150 



ECHINODERMATA. 



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 (t), 

 whereby it is rendered tense and prominent. The muscular coats, 

 which invest the exterior of the protruded portion, are likewise de- 

 picted ; the internal layer (&), immediately in contact with the 

 membranous canal continued from the vesicle, is made up of longi- 

 tudinal bands passing from the root of the organ towards the 

 sucker at its extremity, while the outer layer (/) consists of cir- 

 cular fibres, an arrangement evidently adequate to the performance 

 of all required movements. 



The other portions of this diagram represent the feet in differ- 

 ent stages of protrusion : in Jig. 65, 2, c, the vesicle being par- 

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

 distension ; and at b, the sucker is shown in a still more retracted 

 state, the contained fluid having been completely expelled from 

 the muscular tube, and driven back into the vesicle, which is dis- 

 tended to the utmost. 



Fig. 65. 



The fluid that thus fills the suckers, and performs so important 

 a part in causing all their movements, is not secreted by the vesi- 

 cles in which it is contained, but is conveyed into them by a 

 special vascular apparatus, (g, /, ) from which branches are given 

 off to each tube. The nature of the fluid, however, and the ar- 

 rangement of the vessels through which it flows, will be more 

 properly discussed hereafter. 



