The Origin of Vertebrates

by Walter Holbrook Gaskell

CHAPTER IX

THE PROSOMATIC SEGMENTS OF AMMOCŒTES

The prosomatic region in Ammocœtes.—The suctorial apparatus of the adult Petromyzon.—Its origin in Ammocœtes.—Its derivation from appendages.—The segment of the lower lip or metastomal segment.—The tentacular segments.—The tubular muscles.—Their segmental arrangement.—Their peculiar innervation.—Their correspondence with the system of veno-pericardial muscles in Limulus.—The old mouth or palæostoma.—The pituitary gland.—Its comparison with the coxal gland of Limulus.—Summary.

In the last chapter it was seen not to be incompatible with both the anatomical and morphological evidence to look upon the trigeminal nerves as having originally supplied the seven prosomatic pairs of appendages of the invertebrate ancestor, the foremost of which, the cheliceræ, and the four pairs of endognaths dwindled away and became insignificant, leaving as trace of their former presence the descending root of the Vth nerve; while the two hindmost pairs, the ectognaths and the chilaria, or metastoma, remained vigorous and developed, leaving as proof of their presence the nucleus masticatorius. 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. Not possessing jaws, they feed by suction on the living animal, a method of feeding which gives them no more claim to be classed as parasitic animals than the whole group of spiders which feed in a similar manner on living flies.

The Origin of the Suctorial Apparatus of Petromyzon.

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.

Turn now to the full-grown larval form, Ammocœtes, an animal in the case of Petromyzon Planeri as large as the full-grown Petromyzon, and seek for this musculature. There is, apparently, no sign of it, no suctorial apparatus whatever, only, as already mentioned, an oral chamber bounded by the lower and upper lips and the remains of the septum between it and the respiratory chamber—the velar folds. Attached to its walls a number of tentacles are situated, which form a fringe around and within the mouth. Most extraordinary is the contrast here between the larval and the adult stages; in the former, no sign of the suctorial apparatus, but simply tentacles and velar folds; in the latter, no sign of tentacles or of velar folds, but a massive suctorial apparatus.

In order, then, to understand the origin of the muscles of mastication, it is necessary to study the changes which occur at transformation, and thus to find out how the suctorial apparatus of the adult arises. This most important investigation has been undertaken by Miss Alcock, and owing to the kindness of Mr. Millington, of Thetford, we have been able to obtain a better series in the transformation process than has ever been obtained before. Miss Alcock has not yet published her researches, but has allowed me to make use of some of her facts.

An enormous proliferation of muscular tissue takes place with great rapidity during this transformation, which causes the disappearance of the tentacles, and gives origin to the suctorial apparatus. The starting point of this proliferation can be traced back in all cases to little groups of embryonic tissue found below the epithelial lining of the oral chamber in Ammocœtes. Of these groups the most conspicuous one is situated at the base of the large median ventral tentacles. Others are situated at the base of the tentacular ridge. 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.

The clue, then, to the origin of the suctorial apparatus and of the nature of the original organs supplied by the trigeminal is afforded in this case, as in all other similar inquiries, by the central nervous system and its outgoing nerves. Here is always the citadel, the fixed seat of government, here is 'headquarters,' from which the answers to all our inquiries must originate.