Walter Holbrook Gaskell

“ Again, we see that, whereas the cranial nerves, viz. the ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and vagus nerves, are wonderfully fixed and constant in all vertebrates, the only shifting being in the spino-occipital region, in fact, at the junction of the cranial and spinal region, the spinal nerves, on the other hand, are not only remarkably variable in number in different groups of animals, but that even in the same animal great variations are found, especially in the manner of formation of the limb-plexuses. ”

“ We see how, in Ammocœtes, the first formed parts of the skeleton are the branchial bars and the basi-cranial system, while the rudiments of the vertebræ do not appear until the Petromyzon stage. We see how, with the elongation of the animal by the later addition of more and more spinal segments, organs, such as the heart, which were originally in the head, travel down, and the vagus and lateral-line nerves reach their ultimate destination. ”

“ Comparison of the Cranial and Spinal Segmental Nerves.
It has often been held that the arrangements of the vertebrate nervous system differ from those of other segmented animals in one important particular. The characteristic of the vertebrate is the origin of every segmental nerve from two roots, of which one contains the efferent fibres, while the other possesses a sensory ganglion, and contains only afferent fibres. ”

“ On the other hand, the cranial somatic muscles are the eye-muscles, the formation of which resembles that of the visceral muscles, and not of the spinal somatic. Their direction is not longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to the somatic trunk-muscles, and must, therefore, form a separate group to themselves. Thus the striated musculature of the Ammocœtes must be divided into (1) the visceral muscles; (2) the longitudinal somatic muscles; and (3) the dorso-ventral somatic muscles. ”

“ Each of these appendages possesses its own mixed nerve, glossopharyngeal or vagus, its own cartilage, its own set of visceral muscles, its own sense-organs, just as do the respiratory appendages of Limulus.
The branchial unit in the vertebrate is not the gill-pouch, but the branchial bar or appendage between the pouches. Embryology shows how each such appendage grows inwards, how a cœlomic cavity is formed in it, similarly to the ingrowing of the branchial appendage in scorpions. ”

“ Similarly to the cranial region of the vertebrate, the nerves which supply the somatic segments arise from separate sensory and motor roots, while the single nerve which supplies each appendage contains all the fibres for the appendage, both motor and sensory. ”

“ Not only does the vertebrate heart differ from that of the invertebrate, in that it is branchial while the latter is systemic, but also it is unique in its mode of formation in the embryo. In the Appendiculata the heart is formed as a single organ in the mid-dorsal line by the growth of the two lateral plates of mesoblast dorsalwards, the heart being formed where they meet. In Mammalia and Aves, the heart and ventral aorta commence as a pair of longitudinal veins, one on each side of the commencing notochord. ”

“ From the walls of this cavity the branchial muscles and cartilaginous bar are formed.
Now, from an embryological point of view, the vertebrate shows that it is a segmented animal by the formation of somites, which consist of a series of divisions of the cœlom, of which the walls form a series of muscular and skeletal segments. In the head-region, as already mentioned, such cœlomic divisions form two rows—a dorsal and a ventral set. From the walls of the dorsal set the somatic musculature is formed. From those of the ventral set the branchial musculature. ”

“ The evidence of embryology points directly to the greater phylogenetic antiquity of the cranial region, for we see how, quite early in the development, the head is folded off, and the organs in that region thereby completed at a time when the spinal region is only at an early stage of development. We see how the first of the trunk somites is formed just posteriorly to the head region, and then more and more somites are formed by the addition of fresh segments posteriorly to the one first formed. ”

“ In the cranial region there is evidence of a double set of segments, which may be called somatic and splanchnic. The somatic segments, consisting of the outer skin and the body musculature, are doubly innervated as are those of the spinal cord by a series of ventral motor roots, the oculomotor or IIIrd nerve, the trochlear or IVth nerve, the abducens or VIth nerve, and the hypoglossal or XIIth nerve, and by a series of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve. ”

“ From the mesoblastic tissue, between each gill-pouch, there is formed a supporting cartilaginous bar, to which are attached a system of branchial muscles, with their nerves and blood-vessels. These cartilaginous bars, in all fishes above the Cyclostomata, form a supporting framework for the internal gill-slit, so that the gills are situated externally to them; the more primitive arrangement is, as already mentioned, a system of cartilaginous bars, extra-branchial in position, so that the gills are situated internally to them. ”

“ We do not know how the palæostracan sea-scorpions breathed; they resemble the scorpion of the present day somewhat in form, but they are in many respects closely allied to Limulus. The present-day scorpion is a land animal, and the muscles by which he breathes are dorso-ventral somatic muscles, while those of Limulus are the appendage-muscles. ”

“ The somatic muscles innervated by the spinal set of nerves, including in this term the spino-occipital or so-called hypoglossal nerves, are in Ammocœtes most sharply defined from all the other muscles of the body. They form the great dorsal and ventral longitudinal body-muscles, which extend dorsally as far forward as the nose and are developed embryologically quite distinctly from the others, being formed as muscle-plates (Kästchen) . ”

“ Finally, it has been shown that the segments of which a vertebrate is composed are recognizable in the embryo by the segmented manner in which the musculature is laid down, and van Wijhe has shown that in the cranial region two sets of muscles are laid down segmentally, thus forming a dorsal and ventral series of commencing muscular segments. ”

