130 INTRODUCTION TO CLASSIFICATION. 



The embryo is free and ciliated, and developes within 

 itself a second larval form, which becomes fixed by a peduncle. 



The Crinoids are so different from the other living Echino- 

 dennata that they .will probably have to form a distinct 

 primary division, or sub-class of the class ; and this may possibly 

 be the case with some of the following extinct forms. 



6. The CYSTIDEA. The body spheroidal or ovoidal, inclosed 

 in suturally united plates, and stalked. At the opposite end 

 to the stalk is a terminal aperture, surrounded by arms ; and on 

 the side of the body another larger aperture, closed by a pyramid 

 formed of triangular plates. 



7. The EDKIASTEKIDA. The body depressed, hemispheroidal, 

 inclosed in suturally united plates, but not stalked. In the 

 centre of the convex face is an aperture from whence five am- 

 bulacra radiate : between two of these is a pyramid like that of 

 the Cystidea. 



8. The BLASTOIDEA. The body is inclosed within suturally 

 united plates, and stalked. There is no pyramid. A central 

 aperture appears opposite the stalk, whence five ambulacra with 

 a very complicated structure radiate ; in four of the re-entering 

 angles, between the ambulacra, is a pair of small apertures ; and 

 in the fifth, two small apertures, with a larger one between them, 

 are placed. 



These three groups have been extinct since the latter part 

 of the palaeozoic epoch. 



