vii PHYLUM ECHINODERMATA 173 



In the Heart-urchins (Spatangus) the body is heart-shaped 

 instead of globular, and in the Cake-urchins (represented by 

 our Echinarachnius parma) it is flattened and disc-like. In 

 most respects, however, these irregular sea-urchins are very 

 closely allied to the ordinary or regular forms, and with the 

 latter they constitute the third class of Echinodermata, the 

 Echinoidea. 



4. THE HOLOTHUHOIDEA 



Also widely different from the starfishes in the general 

 form of the body are the Holothurians (class Holothuroidea) . 

 Some of these are known as sea-slugs from their slug-like 

 appearance, others as sea-cucumbers. One is termed the 

 " cotton-spinner " from the cottony filaments which it dis- 

 charges when irritated or removed from the water. Certain 

 large tropical forms which abound on coral reefs in the Pacific, 

 are used as food and form the object of a fishing industry in 

 connection with which they are known as Beche-de-mer 

 or trepang (Holothuria edulis) . Our Floridan trepang is 

 H. flo7-idana ; a more northern form is Pentacta frondosa. 

 A Holothurian (Fig. 97) is roughly comparable to a sea- 

 urchin the body of which has been drawn out in the direc- 

 tion of the line joining mouth and anus, so that it has 

 assumed a long and slender form. But there is only 

 exceptionally a rigid shell of plates, the body-wall being 

 nearly always flexible and sometimes quite soft and sup- 

 ported by calcareous spicules, and usually one side, habitu- 

 ally directed downwards, is often modified as a ventral 

 surface. A circlet of tentacles surrounds the mouth. Five 

 regular zones of tube-feet sometimes run from mouth to 

 anus : sometimes those on the dorsal surface may be modi- 

 fied ; sometimes the tube-feet are scattered over the entire 



