accompanying drawing (Fig. 103).

Of these seven pairs of appendages, the significance of the first and the last has been matter of dispute. With respect to the first pair, or the cheliceræ, the question has arisen whether their nerves belong to the infra-oesophageal group, or are in reality supra-oesophageal.

It is instructive to observe the nature and the anterior position of this pair of appendages in the allied sea-scorpions, especially in Pterygotus, where the only chelate organs are found in these long, antennæ-like cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to be represented by the small non-chelate antennæ seen in Fig. 8, B and C (p. 27), taken from Woodward. If such is the case, then these figures show that a pair of appendages is missing in each {236}of these forms, for they possess only five free prosomatic appendages instead of six, as in Limulus and in Pterygotus. Similarly, Woodward only allowed five appendages for Pterygotus, so that his restorations were throughout consistent. Schmidt, in _Pterygotus osiliensis_ has shown that the true number was six, not five, as seen in his restoration given in Fig. 8, A (p. 27).

The gnathic bases of the appendages have been separated from those of the other side to show the promesosternite or endostoma (_End._).]

With respect to Eurypterus, Schmidt figures an exceedingly minute pair of antennæ between the coxal joints of the first pair of appendages, thus making six pairs of appendages. Gerhard Holm, however, in his recent beautiful preparations from Schmidt's specimens and others collected at Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus were of the same kind as those of Limulus. I reproduce his figure (Fig. 104) showing the small chelate cheliceræ (1) overhanging the mouth orifice, just as in Limulus or in Scorpio.

{237}So, also, since Woodward's monograph, Laurie has discovered in _Slimonia acuminata_ a small median pair of chelate appendages exactly corresponding to the cheliceræ of Limulus, or of Eurypterus, or of Scorpio. We may, therefore, take it for granted that such was also the case in Stylonurus, and that the foremost pair of prosomatic appendages in all these extinct sea-scorpions were in the same position and of the same character as the cheliceræ of the scorpions.

In the living scorpion and in Limulus the nerves to this pair of appendages undoubtedly arise from the foremost prosomatic ganglia, and the reason why they appear to belong to the supra-oesophageal brain-mass has been made clear by Brauer's investigations on the embryology of Scorpio; for he has shown that the cheliceral ganglia shift from the ventral to the dorsal side of the oesophagus during development, thus becoming pseudo-supra-oesophageal, though in reality belonging to the infra-oesophageal ganglia. This cheliceral pair of appendages is, in all probability, homologous with the second pair of antennæ in the crustacea.

{238}I conclude, then, that the cheliceræ must truly be included in the prosomatic group, but that they stand in a somewhat different category to the rest of the prosomatic appendages, inasmuch as they take up a very median anterior and somewhat dorsal position, and their ganglia of origin are also exceptional in position.

Next for consideration come the chilaria (7 in Fig. 103), which Lankester did not consider to belong to appendages at all, but to be a peculiar pair of sternites. Yet their very appearance, with their spinous hairs corresponding to those of the other gnathites and their separate nerve-supply, all point distinctly to their being a modified pair of appendages, and, indeed, the matter has been placed beyond doubt by the observations of Kishinouye, who has found embryologically that they arise in the same way as the rest of the prosomatic appendages, and belong to a distinct prosomatic segment, viz. the seventh segment. In accordance with this, Brauer has found that in the scorpion there is in the embryo a segment, whose appendages degenerate, which is situated between the segment bearing the last pair of thoracic appendages and the genital operculum--a segment, therefore, comparable in position to the chilarial segment of Limulus.

Coming now to the five locomotor appendages, we find that they resemble each other to a considerable extent in most cases, with, however, certain striking differences. Thus in Limulus they are chelate, with their basal joints formed as gnathites, except in the case of the fifth appendage, in which the extremity is modified for the purpose of digging in the sand. In Pterygotus, Slimonia, Eurypterus, the first four of these appendages are very similar, and are called by Huxley and Woodward endognaths; in all cases they possess a basal part or sterno-coxal process, which acts as a gnathite or foot-jaw, and a non-chelate tactile part, which possesses no prehensile power, and in most cases could have had no appreciable share in locomotion, called by Huxley and Woodward the palpus. These small palps were probably retractile, and capable of being withdrawn entirely under the hood. The fifth appendage is usually different, being a large swimming organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is known as the ectognath.

