The Origin of Vertebrates

by Walter Holbrook Gaskell

The Tentacular Segments and the Upper Lip.

Anterior to this metastomal segment, Fig. [116] shows a group of visceral muscles, m₁, and yet again a muco-cartilaginous bar, sk₁, but, as already stated, no tubular muscles. These visceral muscles indicate the presence in front of the lower lip-segment of one or more segments of the nature of appendages. The muscles in question (m₁) are the muscles of the upper lip, the skeletal elements form a pair of large bars of muco-cartilage (sk₁), which start from the termination of the trabeculæ, and pass ventralwards to fuse with the muco-cartilaginous plate of the lower lip (Figs. 117 and 118). This large bar forms the tentacular ridge on each side, and gives small projections of muco-cartilage into each tentacle. In addition to this tentacular bar, a special bar of muco-cartilage exists for the fused pair of median tentacles, the so-called tongue, which extends in the middle line along the whole length of the lower lip, being separated from the muco-cartilaginous plate of the lower lip by the muscles of the lower lip. This tongue bar of muco-cartilage joins with the muco-cartilage of the lower lip at its junction with the thyroid plate, and also with the tentacular bar just before the latter joins the muco-cartilaginous plate of the lower lip. This arrangement of the skeletal tissue suggests that the pair of tentacles known as the tongue stand in a category apart from the rest of the tentacles; a suggestion which is strongly confirmed by the separate character of its nerve-supply, as already mentioned.

For three reasons, viz. the separateness both of their nerve-supply and of their skeletal tissue, and the importance they assume at transformation, this pair of ventral tentacles must, it seems to me, be put into a separate category from the rest of the tentacles. On the other hand, the innervation of the rest of the tentacles by a single nerve which sends off a branch as it passes each one, together with the concentration of their skeletal elements into a single bar, with projections into each tentacle, points directly to the conclusion that these tentacles must be considered as a group, and not singly.

I suggest that these tentacles are the remains of the ectognaths and endognaths; the tongue representing the two ectognaths, and the four tentacles on each side the four pairs of endognaths.

As we see, this method of interpretation attributes segmental value to the tentacles, a conclusion which is opposed to the general opinion of morphologists, who regard them as having no special morphological importance, and certainly no segmental value. On the other hand, the importance of the pair of ventral tentacles, the 'tongue' of Rathke, which lie in the mid-line of the lower lip, has been shown by Kaensche, Bujor, and others, all of whom are unanimous in asserting that at transformation they are converted into that large and important organ the piston or tongue of the adult Petromyzon. It is supposed that the rest of the tentacles vanish at transformation, being absorbed; they appear to me rather to take part in the formation of the sucking-disc, so that I am strongly inclined to believe that the whole of the remarkable suctorial apparatus of Petromyzon is derived from the tentacles of Ammocœtes. In other words, on my view, a conversion of the prosomatic appendages into a suctorial apparatus takes place at transformation, just as is frequently the case among the Arthropoda.

It is to the arrangement of the muscles that we look for evidence of segmental value. As long as it was possible to look upon these tentacles as mere sensory feelers round the mouth entrance, it was natural to deny segmental value to them. Matters are now, however, totally different since Miss Alcock's discovery of the rudimentary muscles at the base of the tentacles and their development at transformation. If these muscles represent some of the appendage muscles belonging to the foremost prosomatic segments just as the ocular muscles represent the dorso-ventral somatic muscles of those same segments, then we may expect ultimately to be able to give as good evidence of segmentation in their case as I have been able to give in the case of these latter muscles; for the two sets of muscles are curiously alike, seeing that the eye-muscles do not develop until transformation, but throughout the Ammocœtes stage remain in almost as rudimentary a condition as the tentacular muscles.

Another difficulty with respect to the tentacles is the determination of the number of them, owing to the fact that in addition to what may be called well-defined tentacles a large number of smaller tactile projections are found on the surface of the upper lip, as is seen in Fig. [115]. In the very young condition, 7 or 8 mm. in length, it is easier to make sure on this point. At this stage they may be spoken of as arranged in two groups: an anterior small group and a posterior larger group. The anterior group consists of a pair of very small tentacles and a very small median tentacle, all three situated quite dorsally in the front part of the upper lip. The posterior group, which is separate from the anterior, consists of five pairs of much larger tentacles, the most ventral pair in the mid-line ventrally on the lower lip being fused together to form the large ventral median tentacle or tongue already mentioned. This pair, according to Shipley, is markedly larger than the others. There are, therefore, five conspicuous tentacles on each side, and in front of them a smaller pair and a small median dorsal one. In the very young condition the accessory projections above-mentioned are not present, or at all events are not conspicuous, and the tentacles are also markedly larger in comparison to the size of the animal than in the older condition, where they have distinctly dwindled.

This posterior group of five conspicuous tentacles is the one which I suggest represents the four endognaths and one ectognath. What the significance of the small anterior group is, I know not. It is possible that the cheliceræ are represented here, for they are situated distinctly anterior to the other group; I know, however, of no sign of a markedly separate innervation to these most dorsal tentacles such as I should have expected to find if they represented the cheliceræ.

The muscles of the upper lip, which distinctly belong to the visceral and not to the somatic musculature, form part of the foremost segments, and in these muscles the tentacular nerve reaches its final destination. From their innervation, then, they must have belonged to the same appendages as the tentacles supplied by the tentacular nerve, i.e. to the endognaths. What conclusion can we form as to the probable origin of the upper lip of Ammocœtes? Since the oral chamber was formed by the forward growth of the metastoma, i.e. the lower lip of Ammocœtes, it follows that the upper lip is the continuation forwards of the original ventral surface of such an animal as Limulus or a member of the scorpion group, where there is no metastoma, and corresponds to the endostoma, as Holm calls it, of Eurypterus. This termination of the ventral surface in all these animals is made up of two parts: (1) Of sternites composing the true median ventral surface of the body, called by Lankester the pro- and meso-sternites; and (2) of the sterno-coxal processes of the foremost prosomatic appendages, called in the case of Limulus gnathites, because they are the main agents in triturating the food previously to its passage into the mouth. In Limulus, a conjoined pro-mesosternite forms the median ventral wall to which the sterno-coxal processes are attached on each side, and in Phrynus and Mygale a well-marked pro-sternite and meso-sternite are present, forming the posterior limit of the olfactory opening. In Buthus and the true scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th prosomatic appendages take part in surrounding the olfactory tubular passage; in Thelyphonus only the processes of the 2nd pair of prosomatic appendages play such a part, the pro-sternite not being present (cf. Fig. [97]).

Seeing, then, what a large share the sterno-coxal processes of one or more of these prosomatic appendages plays in the formation of this endostoma, and seeing also that the nerve which supplies the upper lip-muscles in Ammocœtes is the same as that supplying the tentacles which are attached to the upper lip, it appears to me more probable than not that the muscles in question are the vestiges of the sterno-coxal muscles. These muscles differ markedly in their attachments from the muscles of the lower lip, for whereas the latter resemble the tergo-coxal group in their extreme dorsal attachment, the former resemble the sterno-coxal group in their attachment to what corresponds to the endostoma.