most obvious; enough, however, to establish our point that Apus may have been derived—at least so far as its musculature is concerned-from such an Annelid as we have described. We thus find that the musculature confirms what we learnt from our study of the outer organisation and of the appendages.

SECTION V

THE NERVOUS SYSTEM

THE nervous system of Apus does not at first sight seem to support our theory as obviously as does the musculature. This, however, is the case only at first sight. A closer study of it, and a comparison of it with that of an Annelid modified by having its five anterior segments bent in the way assumed, leave but little doubt concerning its origin. The central nervous system of Apus can in fact be shown to be the central nervous system of a bent Annelid adapted to the necessities of a new manner of life; the principal modification being due to the migration of the eyes on to the dorso-frontal surface.

Figure 17 shows the general type of the nervous system of a carnivorous Annelid, such as the ancestor of Apus may be supposed to have possessed. The longitudinal commissures may perhaps have been somewhat wider apart. We find the brain in the 1 See however P. 80.

prostomium giving off two pairs of nerves to the two pairs of eyes, and connected by œsophageal commissures with the infra-oesophageal ganglion in the first segment. From this ganglion the nerves to the first antennæ diverge; they may perhaps have been united for some distance with the œsophageal commissures. It is even possible that their

FIG. 17.-Diagram of the first five segments of a carnivorous Annelid to show the arrangement of the nervous system, from above. b, brain; eg, anterior pair of eyes on the prostomium; e1, posterior ditto: a1, first antennæ ; a2, antennal parapodium of the second segment.

ganglia may have moved forwards along the commissures towards the brain, as in many Annelids we find the antennæ moved forwards till they appear to be projecting from the posterior edge of the prostomium. The second antennæ, belonging to the second segment, would receive their nerves from the second ventral ganglion, then would follow the nerves to the parapodia of the third segment, &c., in order.

Just as the sharp bending of the head led to a condensation of the ventral musculature into the sinewy mass above described, so it would naturally lead to a fusing of the anterior ventral ganglia, as shown in Fig. 18. We should thus expect to find at

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FIG. 18.-Anterior end of the same, bent as in Fig. 1 to show the change in the central nervous system due to the bending of the body.

least the first three or four pairs of ganglia of the ventral chain fused to form one infra-œsophageal ganglion; the outgoing nerves, however, would remain distinct, except perhaps the first antennal nerve, which, as we have said, might have been fused for a short way with the œsophageal commissures, or might even, as

above stated, come almost direct from the brain.

The

other changes, brought about by the bending of the segments, would be the disappearance of the longitudinal commissures between the four or five fused ganglia, and perhaps a fusion of at least some of their transverse commissures. We shall see in the second part of this book, when we come to compare Limulus and Apus, that the nervous system of the former, though showing certain special modifications of its own, corresponds, to a remarkable degree, with that of such a bent Annelid, and thus shows even a more primitive state than that of Apus.

Now let us consider the modification such a central nervous system would undergo owing to the gradual migration of the eyes on to the dorsal surface. Figs. 19 and 20 are two diagrams to illustrate the change; Fig. 19 supposing the ganglion for the first antenna to come from the infra-oesophageal ganglion, Fig. 20 supposing this ganglion to have already migrated along the commissures to near the brain. The brain, following the eyes, would divide the original œsophageal commissures (a) longitudinally, thus producing two œsophageal commissures, one (a) in its original position, innervating the œsophagus and the upper lip, and the other (a) carrying the brain and the eyes.

This origin of the two œsophageal commissures in Apus is especially interesting because it explains the origin of the sympathetic nervous system in the Crustacea. Reserving, however, this point for the present, we have to consider the more difficult problem relating to the position of the antennal nerves, and how they