“ Each possesses its own afferent branchial blood-vessel from the ventral aorta, and its own efferent vessel to the dorsal aorta (Fig. 65, a. br. and v. br.) . Each possesses its own segmental nerve, which supplies its own branchial muscles and no others with motor fibres, and sends sensory fibres to the general surface of each appendage, as also to the special sense-organs in the shape of the epithelial pits (S., Fig. 65) arranged along the free edges of the diaphragms; each of these nerves possesses its own ganglion—the epibranchial ganglion.
Fig. 63.—Ventral half of Head-region of Ammocœtes. ”

“ Nowhere else in the whole body of Ammocœtes is such a column of fat to be found. It is not skeletogenous tissue with cells of the nature of cartilage-cells, as Gegenbaur thought and as Balfour has depicted (Vol. II., Fig. 315) in his 'Comparative Embryology,' as though this tissue were a part of the vertebral column, but is simply fat-cells, such as might easily have taken the place of some other previously existing organ. ”

“ As already pointed out, the brain-region of the vertebrate is comparable to the supra-œsophageal and infra-œsophageal ganglia of the invertebrate, and it has been shown (p. 54) how, by a process of concentration and cephalization, the foremost region of the infra-œsophageal ganglia becomes the prosomatic region, and is directly comparable to the trigeminal region in the vertebrate; while the hindermost region is formed from the concentration of the mesosomatic ganglia, and is directly comparable to the medulla oblongata, i.e. to the vagus region of the vertebrate brain. ”

“ Nestler then goes on to describe the arrangement in Ammocœtes in the same terms, although there is no bronchus or gill-pouch, but only an open chamber into which these gill-bearing diaphragms project, which open chamber serves both for the passage of food and of the water for respiration. This is manifestly the wrong way to look at the matter: the adult form is derived from the larval, not vice versâ, and the transformation process shows exactly how the gills, in Rathke's sense, come together to form the bronchus and so make the gill-pouches of Petromyzon. ”

“ The surface ornamentation is represented on the first segment posterior to the branchial segments. The opercular appendage is marked out by dots. Let us consider dispassionately whether such a suggestion is a priori so impossible as it at first appears. One of the principles of evolution is that any change which is supposed to have taken place in the process of formation of one animal or group of animals from a lower group must be in harmony with changes which are known to have occurred in that lower group. ”

“ Unfortunately, there is at present no likelihood of any corresponding transitional stage being discovered living on the earth in which both the dorsal arthropod alimentary canal and the ventral vertebrate one should simultaneously exist in a functional condition ”

“ In its structure there is not the slightest sign of anything of the nature of a heart; it is a solid mass of closely compacted cells, and the cells are all very full of fat, staining intensely black with osmic acid. ”

“ 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 ”

“ Although the evidence of such appendages has entirely disappeared in the higher vertebrates, together with the disappearance of branchiæ, and is not strikingly apparent in the higher gill-bearing fishes, yet in Ammocœtes, so great is the difference here from all other fishes, it is natural to describe the pharyngeal or respiratory chamber as a chamber into which a symmetrical series of respiratory appendages, the so-called diaphragms, are dependent. ”

“ Indeed, markings have been found on the internal side of the segments 1-5, Fig. 62, which are supposed to indicate branchiæ, and these segments are therefore supposed to have borne the branchiæ. Up to the present time no indication of gill-slits has been found, and we cannot say with certainty how these animals breathed. Further, in the Upper Silurian of Lesmahago, Lanarkshire, a scorpion (Palæophonus Hunteri) , closely resembling the modern scorpion, has been found, which, as Lankester states, was in all probability aquatic, and not terrestrial in its habits. How it breathed is unknown ”

“ This secondary area of proliferation coincides with the area of the primitive streak, and he has shown, in a subsequent paper, by means of the insertion of sable hairs into the unincubated blastoderm of the chick, that a hair inserted into the centre of the blastoderm appears at the anterior end of the primitive streak, and subsequently is found at the level of the most anterior pair of somites. ”

“ Of these, the constrictor muscle (Fig. 71, m. con. str.) is in close contact with its cartilaginous bar, while the adductor (Fig. 71, m. add.) is attached to the cartilage only at its origin and insertion, and the tubular muscles (Fig. 71, m. con. tub.) have nothing whatever to do with the cartilage at all, being attached ventrally to the connective tissue in the neighbourhood of the ventral aorta (V.A.) , and dorsally to the mid-line between the dorsal aorta (D.A.) and the notochord. ”

“ These muscular bands, to which Lankester and Benham have given the name of 'veno-pericardial muscles,' are so different in appearance from the rest of the muscular substance, that Milne-Edwards did not recognize them as muscular, but called them 'brides transparentes.' Blanchard speaks of them in the scorpion as 'ligaments contractiles,' and considers that they play an important part in assisting the pulmonary circulation ”

“ Branchiæ considered as internal branchial appendages.—Innervation of branchial segments.—Cranial region older than spinal.—Three-root system of cranial nerves, dorsal, lateral, ventral.—Explanation of van Wijhe's segments.—Lateral mixed root is appendage-nerve of invertebrate.—The branchial chamber of Ammocœtes.—The branchial unit, not a pouch but an appendage.—The origin of the branchial musculature.—The branchial circulation.—The branchial heart of the vertebrate.—Not homologous with the systemic heart of the arthropod.—Its formation from two longitudinal venous sinuses.—Summary. ”