100 STARFISH, SEA URCHIN, BRITTLE STARS 



exactly the reverse of that in the starfish. The arms of the 

 crinoids are often many times branched and the branches 

 bear short, lateral projections called pinnulse that serve to 

 give the animal a feathery appearance. . 



Reproduction and development. — The echinoderms re- 

 produce by eggs, not by budding, and the sexes are separate 

 in the majority of species. In one species, at least, the 

 female may be distinguished from the male by a difference 

 in color. In this case the female is decidedly bluish in 

 color while the male is reddish brown and presents a 

 marked contrast to the former sex. The eggs and sperm 

 cells are extruded into the water and there the sperm cells 

 find the eggs and fertilize them. The egg then hatches 

 into a tiny organism, the larva, which has cilia, is free- 

 swimming, and in no way resembles the adult. The larvse 

 of these animals pass through remarkable changes in 

 their growth to adults and when first discovered, were 

 thought to be distinct species of marine animals and 

 were given various names. For example, the larvae of 

 sea urchins and brittle stars were called Pluteus, while the 

 larvse of the starfish were called Brachiolaria and Bipin- 

 naria. 



Regeneration of lost parts. — Like the hydra, many of 

 the echinoderms are remarkable for their ability to repro- 

 duce parts of the body that may have been broken or thrown 

 off. Certain of the starfish and man> br'^tle stars, when 

 molested, will throw off pieces of their arms or sometimes 

 all of their arms. Indeed it is difficult to obtain these par- 

 ticular species whole. Under these conditions the central 

 disk will produce new arms and sometimes an arm will 

 produce a new central disk and the other arms. Stranger 

 still, in some sea cucumbers a part or even all of the ali- 



