192 BRITISH FOSSIL CRUSTACEA. 



inflecting, and straightening out of the two carapaces with the pushing-purchase exerted 

 by the tail-spine, accomplish both digging and subterranean progression. Hence the 

 King-Crab is worthy to be called the ' Marine Mole.' 1 



§ 2. "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. XXXVI, 

 fig. 1, a b), one from below (PI. XXXV, fig. 1, a /3), and one from the side (PI. XXXIV, 

 fig. 1, a j8) ; 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 

 determinations have weight, as will 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. XXXIV, XXXV, and 

 XXXVI, a), and which, were the tube straightened and the mouth brought to its 

 ordinary position at the forepart of the body, would be superior or dorsal in position, is 

 an oblong mass, concave where applied to the tube (PI. XXXVI), convex on the opposite 

 side (PI. XXXV), 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 XXXIV, fig. 1, /3). There is 

 no trace of lateral bipartition of the supercesophageal 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 PI. XXXV, fig. 1, it seems to extend rather 

 further 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 (PL XXXIV, fig 1, (5). 

 Two commissural bands unite the lateral parts of the ring PI. XXXVI). The ganglions 

 (Pis. XXXIV, and XXXV, 3, £, l, v, 9) are confined to the thoraeetral region. 



" Ocellar Nerve, n a. — The first pair of nerves is the • ocellar ' (Pis. XXXIV, 

 XXXV, and XXXVI, n a). They rise, with an interval of their own diameter, from 

 the fore and upper part of the brain (PI. XXXVI a), diverge with a gentle curve as they 

 advance, bend round the front convexity of the stomach (PI. XXXIV, fig. 1, n a), and 

 ascend, converging to terminate each in its ocellus (id., a, I). 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 XXXVI), 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 upwards, crosses the gizzard near the pylorus (PI. XXXIV), then more abruptly 

 bends outward (PL XXXVI), coasting round the apodeme (ii) of the second limb, and 

 retrograding obliquely to the compound eye, near which the nerve divides into a larger 



1 'The "Horse-Foot Crab," 'Limulus pohjphemus,' by the Rev. S. Lockwood, Ph.D., the American 

 Naturalist, vol. iv, 1870, p. 257. 



