

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 synmietry. 



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



1. Erhinoidea (Sea-urchins). 



2. Asteroidea (Star-fishes). 



3. Ophiaroidea (Sand-stars and Brittle-stars). 



4. Orinoidea (Feather-stars). 



5. Cystoidea (extinct). 

 G. Blastoidea (extinct). 



7. Holothuroidea (Sea-cucund>crs), 



Order I. Echisoidea. 



The animals included in this order vary from the sha])e 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 chaiactei'ised by the fact that 

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

 numerous calcareous ]jlates, usually immovably jciinted together so 

 as to form a kind of box. The intestine is convcjluted, and tlici'e is 

 a distinct vent, or anal aperture. 



The test of a Sea-ui-cliin, a-s just said, consists of many calcareous 

 ])lates 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. Gl, A, ?') the jilates are of large 

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

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

 plates are of small size, and ai'e perforated by little ajiertures 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 ])lates, which 

 collectively make up the greater ])art of the test, theie are other 

 plates placed in the leathery skin loiuid 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. Gl) is composed of two sets of 

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

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

 "eye," and known by the name of "ocular ])lates." One of the 

 genital plates is also larger than the others, and cariies a spjongj' mass 

 which is called the " madreporiform t)d:iei'cle," and which jn-otects the 

 entrance of the water-vascular or ambulacr'al system. The whole of 



