648 THE INVERTEBRATA 



Typical members of this order live buried at some depth in the 

 sand and move, not by means of their tube feet, but by ploughing 

 their way with numerous, curved, flattened spines. In such forms 

 the body has a heart-shape, owing to the depth of the anterior am- 

 bulacrum, which differs from the rest and has special tube feet, 

 capable of great elongation and provided with fringed discs. These 

 gather sand rich in food, which is then pushed into the mouth by 

 stout buccal tube feet. 



Spatangus and Echinocardium (Fig. 454) are typical members of 

 the order, found in British waters. Echinocardium comes into shallower 

 water than Spatangus, burrows deeper, and differs in respect of the 

 arrangement of the spines. 



-esp.tf. 



buctf. 



Fig. 454. Echinocardium cordatum. A, From the aboral; B, From the oral 

 side, buc.tf. buccal tube feet, where the ambulacra converge upon the 

 mouth ; resp.tf. respiratory tube feet at the side of a petal : both kinds much 

 contracted. 



Class HOLOTHUROIDEA 



Sausage-shaped Echinodermata, without arms ; without recognizable 

 abambulacral area ; usually without external madreporite in the adult ; 

 with the ambulacral grooves covered ; some of the tube feet modified 

 into tentacles around the mouth, and some or all of the rest, if present, 

 provided with suckers ; a muscular body wall containing very small 

 ossicles; no spines; and no pedicellariae. 



The typical form of body of the Holothuroidea is well seen in the 

 members of the widely distributed genus Holothuria (Fig. 457 B), to 

 which the familiar British "cotton spinner" belongs. It is such as 

 would result if in a regular echinoid the ossicles were reduced and 

 the body drawn out in the oro-anal axis, the madreporite with the 



