OF THE AMERICAN KING-CRAB. 477 
The second ventral pair (» xrv, ib.) is chiefly distributed to the fibres of the flexor 
muscles of the tail-spine, arising from and occupying the soft, rather tumid tract, which 
resembles a leaf-foot soldered down to form the covering of the hindmost part of the 
ventral surface of the thoracetron. i 
The third pair of principal nerves from the terminal ganglion represents a bifid con- 
tinuation of the neural axis (Pl. XXXVIII., /). After a course of about three lines, 
each sends off a nerve (n xv, Pl. XXXVIII.) belonging to the ventral series, which 
supplies the hindmost or postanal region of the abdomen affording the articular surface 
for the tail-spine. 
After sending off the above nerve, each continuation of the chord forms an oblong loop 
(ib. x), which, prior to the removal of the vascular sheath, looks like a ganglionic swell- 
ing*; beyond which the chord (A, Pls. XXXVII., XXXVÍIL. continues along the side 
of the tail-joint, and, on entering the cavity of the tail-spine (0), resolves itself into a 
fasciculus of fine nerves (ib. pl), resembling the ‘cauda equina” of anthropotomy. But 
in this bundle a principal filament, or continuation of the chord (Pl. XXXVII. a), can 
be traced about a third of the way down the spine. "These nerves seem to constitute the 
major part of the tissues in the hollow of the spine, and render a marvellous supply of 
neurine to so hard, inflexible, and seemingly insensible a part. 
Each chord, A, from the ganglionie loop sends off nine nerves, four directed toward 
the ventral (Pl. XX XVIII. fig. 1, pl), four towards the dorsal (Pl. XXXVII. fig. 1, a 1-4) 
region of the spine: the ninth nerve being of larger size, claims to be the continuation 
of the bifid neural axis. If the dorsal and ventral divisions be regarded as those of four 
nerves serially homologous with such divisions of antecedent primary pairs, they would 
indicate as many segments coalesced in the fore part of the spine. The ninth nerye and 
its divisions supply in a similar way the rest of the tail-spine. 
Are the phenomena of this final part of the nervous system of Limulus devoid of homo- 
logical significance? It seems to be otherwise, for any thing that one can see needing 
such supply of nerves in the interior of the hollow spine. All, however, that embryology 
has yet shown of the development of this part is, that in the interval between exclusion 
and the first moult it buds out of the posterior part of the * thoracetron,’ does not shrink 
up to it, and only feeble or doubtful traces of a segmentation have been noticed in the 
embryonal but late-growing *pleon. Nerve precedes crust in blastemal differentiation : 
the earlier tissue obeys the type, the later tissue the adaptive departure therefrom. A 
superficial glance catches the result as a ‘spinous process ;’ deeper insight discerns the 
body-joints masked by the outer connation. Neither development, rightly understood, 
nor adult structure gives any countenance to the notion that the tail-spine of Limulus 
is a mere process or appendage growing from the dorsal part only of the terminal segment 
of the thoracetron. 
Seeing the relations of the pleonal nerves as continuators of the neural axis, and the 
like relation of the artery of the spine to the dorsal vessel, I long ago concluded the spine 
itself to be a continuation of the series of body-segments, to be serially homologous 
therewith, and not with their ‘appendages.’ The coccygeal style of the frog’s endo- 
* As represented by Van der Hoeven, op. cit. pl. iii. fig. 2c. 