Finally, in _Drepanopterus Bembycoides_, as stated by Laurie, all five locomotor appendages are built up after the same fashion, the last one not being formed as a paddle-shaped organ or elongated as {239}in Stylonurus, but all five possess no special locomotor or prehensile power. According to Laurie this is a specially primitive form of the group.

It is significant to notice from this sketch that with the absence of special prehensile terminations such as chelæ, or the absence of special locomotor functions such as walking or swimming, these appendages tend to dwindle and become insignificant, taking up the position of mere feelers round the mouth, and at the same time are concentrated and pressed closely together, so that their appendage-nerves must also be close together.

This sketch therefore shows us that--

Of the six foremost prosomatic appendages, the cheliceræ and the four endognaths were, at the time when the vertebrates first appeared, in very many cases dwindling away; the latter especially no longer functioned as locomotor appendages, but were becoming more and more mere palps or tentacles situated round the mouth, which could by no possibility afford any help to locomotion.

On the contrary, the sixth pair of appendages--the ectognaths--remained powerful, being modified in many cases into large oar-like limbs by which the animal propelled itself through the water.

It is a striking coincidence that those ancient fishes, Pterichthys and Bothriolepis, should have possessed a pair of large oar-like appendages.

At this time, then, in strong contrast to the endognaths, the ectognaths, or sixth pair of appendages, remained strong and vigorous. What about the seventh pair, the chilaria of Limulus?

Of all the prosomatic appendages these are the most interesting from the point of view of my theory, for whereas in the scorpion of the present day they have dwindled away and left no trace except in the embryo, in the sea-scorpions of old, far from dwindling, they had developed and become a much more important organ than the chilaria of Limulus.

In all these animals a peculiarly striking and unique structure is found in this region known by the name of the metastoma, or lip-plate (Figs. 8 and 104 (7)); it is universally considered to be formed by the fusion of the two chilarial appendages.

All observers are agreed that this lip-plate was freely movable. Nieskowski considers that the movement of the metastoma was entirely in a vertical direction, whereby the cleft which is seen {240}between the basal joints of all the pairs of locomotor appendages could be closed from behind. Woodward says it no doubt represents the labium, and served more effectually to enclose the posterior part of the buccal orifice, being found exteriorly to the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with Nieskowski, and looks on the mestasoma as forming a lower lip within which the bases of the ectognaths worked.

Quite recently Gerhard Holm has worked over again the very numerous specimens of _Eurypterus Fischeri_, which are obtainable at Rootziküll, and has thrown new light on the relation of the metastoma to the mouth-parts. His preparations show clearly that the true lower lip of Eurypterus was not the metastoma, for when the metastoma is removed another plate (_End._, Fig. 105, B) situated {241}internally to it is disclosed, which, in his view, corresponds to the sternite between the bases of the pro-somatic appendages in Limulus, _i.e._ to the sternite called by Lankester, the pro-mesosternite (_End._, Fig. 103). This inner plate formed with the metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed posteriorly, within which the bases of the ectognaths worked. In other words, the removal of the metastoma discloses in Eurypterus the true anterior ventral surface of the animal which corresponds to that of Limulus, or of the scorpion group, with its pro-mesosternite and laterally attached gnathites or sterno-coxal processes. To this inner plate or pro-mesosternite Holm gives the name of _endostoma_.

To the anterior edge of the endostoma a thinner membrane is attached which passes inwards in the direction of the throat, and forms, therefore, the lower lip (_Hyp._, Fig. 105, B) of the passage of the mouth (_olf. p._). This membrane bears upon its surface a tuft of hairs, which he thought were probably olfactory in function. Consequently, in his preliminary communication, he describes this lower lip as forming, in all probability, an olfactory organ; in his full communication he repudiates this suggestion, because he thinks it unlikely that such an organ would be situated within the mouth. I feel sure that if Holm had referred to Croneberg's paper, and seen how the true mouth in all the scorpion group is situated at the base of an olfactory passage, he would have recognized that his first suggestion is in striking accordance with the nature of the entrance to the mouth in other scorpions.

