ECHINODERMATA. 11 



cally laid open. In this specimen the animal has contracted 

 so much, that the whole of the tract round the mouth 'the 

 peristomial space/ with the tentacles, has been withdrawn 

 within the body-walls, and hangs down like a bag, which 

 has been laid open so as to expose the tentacles, of a black 

 colour. 



Five pairs of longitudinal muscular bands, radiating from 

 mouth to anus, are seen to divide the body-walls into a corre- 

 sponding number of crescent-shaped divisions, in which the 

 muscular fibres, which encircle the animal transversely, are well 

 seen. In the interval between the two elements of each pair of 

 muscular bands lies the longitudinal water- vascular ambulacral 

 vessel. In this family the Dendrochirotce each longitudinal 

 muscle gives off a long slip which inserts itself into the corre- 

 sponding radial ossicle of the calcareous ring encircling the 

 mouth, and acts as a retractor. The water- vascular ring sur- 

 rounds the muscular pharynx, a little way posteriorly to the 

 calcareous ring. In the centre of the preparation are the 

 muscular pharynx and stomach. On the exterior may be 

 noticed the longitudinal rows of apertures for the tube-feet, 

 which, however, are now partially retracted. 



Rolleston, p. 145 sq., and p. cxlvii. 



19. A specimen of Psolus phantapus with part of the dorsal 

 and ventral surfaces removed. The animal is contracted as in 

 the last preparation. The longitudinal and retractor muscles, 

 and the calcareous ring with its two Polian vesicles, are well 

 seen. 



On the ventral side of the preparation is seen the whole of 

 the digestive tract, spread out upon a sheet of mica. The anal 

 end receives the ducts of the so-called respiratory trees, which 

 in the preparation depend downwards from the cloaca, one on 

 each side. These organs consist of hollow stems carrying some- 

 what scanty ramifications; they reach a considerable distance 

 into the body, to the walls of which they are attached by 

 mesenteries. 



On the dorsal surface is the generative gland a bundle of 

 simple unbranched caeca of various lengths. The mesentery, by 

 which it was attached to the body-walls, has been removed. A 

 single efferent duct passes from these ca?ca to open inside the 



