THE ECHINOIDEA 



and below it. Genera Echinothuria, S. P. Woodw. ; from the Chalk ; 

 Asthenosoma, Grube (Fig. XXIX.), a living genus with large " Stewart's 

 organs " and powerful radial muscles ; and Pliormosoma, Wy v. Thomson, 

 with rudimentary Stewart's organs and without powerful radial muscles. 

 In the Echinothurinae the reduction in the calcification of the test, 

 which had begun in Pelanechinus, has been carried so far, that all the 

 ambulacra! plates are disunited, but are held together by strong muscular 

 lining. In Asthenosoma there is, in addition, a series of powerful radial 

 muscles (Fig. XXX.), which give a panting motion to the test. The 



FIG. XXIX. 



Asthenosmna hystrix. Oral surface ; the tips of tin' .jaws an- sei-n protruding through 

 the peristomial membrane. 



spines are covered by epithelium, and when handled can inflict a sharp 

 sting. 



SUB-ORDER 4. ECHININA. 



Diademoida in which the ambulacral plates typically consist of an 

 aboral and adoral primary, between which are one or more denii-plates. 

 The sub-order includes a series of Echinoids, in the simplest of which the 

 compound ambulacral plates consist of three primaries, and are separated 

 from one another by free primaries. 



In the sub-order Arbacina the plates of the test are often fixed 

 together by sockets and knobs (Fig. XXI. 2), while in some genera, sucli 

 as Glypticus, there is a great development of the subsidiary ornament. In 



