THE DEVELOPMENT OP ASTEKINA GIBBOSA. 293 



Fig. 149.— Tube-foot of the larva shown in Figs. 62—69. 



Fig. 150.— Tube-foot of the star-fish from which Figs. 141 and 146 are 

 taken, nerv. Nervous tissue under the sensory epithelium at the tip. muse. 

 Muscular tails of hydrocoele epithelial cells. 



PLATE XXII. 



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, sanff. circ. Oral "blood" ring. 

 muse. amb. Ambulacral muscles, ph. Perihsemal space. 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. 

 Fig. 155. — Transverse section of the radial nerve-cord of a young Asterias, 

 to show the feeble development of coelomio nervous system. 



Fig. 156. — Similar section of nerve-cord of an Ophiurid, to show the great 

 ganglia of the coelomic nervous system. 



Fig. 157. — Diagram of the hypothetical ancestor of Asterids and Crinoids. 

 The hjdroccele 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 hydrocoele and left posterior ccelom 

 to form rings. The hydrocoele encircles the base of the stalk. This drawing 

 does not properly represent the oblique position which the disc acquires in 

 reference to the stalk. The mouth ought to be half turned towards the 

 observer. 



Fig. 159. — Diagram of stage in evolution of Crinoids. Notice that the 

 hydrocoele 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. 



