ECHINODERMATA. 99 



the Pluteus, and finally envelops that organ ; while a new mouth is 

 formed, and a large part of the provisional embryo is absorbed. 

 The adult further produces a fresh skeleton for itself, and now 

 exhibits conspicuous radial symmetry. 



The Echinoderniata are divided into seven orders, as follows : — 



1. Efhinoidea (Sea-urchins). 



2. Asteroidea (Star-fishes). 



3. Op/iiufoidea (Sand-stars and Brittle-stars). 



4. Crinoidea (Feather-stars). 



5. Cystoidea (extinct). 



6. Blastoidea (extinct). 



7. Holothuroidea (Sea-cucumbers). 



Order I. Echinoidea. 



The animals included in this order vary from the shape of a sphere 

 or globe to that of a disc, and they are all commonly known as " Sea- 

 urchins " or " Sea-eggs." They are all characterised by the fact that 

 the body is encased in a "test" or "shell" (fig. 61) composed of 

 numerous calcareous plates, usually immovably jointed together so 

 as to form a kind of box. The intestine is convoluted, and there is 

 a distinct vent, or anal aperture. 



The test of a Sea-urchin, as just said, consists of many calcareous 

 plates accurately fitted together, and united by their edges, or, in a 

 few cases, overlapping one another. In all living forms the test is 

 composed of ten zones of plates, each zone consisting of a double 

 row. In five of these zones (fig. 61, A, i) the plates are of large 

 size, and are perforated by no apertures. These are termed the 

 " interambulacral areas.'' In the other five zones (fig. 61, A, a) the 

 plates are of small size, and are perforated by little apertures for 

 the emission of delicate locomotive suctorial tubes (the so-called 

 "ambulacral tube-feet"). These zones are therefore called the 

 " ambulacral areas.'' Besides these main rows of plates, which 

 collectively make up the greater part of the test, there are other 

 plates placed in the leathery skin round the mouth and vent. The 

 most important of these form a kind of disc, which is placed at the 

 summit of the shell. This disc (fig. 61) is composed of two sets of 

 plates — one called the " genital plates," perforated for the ducts of 

 the reproductive organs ; the other set smaller, carrying each a little 

 "eye," and known by the name of "ocular plates." One of the 

 genital plates is also larger than the others, and carries a spongy mass 

 which is called the " madreporiform tubercle," and which protects the 

 entrance of the water-vascular or ambulacral system. The whole of 



