ECHIXODERMATA. 



It is, however, upon the ventral aspect of the Asterias that the 

 skeleton assumes its most perfect developement ; the floor of every 

 ray is made up of a continuous series of detached pieces, or verte- 

 brae, as they are generally called, fitted to each other and united by 

 a strong ligamentous substance, so as to form a succession of joints, 

 upon which the flexibility of the ray depends. The pieces around 

 the mouth constitute a strong circular framework enclosing the oral 

 aperture, from which, as from a centre, the rest of the skeleton 

 radiates. The joints forming the floor of the ray succeed to this ; 

 these are partially represented in Jig. 67, where the soft parts 

 having been removed from the ray marked &, their general arrange- 

 ment is displayed. 



The vertebrae thus exposed are individually composed of 

 several pieces, and each is articulated by oblique facets to those 

 which precede and follow it ; a kind of union which admits of 

 considerable motion, and provides for the flexibility of the ray, 

 so as to render it capable of executing those movements which 

 are requisite for the purpose of progression, or of seizing prey. 

 The connection of the vertebras is effected in such a manner, 

 that between each pair of calcareous plates minute orifices are left, 

 which in the entire state of the ray are seen to be arranged in a 

 quadruple series ; these holes give passage to the locomotive 

 suckers, and from this circumstance have been named the am- 

 bulacral holes, while the furrows seen upon the ventral surface 

 into which they open are designated the ambulacra! grooves 

 (fig- 64). 



(190.) The singular organs which, at the will of the animal, are 

 protruded through the ambulacral apertures, forming the principal 

 agents whereby, in the generality of species, locomotion is effected, 

 next require our notice. In the annexed figure (Jig. 64) they 

 are seen fully extended, projecting for some distance beyond the 

 margins of the ambulacral grooves which occupy the centre of each 

 ray, every one of them being furnished at its extremity with a 

 sucking disc, adapted to take firm hold upon any smooth surface. 

 The mechanism by which these suckers, or feet, as they are usually 

 called, are extended from the body and again retracted, is very 

 simple. That portion of each foot which is external to the shell 

 is a muscular tube, closed at one extremity, namely, that where- 

 unto the sucker is appended ; whilst, by the opposite, it communi- 

 cates through the corresponding ambulacral hole with a globular 

 contractile vesicle situated within the body of the animal. Both 



