DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERIDJE. 113 



porting the vertebral plates. The first ambulacral arch, however, does not rest directly 

 on these, there being between them two intermediate pieces {b, c), which are each con- 

 nected to the corresponding ones of the opposite side by fibrous tissue stretched across. 

 The interambulacral cord either lies below or on a level with the uppermost of these 

 ligaments. I have failed to find nerve-cells in this cord, owing to its extreme delicacy. 



2. Crihella oculata. 



The nervous system of this species is on the whole very distinct, and may be examined 

 with comparative ease; but the great amount of pigment deposited in the integument 

 renders it much less transparent than in Solaster paj)posa. 



The ambulacral cord occupies the same position as in the preceding species, lying 

 between the integument and the inferior ligament*. The ambulacral arches do not pre- 

 sent such well-marked angles l^etween the vertical and oblique portions of their plates {a a) ; 

 the latter are therefore less irregular in form. The triangular space filled up by the 

 superior muscle (d) is well marked; but the fulcrum {ax), upon which the plates move 

 as levers, is not so extensive. The tubercles (c c), giving attachment to the inferior muscle 

 on each side, are prominent. The muscle itseK is thick, and has among its fibres nume- 

 rous fusiform nucleated cells. The water-vessel {b) lies upon it. The vertebral plates 

 rest on two calcareous masses {e,f) on each side, the innermost of which (<?) is rounded, 

 and gives articulation to a double row of long club-shaped spines. The outer mass (/), 

 more or less triangular, bears on its inferior surface two other rounded pieces. Of these, 

 the innermost {e') affords attachment to spines. The edges of the plates are comparatively 

 thick, but with their intervertebral spaces of less width than in the Solaster. Upon the 

 caecal surface rest the vesicles {i i) ; and to the ambulacral aspect are connected the mus- 

 cles of the suckers {k k). The integument [h) covering the nerve-cord is highly coloured 

 by a dark orange-red pigment, with which, when seen in a vertical section, the clear nerve- 

 structure lying immediately above forms a great contrast. The same deej) colour persists 

 in the integumentary covering of the plates and the spines. 



The nerve-cells occupy the same position, and present an appearance and variations in 

 size similar to those in the Solaster. The nerve-filaments resemble in their distribution 

 the corresponding structure in the Solaster. The delicate fibrils radiating from the cen- 

 tral nerve-mass into the integument are here, owing probably to the amount of pigment, 

 remarkably distinct. In some specimens, where the integument happened to be torn, I 

 observed these fibrils terminate in a rounded nucleus or cellule-like bodyf. But I have 

 failed, in this case as tu the other, to satisfy myself as to whether they are or are not 

 connected with nerve-cells. The transparent lines or streaks on the suckers are also very 

 well marked. 



The nerve-cells extend as far as the optic bulb. When the animal is laid on its dorsum 

 and the point of the ray examined, the bulb is found to rest upon calcareous pieces, simi- 

 lar to those in the Solaster, and evidently formed of the coalesced ultimate vertebral arches. 

 They bear a great number of spines, which are much shorter than those of the neighbouring- 

 arches. Indeed the spines here become so suddenly short, that the ray-point seems at 



* Plate XIV. fig. 1. (/. t IWcl. fig. 2. 



VOL. XXIII. Q 



