The Origin of Vertebrates

by Walter Holbrook Gaskell

In Lankester's words, one of the characteristics of the Osteostraci (Cephalaspis, Auchenaspis, etc.), as distinguished from the Heterostraci (Pteraspis), are the large orbits placed near the centre of the shield. The apparent exception of Thyestes mentioned by him is no exception, for orbits of the same character have since been discovered, as is seen in Rohon's figure (Fig. [14]). In Fig. [110], C, I give an outline of the frontal part of the head-shield of a Cephalaspid, in which I have drawn the eye-muscles as in the other two figures.

Although all the members of the Osteostraci possess large lateral eyes towards the centre of the head-shield, the other group of ancient fishes, the Heterostraci, are characterized by the presence of lateral eyes far apart, situated on the margin of the head-shield on each side (cf. Fig. 142, o, p. [350]).

So, also, on the invertebrate side, the lateral eyes of Pterygotus and Slimonia are situated on the margin of the prosomatic carapace, while those of Eurypterus and Stylonurus are situated much nearer the middle line of the prosomatic carapace.

Next comes the question of the superior oblique muscle and the trochlearis nerve. Why does this nerve (n.IV. in Fig. [106], C and D) alone of all the nerves in the body take the peculiar position it always does take? The only suggestion that I know of which sounds reasonable and worth consideration is that put forward by Fürbringer, which is an elaboration of the original suggestion of Hoffmann. Hoffmann suggested in 1889 that the trochlearis nerve represented originally a nerve for a protecting organ of the pineal eye, which became secondarily a motor nerve for the lateral eye as the pineal eye degenerated. Fürbringer differs from Hoffmann in that he considers that the nerve was originally a motor nerve, and was not transformed from sensory to motor, yet thinks Hoffmann's suggestion is in the right direction.

He points out that the crossing of the trochlearis is not a crossing of fibres between two centres in the central nervous system, but may be explained by the shifting of the peripheral organ, i.e. the muscle, from one side to the other, and the nerve following this shift. Consequently, says Fürbringer, the course of the nerve indicates the original position of the muscle, and therefore he imagines that the ancestor of the superior oblique muscle was a muscle the fibres of which were attached in the mid-dorsal line, and interlaced with those of the other side, the two muscles thus forming an arch through which the nervous system with its central canal passed. Then, for the sake of getting a more efficient pull, the crossing muscle-fibres became more definitely attached to the opposite side of the middle line, and finally obtained a new attachment on the opposite side, with the obliteration of the muscular arch; the nerve on each side, following the shifts of the muscle, naturally took up the position of the original muscular arch, and so formed the trochlear nerve, with its dorsal crossing. This explanation of Fürbringer's was associated by him with movements of the median pineal eyes, the length of their nerve, according to him, even yet indicating their previous mobility. This assumption is not, it seems to me, necessary. The length of the nerve is certainly no indication of mobility, for in Limulus and the scorpion group the nerve to each median eye is remarkably long, yet these eyes are immovably fixed in the carapace. All that is required is a pair of dorso-ventral muscles belonging to the segment immediately following the group of segments represented by the oculomotor nerves, the fibres of which should cross the mid-dorsal line at their attachment; for, seeing that the lateral eyes were originally so near this position, it follows that such muscles might form part of the muscular group belonging to the lateral eye without having previously moved the pineal eyes. In fact, Fürbringer's explanation requires as starting-point that the pair of muscles which ultimately become the superior oblique should have the exact position of the pair of dorso-ventral muscles in the scorpion, called by Miss Beck the anterior dorso-plastron muscles (63), which I have named the oblique muscles. Here, and here only, do we find an interlacement, across the mid-dorsal line, of the fibres of attachment of the muscles on the two sides, in consequence of which this pair of muscles is described by her as forming an arch encircling the alimentary canal and dorsal vessel. If, then, as I have previously argued, the primitive plastron formed a pair of trabeculæ, and the nervous system grew round the alimentary canal, such an arch would encircle the tubular central nervous system of the vertebrate.

Still more striking is this pair of muscles (63) in Phrynus (Fig. 108), where we see how the arch formed by them almost touches the posterior extremity of the supra-œsophageal brain-mass, crossing, therefore, over the beginning of the stomach region of the animal. The angle formed by the arch is much more obtuse than that formed in Scorpio, so that an actual crossing of the muscle-fibres has taken place at the point of attachment to the carapace. Also, only the part nearest the carapace is muscular, the rest forming a long tendinous prolongation of the plastron wall (the primordial cranium), as seen in the figure.

This muscle-pair is, as it should be, the pair of dorso-ventral muscles belonging to the segment immediately following on the group of segments represented by the recti muscles, i.e. according to previous argument, the segment belonging to the sixth pair of locomotor appendages or ectognaths; a muscle, therefore, which would arise in the vertebrate from the mandibular, and not from the premandibular cavity. A similar muscle probably existed in Eurypterus (M.obl. in Fig. [106], B), and, as in the case of the formation of the oculomotor group, derived from the recti group of the scorpion, would form the commencement of the superior oblique muscle in Thyestes and Tremataspis.

Fig. 112.—A, Diagram of Position of Oblique Muscle in Scorpion; B, Diagram of Transition Stage; C, Diagram of Superior Oblique Muscle in Vertebrate.

l.e., lateral eyes; c.e., central eyes; C.N., central nervous system; Al., alimentary canal; c., aqueductus Sylvii.