174 



ECHINODEEMATA. 



tained fluid is forced back again into the internal vesicle, and the whole 

 foot collapses. The arrangement referred to will be easily intelligible 

 on reference to the annexed rough diagram, which represents a lon- 

 gitudinal section of one of the rays of the Asterias depicted above. 

 The internal vesicles (fig. 88, 1, Ji) occupy the floor of each segment of 

 the body ; and when viewed from above (fig. 90, d), the entire series 

 resembles strings of transparent beads placed above the rows of ambu- 

 lacral apertures, through which they communicate with the tubular 

 feet (fig. 88, 1, g}. In fig. 88, 2, three of these organs are represented 



Fig. 88. 



1. Diagrammatic section of a Star-fish : , mouth ; 6, stomach ; c, intestiniform caeca ; d, dorsal 

 surface ; e, ambulacral plates ; f, ovarium ; 17, tubular feet ; h, internal vesicles. 2. Diagram re- 

 presenting the suckers in different states of extension : a, ambulacral plates ; 6, c, d, internal 

 vesicles ; <?,/, g, vascular system ; i, tubular foot, laid open to exhibit k, I, its muscular coats. 



in different states of extension, and their whole structure is developed. 

 The foot (d) is shown protruded to its full extent ; the vesicle, much 

 contracted, has forced the fluid which it contained into the external 

 tube (i), whereby it is rendered tense and prominent. The muscular 

 coats that invest the exterior of the protruded portion are likewise de- 

 picted ; the internal layer (lc\ immediately in contact with the mem- 

 branous canal continued from the vesicle, is made up of longitudinal 

 bands passing from the root of the organ towards the sucker at its 

 extremity, while the outer layer (Z) consists of circular fibres, an 

 arrangement evidently adequate to the performance of all required 

 movements. 



(455.) The other portions of this diagram represent the feet in dif- 

 ferent stages of protrusion : in fig. 88, 2, c, the vesicle being partially 

 contracted, the tubular portion is seen in a medium state of distention ; 

 and at Z>, the sucker is shown in a still more retracted state, the con- 



