19 



After sending off the above nerve, each continuation of the chord forms an oblong loop 

 (ib. k), which, prior to the removal of the vascular sheath, looks like a ganglionic swell- 

 ing* ; beyond which the chord (X, Pis. II a., IV.) continues along the side of the tail- 

 joint, and, on entering the cavity of the tail-spine (c), resolves itself into a fasciculus 

 of fine nerves (ih. ^;^), resembling the ' cauda equina ' of anthropotomy. But in this 

 bundle a principal filament, or continuation of the chord (PI. II a. ^i), can be traced 

 about a thu'd of the way doAvn the spine. These nerves seem to constitute the major 

 part of the tissues in the hollow of the spiae, and render a marvellous supply of neurine 

 to so hard, inflexible, and seemingly insensible a part. 



Each chord, A, from the ganglionic loop sends off nine nerves, four directed toward 

 the ventral (PI. IV. fig. 1, j^l), fom- towards the dorsal (PL II A. 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 nerve and its divi- 

 sions supply in a similar way the I'est of the taU-spine. 



Are the phenomena of this terminal part of the nervous system of Limulus devoid of ho- 

 mological 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. AU, 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- 

 skeleton t is analogous to the taU-spine of the King-crab's exoskeleton. The antecedent 

 part of the thoracetron (b") wherewith the spine is articulated has no limbs. Is it also 

 part of the pleon ? and does the postganglionic part of the neural axis indicate the 

 extent of such part ? 



In anatomizing, in 1843, my first-received specimens of Limulus, the details of the 

 nervous system were followed out by my then anatomical assistant, Mr. Heniy Goadby, 

 and well exemplify his peculiar skiU. and patience. 



The nearest approaches to the type of nervous system above described we found to be, 



* As represented by Van der Hoeven, op. cit. pi. iii. fig. 2 c. 

 t Owen, ' Anatomy of Vertebrates,' vol. i. p. 49, fig. 44 c. 



D 2 



