124 MANUAL OP ZOOLOGY. 



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 con- 

 nected, 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 EcMnoidea 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 all recent members of the order the test is com- 

 posed of twenty rows of these plates, arranged in ten alter- 

 nating double rows of plates, which pass from the one pole of 

 the animal to the other. Five of these double rows are com- 

 posed of large plates, which are not perforated by any apertures 

 (Jig. 32) ; the zones formed by these imperforate plates being 

 termed the ' inter- ambulacral areas.' The other five double 

 rows of plates alternate regularly with the former, and are 

 termed the 'ambulacral areas,' or ' poriferous zones.' Each of 

 these zones is composed of two rows of small plates, which 

 are perforated by minute apertures for the emission of the 

 4 ambulacral tubes,' or ' tube-feet.' Growth of the test is carried 

 on by additions made to the edge of each individual plate, by 

 means of an organised membrane which passes between the 

 sutures, where the plates come into contact with one another. 

 The plates of the test are studded with large tubercles, which 

 are more numerous on the inter-ambulacral areas than on the 

 ambulacral, and are wanting on all the plates which do not 

 belong to either area. These tubercles carry spines, used 

 defensively and in locomotion, which are articulated to their 

 apices by means of a sort of 'universal,' or ' ball-and-socket,' 

 joint. Occasionally a small ligamentous band passes between 

 the head of the tubercle and the centre of the concave arti- 

 cular surface of the spine, thus closely resembling the ' round 

 ligament' of the hip-joint of man. Besides the main rows of 

 plates just described, other calcareous pieces go to make up 

 the test of an Echinus. The mouth is surrounded by a coria- 

 ceous buccal membrane, which contains a series of small 

 calcareous pieces, known as the ' oral plates ; ' whilst a cor- 



