146 



ANIMAL FORMS 



and a portion of the locomotor system ; but some species, at 

 least, renew them again. In some of the starfishes and 



brittle-stars portions of the body 

 appear to be voluntarily de- 

 tached and to develop into new 

 individuals, and it is thought 

 that such self-mutilation is a 

 normal method of reproduction. 

 140. Locomotor system. One 

 of the most characteristic and 

 remarkable features of the echi- 

 noderms is the water-vascular 

 system, a series of vessels con- 

 taining water which serve in the 

 process of locomotion. Their 

 arrangement and mode of opera- 

 tion are, with slight modifica- 

 tions, the same throughout the 

 group, and may be readily un- 

 derstood from their study in 

 the starfish. 



On the dorsal surface of a 

 starfish, in the angle between 



two of the arms, is a round, slightly elevated, calcareous 

 plate, the madreporic body (Fig. 95, m.p.), which under 

 the microscope appears full of holes, like the " rose " of a 

 watering-pot. This connects with a tube that passes to 

 the opposite side of the body, where it enters a canal 

 completely encircling the mouth. On this ring-canal a 

 number of sac-like reservoirs with muscular walls are at- 

 tached, and from it a vessel extends along the under sur- 

 face of each arm from base to tip. Each of these radial 

 water-mains gives off numerous lateral branches that open 

 out into small reservoirs similar to those located on the 

 ring-canal, and a short distance beyond communicate 

 through the wall of the body with one of the numerous 



FIG. 92. Sea-cucumber (Cucu- 

 maria sp.). Natural size. 



