THE DEVELOPMENT OF ASTERINA GIBBOSA. 411 
taken. xerv. Nervous tissue under the sensory epithelium at the tip. muse. 
Muscular tails of hydroccele epithelial cells. 
PLATE 29. 
Figs. 151—154 show the development of the transverse muscles, which 
extend from one ambulacral ossicle to its fellow of the opposite side. 
Figs, 151 and 152.—Two sections perpendicular to the disc from a star- 
fish, in which R equals ‘4 millimetre. sang. circ. Oral “blood” ring. 
muse. amb. Ambulacral muscles ; the reference line (in Fig. 151) is too 
long. ph. Perihemal space; the reference line (in Fig. 151) is too 
short. Magnified 350 diameters. 
Fig. 153. Similar section from star-fish in which R equals °63 milli- 
metre. 
Fig. 154. Similar section from star-fish of the same size as the preceding, 
but more advanced in the development. 
Fic. 155.—Transverse section of the radial nerve-cord of a young Asterias, 
to show the feeble development of ccelomic nervous system. 
Fig. 156.—Similar section of nerve-cord of an Ophiurid, to show the great 
ganglia of the cceelomic nervous system. 
Fig. 157.—Diagram of the hypothetical ancestor of Asterids and Crinoids. 
The hydroccele is a paired structure. 
Fig. 158.—Diagram of a stage in the evolution of Asterids from this an- 
cestor. Notice the growth of both left hydroccele and left posterior ceelom 
to form rings. The hydroccele encircles the base of the stalk. This drawing 
does not properly represent the oblique position which the dise acquires in 
reference to the stalk. The mouth ought to be half turned towards the 
observer. 
Fic. 159.—Diagram of stage in evolution of Crinoids. Notice that the 
hydroceele is carried entirely away from the stalk. 
These last two diagrams are only hypothetical, in so far as they represent 
as co-existing structures which succeed one another in ontogeny ; otherwise 
they represent the actual fixed stage in both Asterid and Crinoid ontogeny. 
VOL, 38, PART 3.—NEW SER. EE 
