10 



ECHIXODEKMA, 



the madreporite becomes very extensive, and occupies the centre of 

 what remains of the calycinal area. In Ophiuroids the stone-canal 

 ends on one or several of the mouth-shields. Iu Crinoids the water- 

 vascular system opens by one or a number of separate pores. In the 

 Holothurians the stone-canal is connected with the wall of the body 

 in the Elasipoda and a few others, to the mesentery in others, and 

 in many hangs freely into the body-cavity ; in a number of cases 

 there is more than one canal. 



Development. — Although it is possible to understand the diagnosis 

 of an Echinoderm without knowing anything of its life-history, it is 

 not possible to have an intelligent interest in them without desiring 

 to know something of how it comes to be what it is. 



Developed, like all other Metazoa, from an egg, there very 

 rarely appears to be that mode of development without the inter- 

 ference of the male element, which is seen in various Arthropoda. 

 The sexes are generally but not always separate. There is often 

 some kind of sexual congress though no copulation. The fertilized 

 ovum undergoes division, and gives rise to a larva which is ciliated 

 all over; these cilia then become arranged in one curving band (as 

 in Auricularia, the typical larva of Holothurioids), or in two, as in 

 Bipinnaria (the typical larva of Asteroids), or in several circlets as in 

 the larva of Antedon. All these larva? are bilaterally symmetrical. 

 They generally become very remarkable in shape owing to the 

 development of lobes which may unite to form an organ larger than 



Developing larviw 



Pluteus, 



Bipinnaria. 



the young itself, or several lobes may form processes, or within the 

 lobes rods may be developed and give rise to bodies compared, not 

 inaptly, to a painter's easel {Pluteus). These larva? may, as in the 

 case of Bipinnaria asterigera, be more than an inch in size, and 

 sometimes they swarm in the sea in great number. 



