OF THE AMERICAN KING-CRAB. 473 
doubling up or inflecting and straightening out of the two carapaces, with the pushing- 
purchase exerted by the tail, accomplish both digging and subterranean progression. 
Henee the King-crab is worthy to be called the * Marine Mole” ” *, 
$ 4. Nervous System.—The chief part of the neural axis is in the form of an elliptic ring 
girting the cesophagus. Of this ring three views are given—one from above (Pl. XXXIX. 
fig. 1, a, B), one from below (Pl. XXXVIII. fig. 1, a, 8), and one from the side (Pl. 
XXXVII. fig. 1, a, B), 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 cesophagus (Pls. XXXVII., XXXVIII., 
XXXIX. 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 (Pl. XXXIX.), convex on the oppo- 
site side (Pl. XXXVIIT.), 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. XXXVII. fig. 1, 9). 
There is no trace of lateral bipartition of the superæsophageal or cerebral part of the 
neuralaxis. The substance of the ring shows the same axial or longitudinal extent behind 
as before the cesophagus; viewed from below, as in Plate XXXVIII. 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 cesophagus; but this is transversely; vertically 
the substance there is equal to that of the hind part of the ring (Pl. XXXVII. fig. 1, 3). 
Two commissural bands unite the lateral parts of the'ring (Pl. XXXIX.). The ganglions 
(Pls. XXXVII., XXXVIII., 8, e, C, n, 0) are confined to the thoracetral region. 
Ocellar Nerve, n a—The first pair of nerves is the ‘ocellar’ (Pls. XXXVII., 
XXXVIII, XXXIX.,» o). They rise, with an interval of their own diameter, from the 
fore and upper part of the brain (Pl. XXXIX. a), diverge with a gentle curve as they 
advance, bend round the front convexity of the stomach (Pl. XXXVII. fig. 1, 2 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. XXXIX.), where the special quality of 
an optie 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. XXXVIL), then more abruptly bends outward 
(Pl. XXXIX.), coasting round the apodeme (11) 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. XXX VIII. fig. 2, d) soon expands, and 
resolves itself into a fasciculus of nervules, which subdivide, and finally supply or form 
the retinæ 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 hefore as and 
* The American Naturalist,' 8vo, vol. iv. 1870, p. 257. 
VOL. XXVIII. 38 
