Walter Holbrook Gaskell

“ Throughout the branchial region the striated muscles of each segment are strictly supplied by the nerve of that segment, and, as already described, each segment is as carefully mapped out in its innervation as it is in any arthropod appendage. One exception occurs to this orderly, symmetrical arrangement: a nerve arises in connection with the facial nerve, and passes tailwards throughout the whole of the branchial region, giving off a branch to each segment as it passes. ”

“ 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. ”

“ From this homology two conclusions of importance follow: (1) the old mouth, or palæostoma, of the vertebrate was situated at the end of this tube, therefore, at the termination of the infundibulum; (2) the upper lip, which by its growth, brings the olfactory tube from a ventral to a dorsal position, was originally formed by the foremost sternites or endostoma, or else by the sterno-coxal processes of the second pair of prosomatic appendages of the palæostracan ancestor. ”

“ Evidence in favour of this suggestion and of the nature of the dwindling process is afforded when we examine what the trigeminus does supply in Ammocœtes. In all vertebrates this nerve supplies the great muscles of mastication which, in all gnathostomatous fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws; they take in their food by attaching themselves to their prey and by means of rasping teeth situated in serried rows within the circular mouth, combined with a powerful suctorial apparatus, they suck the juices of the fish they feed upon. ”

“ The tongue-nerve supplies in Ammocœtes the rudimentary muscles which pass laterally from the base of the large ventral tentacle to the wall of the throat, and even in Ammocœtes must possess some power of moving that tentacle.
At transformation these muscles proliferate and develop enormously, and form the bulk of the large basilar muscle which surrounds the throat ventrally and laterally, and is the most bulky muscle in the suctorial apparatus. ”

“ Whatever was the condition of the digestive tracts at the transition stage between the arthropod and the vertebrate, the original mouth-opening at the base of the olfactory tube was ultimately closed. The method of its closure was exceedingly simple and evident. The membranous cranium was in process of formation by the extension of the plastron laterally and dorsally; a slight growth of the same tissue in the region of the mouth would suffice to close it and thus separate the infundibulum from the olfactory tube. ”

“ Further, although this extraordinary change takes place in the peripheral organ, no marked difference occurs in the arrangement of the nerves issuing from the trigeminal motor centre, no new nerves are formed to supply the new muscles, but every motor nerve-fibre and the motor cell from which it arises increases enormously in size, and these giant nerve-fibres thus formed split into innumerable filaments corresponding with the proliferation of the muscular elements. ”

“ Then again, although at present there is no evidence to connect the pericardial nerve in Limulus with this veno-pericardial system of muscles, yet it is extraordinarily significant that in such animals as Limulus and Ammocœtes, in both of which the mesosomatic or respiratory region is so markedly segmental, an intrusive nerve should, in each case, extend through the whole region, giving off branches to each segment. ”

“ 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. ”

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

“ It is the only non-tubular muscle belonging to the velum, and by its contraction it draws the anterior portions of the velar folds apart from each other, and so opens the slit between them, through which the food and mud must pass. ”

“ This powerful suctorial apparatus is innervated entirely by the trigeminal nerve, so that here in its muscular arrangements any original segmental arrangement of the muscles of mastication might be expected to be visible. It consists of a large rod or piston, to which are attached powerful longitudinal muscles; a large muscle, the basilar muscle, which assists the piston in producing a vacuum, and annular muscles around the circular lip. ”

“ Passing to the segment in front, Fig. 116 shows that the group of visceral muscles (m2) corresponds in relative position with respect to the metastomal bar to the hyoid muscles with respect to the opercular bar or to the branchial visceral muscles with respect to each branchial bar. What, then, is this muscular group? The series of sections show that these are the dorso-ventral muscles belonging to the lower lip, which, as seen in Fig. 119 (M.) , form a well-marked muscular sheet, whose fibres interlace across the mid-ventral line of the lower lip. ”

“ The study of Ammocœtes indicates that the velum and lower lip correspond to the metastoma of the Eurypterid, i.e. the chilaria of Limulus, while the large ventral pair of tentacles, called the tongue, correspond to the ectognaths of the Eurypterids, and probably to the oar-like appendages of Ptericthys and Bothriolepis. ”

“ These tentacles are the last remnants of the original foremost prosomatic appendages of the palæostracan ancestor. Like the lateral eyes they do not develop until the transformation comes, but during the whole larval condition their musculature remains in an embryonic condition, and then from these embryonic muscles the whole massive musculature of the suctorial apparatus develops; a sucking apparatus derived from the modification of appendages, as so frequently occurs in the arthropods. ”

“ The analysis of these tentacular segments belonging to the trigeminal nerve presents greater difficulties than that of any of the other cranial segments, owing to the deficiency of our knowledge of what occurs at transformation. Light is required not only on the origin of the new muscles but also on the origin of the new and elaborate cartilages which are newly formed at this time. ”

“ These segmental tubular muscles are found also in the velar folds—the remains of the septum or velum which originally separated the oral from the respiratory chamber. In the branchial region they act with the other constrictors as expiratory muscles, forcing the water out of the respiratory chamber. In the living Ammocœtes, the velar folds on each side can be seen to move synchronously with the movements of respiration, contracting at each expiration ”

“ Miss Alcock has not yet worked out the origin of all these cartilages and muscles, so that we are not yet in a position to analyze the trigeminal supply in Petromyzon into its component appendage elements, an analysis which ought ultimately to enable us to determine from which appendage-muscles the masticating muscles in the higher vertebrates have arisen. ”

“ The Trigeminal Nerve of Ammocœtes.
Striking is the answer. In Fig. 114, Miss Alcock has drawn the distribution of the trigeminal nerve as traced by her through a series of sections. It arises, as is well known, from two separate ganglia, of which the foremost gives rise to a purely cutaneous nerve, the ophthalmic nerve, and the hindmost to three nerves, the most posterior of which is purely cutaneous and passes tailwards over the ventral branchial region, as shown in the figure ”

“ Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system. Br. cart., branchial cartilage; M. con. str., striated constrictor muscles; M. con. tub., tubular constrictor muscles; M. add., adductor muscle; D.A., dorsal aorta; V.A., ventral aorta; S., sense-organs on diaphragm; n. Lat., lateral line nerve; X., epibranchial ganglia of vagus ”

“ For the present I will confine myself to the consideration of this muco-cartilaginous skeleton as evidence of the relationship of Ammocœtes to the Eurypterids, and in the next chapter will show how absolutely the same skeleton corresponds to that of the Cephalaspidæ, so that Ammocœtes is really a slightly modified Cephalaspid, the larval form of which was Eurypterid in character. ”

“ 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. ”

“ Its presence shows that the velar folds, formed as they were by the breaking down of the septum, are in reality part of two segments, viz. the opercular and the metastomal, which have fused together in their basal parts, and by such fusion have caused the inter-relationship between the VIIth and Vth nerves, so apparent in the anatomy of the vertebrate cranial nerves. ”

“ Further, these muscles run along the length of the velar fold, and are attached ventrally on each side of the thyroid gland, so that their ventral attachment also corresponds in position, as regards the hyoid segment, with the ventral attachment of the rest of the tubular muscles as regards their respective segments. ”