15 



§ 4. Nervous System. — The chief part of the neural axis is in the form of an elliptic ring 

 girting the oesophagus. Of this ring three views are given — one from above (PI. V. 

 fig. 1, a, b), one from below (PI. IV. fig. 1, a, &), and one from the side (PI. II A. 

 fig. 1, a, /3), in order to show, besides the shape of the part itself, the precise position of 

 the nerves arising therefrom or connected therewith. For the origin of a nerve is an 

 Important element in determining the homology of the part it supplies ; and such de- 

 terminations have weight, as v>'\\\ be seen, in wider questions, extending, in the case of 

 Limulus, e. g., even to class-affinity. 



The part of the neural axis anterior to the oesophagus (Pis. II., III., V. a), and which, 

 were the tube straightened and the mouth brought to its ordinary position at the fore 

 part of the body, would be superior or dorsal in position, is an oblong mass, concave 

 where applied to the tube (PI. V.), convex on the opposite side (PI. III.), 3 lines in 

 length, and 4 lines in posterior or basal breadth, where its angles are continued into 

 the side parts of the ring (PL II a. fig. 1, j3). There is no trace of lateral bipartition 

 of the superoesophageal or cerebral part of the neural axis. The substance of the ring 

 shows the same axial or longitudinal extent behind as before the oesophagus ; viewed 

 from below, as in Plate III. fig. 1, it seems to extend rather fm*ther before contracting 

 to form the ganglionic chord. The narrowest parts of the ring are at the sides of the 

 oesophagus ; but this is transversely ; vertically the substance there is equal to that of 

 the hind part of the ring (PI. II a. fig. 1, /3). Two commissural bands unite the lateral 

 parts of the ring (PI. V.). The ganglions (Pis. II a., IV., 8, e, I, jj, 0) are confined to the 

 thoracetral region. 



Ocellar Nerve, n a. — The first pair of nerves is the ' oceUar ' (Pis. II., II a., III., IV., 

 v., n a). They rise, with an interval of their own diameter, from the fore and upper part 

 of the brain (PL V. a), diverge with a gentle curve as they advance, bend round the front 

 convexity of the stomach (PL II a. fig. 1, n a), and ascend, converging to terminate each 

 in its ocellus (ib. a 1). The length of the nerve is two inches. 



Ocular Nerve, n a. — The second pair of nerves is the ' ocular ' (ib. n a). Each nerve 

 rises from a small conical process of the brain (PL V.), where the special quality of an 

 optic ganglion may be surmised to dwell. An interval of the basal breadth of the 

 swelling divides the ocular from the ocellar nerve. The ocular nerve curves upward, 

 crosses the gizzard near the pylorus (PL II.), then more abruptly bends outward (PL V.), 

 coasting round the apodeme (ii) of the second limb, and retrograding obliquely to the 

 compound eye, near which the nerve divides into a larger dorsal and smaller ventral 

 chord. The dorsal division (PL IV. fig. 2, d) soon expands, and resolves itself into a 

 fasciculus of nervules, which subdivide, and finally supply or form the retinse of the 

 lenses at the upper and hinder part of the compound eye. The ventral, which is also the 

 anterior division (ib. v), is continued further before expanding and resolving into the 

 plexus which supplies the retinae to the lenses at the lower and fore part of the com- 

 pound eye. 



Gastric Nerves. — Two pairs of nerves arise posterior to the ocular pair. The nerves of 

 the first pair {n 3, PL V.) pass forward, give filaments to the oesophagus and stomach, and 

 are continued on into the hepatic and ovarian substance at the fore part of the carapace. 



