ECHINODERMATA : ECHINOIDEA. 193 



noidea, and Holothuroidea. Of these, the first is to a consider- 

 able extent 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 XVIII. 

 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, usually immovably con- 

 nected, calcareous plates. The intestine is convoluted, and there 

 is a distinct anus. The sexes are distinct, and the larva is plutei- 

 form, and has a calcareous skeleton. * As regards their general 

 anatomy, the " test " of the Echinoidea is composed of numer- 

 ous calcareous plates,*/ which are generally firmly united to 

 one another by their edges, in such a manner that the body 

 of the animal is enclosed in an immovable box. In the singu- 

 lar Urchins, however, which constitute the family of the Echi- 

 nothuridce, the plates of the test overlap one another in an 

 imbricating manner, so that the shell becomes quite flexible ; 

 and the same is the case with some of the Palaeozoic Echinoids. 

 In all living Sea-urchins, and in the great majority of the ex- 

 tinct forms, the test is composed of twenty meridional rows of 

 plates, arranged in ten alternating zones (fig. 93, A), which 

 typically pass from one pole of the shell to the other, and 

 each of which is composed of two similar rows of plates. Five 

 of these double rows are composed of large plates, which are 

 not perforated by any apertures (fig. 93, A and B, ct}\ 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 (fig. 93, A and B, /) is composed of two rows of small 

 plates, which are perforated by minute apertures for the emis- 

 sion of the " ambulacral tubes, " or " tube-feet." In one great 



* The skeleton of the Echinoids is composed of calcined areolar or 

 connective tissue, the fibres of which enclose oval or rounded meshes 

 (fig. 96, B), exhibiting under the microscope an exceedingly characteristic 

 appearance. 



N 





