ANNULOIDA I ECHINODERMATA. 157 



mimication of the ambulacral system with the exterior is 

 doubtful, and still more so in Crinoids. In Holothurids no 

 such communication obtains ; the madreporic canals and their 

 tubercles depending freely from the circular canal into the 

 peri visceral cavity." 



By Professor Wyville Thomson the larva of the Echinoder- 

 mata is termed the " pseud-embryo," since it leads a perfectly 

 independent existence, and the true Echinoderm is usually 

 developed out of a portion only of its substance. The great 

 peculiarity, therefore, in the development of the Echinodermata 

 is found in the possession by the larva of provisional organs, 

 which may be either absorbed or cast off, but which are not 

 converted into the corresponding structures of the adult. 'Thus 

 the Pluteus of an Echinoid possesses a mouth and alimentary 

 canal which are not converted into, and in no way correspond 

 with, the mouth and alimentary canal of the adult. 



The Echinodermata are divided into seven orders viz., the 

 Crinoidta, Cystoidea, Blastoidea, Ophiuroidea, Asteroidea, Echi- 

 noidea, and Holothuroidea. Of these, the first is almost extinct, 

 and the two next are entirely so ; they are really the lowest 

 orders ; but their structure will be better understood if the 

 higher orders are considered first. 



CHAPTER XIX. 



ECHINOIDEA. 



ORDER ECHINOIDEA. The members of this order commonly 

 known as Sea-urchins are characterised by the possession of 

 a subglobose, discoidal, or depressed body, encased in a " test " 

 or shell, which is composed of numerous, immovably connected, 

 calcareous plates. The intestine is convoluted, and there is 

 a distinct anus. The mouth is usually armed with calcareous 

 teeth, and is always situated on the inferior surface of the body, 

 but the position of the anal aperture varies. The larva is 

 pluteiform, and has a skeleton. 



The " test "of the Echinoidea is composed of numerous cal- 

 careous plates, firmly united to one another by their edges, 

 and bearing different names according to their position and 

 function. In one or two exceptional -cases (Echinothurida), 

 though the essential structure of the shell is the same as in the 

 ordinary forms, the plates of the test are so thin, and are so 



