240 



ZOOLOGY 



ill the skin wliere ttiey iiave been formed; from tliem 

 often arise otlier structures — spines, whicli gain a great 



size in certain sea-ur- 



chins and are moved by 

 muscles at their base. 

 Certain spines are mod- 

 ified and have special 

 uses ; such are the little 

 forceps that abound 

 about the mouth and 

 are beheved to help 

 keep the other spines 

 clean. In the sea-cu- 

 cumbers (Holothurians), 

 alone, is the skeleton 

 insignificant, being 

 made up of small spic- 

 ules that take on a va- 

 riety of forms, such as 

 rods, anchors, wheels, 

 etc. (Fig. 231). Since 

 these bodies are rela- 

 tively small, the skin of 

 holothurians is flexilile 

 and often transparent. 

 The Water-vascular 

 System. — The most 



Fig. 2.30. — A sea-urchin that has gained a bi- 

 lateral shape. The mouth is shown at the 

 upper (anterior) end ; the anus at the pos- 

 terior end. .4, dorsal or aboral \'iew ; B, 

 ventral or oral ^-iew. From Parker and 

 Haswell. 



characteristic feature of echinoderms is a system of water-tubes 

 which serve for locomotion. This system opens to the exterior 

 by means of a sieve-like body placed at the aboral pole called 

 the madreporite. Thence a canal runs to a circular canal sur- 