That Eurypterus also possessed a camerostome (_cam._) seems to follow of necessity from its evident affinities both with Limulus and the scorpions. We see, in fact, that the mouth of these old sea-scorpions was formed after the fashion of Limulus, surrounded by masticatory organs in the shape of foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal sagittal section of one of these old palæostracan forms would be obtained by the combination of actual sagittal sections through Limulus and a member of the scorpion group, with, at the same time, a due recognition of Holm's researches. Such a section is represented in Fig. 105, B, in which I have drawn the central nervous system and its nerves, the median eyes (_C.E._), the olfactory organs (_Cam._), the pharynx (_Ph._), oesophagus (_oes._), and alimentary canal (_Al._), but have not tried to indicate the lateral eyes. I have represented the prosomatic appendages by numbers (1-7), and {242}the foremost mesosomatic segments by numbers (8-13). I have placed the four endognaths and the nerves going to them close together, and made them small, mere tentacles, in recognition of the character of these appendages in Eurypterus, and have indicated the position and size of the large ectognath, with its separate nerve, by (6). If among the ancient Eurypterus-like forms, which were living at the time when vertebrates first appeared, there were some in which the ectognaths also had dwindled to a pair of tentacles, then such animals would possess a prosomatic chamber formed by a metastoma or accessory lip, within which were situated five pairs of short tactile appendages or tentacles. If the vertebrate were derived from such an animal, then the trigeminal nerve, as the representative of these prosomatic appendage-nerves, ought to be found to supply the muscles of this accessory lip and of these five pairs of tentacles in the lowest vertebrate.

This prosomatic or oral chamber, as it might be called, was limited posteriorly by the fused metastoma (7) and operculum (8), so that if in the same imaginary animal one imagines that the gill-chambers, instead of being separate, are united to form one large respiratory chamber, then, in such an animal, a prosomatic oral chamber, in which the prosomatic appendages worked, would be separated from a mesosomatic respiratory chamber by a septum composed of the conjoined basal portions of the mesosomatic operculum and the prosomatic metastoma, as indicated in the diagram. In this septum the nerves to the last prosomatic appendage (equivalent to the last part of the trigeminal in the vertebrate) and to the first mesosomatic (equivalent to the thyroid part of the facial) would run, as shown in the figure, close together in the first part of their course, and would separate when the ventral surface was reached, to pass headwards and tailwards respectively.

THE COXAL GLANDS.

One more characteristic of these appendages requires mention, and that is the excretory glands situated at the base of the four endognaths known as the coxal glands. These glands are the main excretory organs in Limulus and the scorpions, and extend into the basal segments or coxæ of the four endognaths, not into those of the ectognaths or the chilaria (or metastoma). Hence their name, coxal {243}glands; and, seeing the importance of the excretory function, it is likely enough that they would remain, even when the appendages themselves had dwindled away. With the concentration and dwindling of the endognaths these coxal glands would also be concentrated, so that in the diagram (Fig. 105) they would rightly be grouped together in the position indicated (_cox. gl._).

Such a diagram indicates the position of all the important organs of the head-region except the special organs for taste and hearing. These, for the sake of convenience, I propose to take separately, in order at this stage of my argument not to overburden the simplicity of the comparison I desire to make with too much unavoidable detail.

THE PROSOMATIC REGION OF AMMOCOETES.

Let us now compare this diagram with that of the corresponding region in Ammocoetes and see whether or no any points of similarity exist.

With respect to this region, as in so many other instances already mentioned, Ammocoetes occupies an almost unique position among vertebrates, for the region supplied by the trigeminal nerve--the prosomatic region--consists of a large oral chamber which was separated from the respiratory chamber in the very young stage by a septum which is subsequently broken through, and so the two chambers communicate.

This chamber is bounded by the lower lip ventrally, the upper lip and trabecular region dorsally, and the remains of the septum or velum laterally and posteriorly. It contains a number of tentacles arranged in pairs within the chamber so as to form a sieve-like fringe inside the circular mouth; of these, the ventral pair are large, fused together, and attached to the lower lip.

All the muscles belonging to this oral chamber are of the visceral type, and are innervated by the trigeminal nerve. In accordance with the evidence obtained up to this point this means that such an oral chamber was formed by the prosomatic appendages of the invertebrate ancestor, similarly to the oral chamber just figured for Eurypterus.

This chamber in the full-grown Ammocoetes is not only open to the respiratory chamber, but is bounded by the large upper lip (_U.L._, Fig. 106, D). On the dorsal surface of this region, in front of the {244}pineal eye (_C.E._), is the most conspicuous opening of the olfactory tube (_Na._), which olfactory tube passes from the dorsal region to the ventral side to terminate blindly at the very spot where the infundibulum comes to the surface of the brain. Here, also, is situated that extraordinary glandular organ known as the pituitary body (_Pit._). A sagittal section, then, in diagram form, of the position of parts in the full-grown Ammocoetes, would be represented as in Fig. 106, D.