5S^ THE INVERTEBRATA 



In life, the Crinoldea 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. 4J0). 



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

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

 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, 

 among the bases of the epithelial cells. The characteristic ossicles of 

 the dermis may be scattered, so as merely to impart a leathery con- 

 sistency to the skin, or united by muscles as a skeleton, or firmly 

 apposed so as to constitute an armour. Some of them usually project 

 as spines, over which the epidermis may presently wear away. Pedi- 

 cellariae (Figs. 408 C, 424) are sets of two or three spines arranged to 

 bite together as pincers. They are of various types, often complicated, 

 but only occur in asteroids and echinoids. 



The alimentary canal differs greatly in the several groups. It is 

 axial in the Asteroidea and Ophiuroidea, coiled in the other classes. 

 It possesses various diverticula in different cases, but not large 

 glands like those which are common in other phyla. The anus is 

 lacking in the Ophiuroidea and a few asteroids, and when present is 

 more or less excentric except in the Holothuroidea. 



The coelom of the adult is present as several distinct systems of 

 spaces, of which the following are the most important: (i) the large 

 perivisceral cavity in which lie all the principal viscera ; (2) the peri- 

 haemal syste?n, consisting of a radial vessel (in asteroids divided 

 longitudinally by a vertical septum in which lies the principal 

 "blood" strand) along each radius, and a ring vessel around the 

 mouth, all lying immediately above the main nerve cords; (3) the 

 aboral sinus system enclosing the genital rachis and gonads (see below) ; 

 (4) the water vascular system (Fig. 411), which lies above the peri- 

 haemal system, and consists of a ring around the mouth, a tube, 



