ECHINODERMATA 625 



In life, the Crinoidea are fastened to the ground by a stalk which 

 arises from the middle of their aboral surface, and, though a few of 

 them break free when they are adult, the mouth is directed upwards 

 by them all. The other existing groups (Eleutherozoa) are free. In 

 the Asteroidea, Ophiuroidea, and Echinoidea the mouth is directed 

 downwards. The Holothuroidea apply one side of the long body to 

 the ground, so that the mouth is directed horizontally (Fig. 435). 



The tube feet {podia), whose function was perhaps originally a 

 sensory or food-collecting one, are (or some of them are) in the 



nrmrinnnnnni^^. ^^^^'I'tnnnniTOP^ 



or 



abo 



2 6r 



abo 



^or 



abo -J 



Fig. 434. Diagrams to show the relative extent of the oral and aboral sur- 

 faces, and to compare the form of body, in the several classes of the Echino- 

 dermata. All the diagrams are in the same morphological position. From 

 Borradaile. i, Asteroidea. 2, Ophiuroidea. 3, Echinoidea. 4, Holothuroidea. 

 5, Crinoidea. abo. aboral surface; or. oral surface. 



Asteroidea, Echinoidea, and Holothuroidea adapted, by the presence 

 of suckers at their ends, to walking. Probably they always subserve 

 respiration, and in the "irregular" echinoids some of them are 

 modified for this function. They may also be modified for seizing 

 food. They are protruded and retracted by alterations of the pressure 

 of the fluid within them by the action of the water vascular system 

 (see below). 



The epidermis is usually ciliated, but not in ophiuroids or, except 

 in the ambulacral groove, in crinoids. Usually, also, it contains gland 

 cells and sense cells, the latter with their bases prolonged into fibrils 

 which enter a plexus, formed by them and by branched nerve cells, 



