10 ECHINODERMA. 
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. In Crinoids the water- 
vascular system opens by one ora 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 larvie 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 larva. 
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. 
