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CHAPTER XXIII. 



DIVISION A.—ECHINOZOA. 



Class I. Echinoidea. 



The members of this class — commonly known as Sea-urchins — are 

 characterised by the possession of a more or less globular, heart- 

 shaped, discoidal or depressed body, encased in a " tes't" or shell, which 

 is composed of numerous calcareous plates, in general immovably con- 

 nected together. The intestine is convoluted, and there is a distinct 

 anus. The mouth is always situated on the inferior aspect of the 

 body, but the position of the vent varies. 



The ambulacral system of the Echinoids consists of its usual 

 parts, and the five radial vessels given off from the circular oeso- 

 phageal ring are situated within the shell of the animal. It follows 

 from this that the tube-feet given off from the radial vessels can 

 only reach the exterior by passing through perforations in the plates 

 of the calcareous test. Hence the outer surface of a Sea-urchin 

 exhibits five longitudinal areas which are " radial " in position and 

 correspond with the five radiating ambulacral vessels, and the plates 

 of which are more or less extensively perforated for the protrusion 

 of the tube-feet. These so-called " ambulacral " or " poriferous " 

 areas are separated by five zones which are " interradial " in position, 

 have imperforate plates, and are known as the " interambulacral " 

 areas. The external opening of the ambulacral system is closed by 

 a spongy or porous plate — the " madreporite " — which is almost in- 

 variably situated on the apex of the test in one of the interradii. 

 In the extinct Echinocystites (Cystocidaris) alone the madreporite 

 is removed from its normal position, and is placed " close to the 

 apical pole." 



As regards the digestive system, the mouth is always placed on 

 the under surface of the body, and may be central or excentric in 

 position. The mouth is sometimes edentulous, but in other cases a 

 complicated apparatus of calcareous teeth is developed. The anus 



