508 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



zoic forms are spherical, and have multiple interambulacral 

 plates and simple. ambulacra. Echinidea of the modern type 

 appear in the Mesozoic strata — the Echinoida first, while the 

 Spatangoida and Clypeastroida are of later date. This order 

 of occurrence agrees with the embryonic development of the 

 two latter groups, which are more nearly spherical when young 

 than subsequently. 



The Crinoidea abound in the Palaeozoic and older Meso- 

 zoic rock's, gradually diminishing in number in later forma- 

 tions. The oldest appear to have all been stalked, and of 

 peculiar and extinct types. 



Three groups are wholly extinct, and are unknown in 

 strata newer than the Carboniferous formation. These are 

 the Cystidea, the Edrioasterida, and the Blastoidea. 



The Cystidea. — In their general characters the Cystidea 

 come very near the Crinoids. Cryptocrinus, the simplest 

 form of the group, possesses a calyx supported on a stem, and 

 composed of five basalia, five parabasalia, and five radialia. 

 An interradial aperture is surrounded by a cone of small 

 plates, termed the pyramid. The antambulacral surface has 

 no pores, but these were present in other genera, and sometimes 

 are scattered irregularly ( Caryocrinus) ; sometimes disposed 

 in pairs (Sphceronites) ; while sometimes they take the form 

 of parallel slits arranged in " pectinated rhombs." The arms 

 were free (Comarocystites), or recurved and closely applied 

 to the calyx. They bore pinnules, which, in consequence of 

 the non-development of the arms, were sometimes sessile on 

 the radialia. In the species with recurved arms, the latter 

 simulate calycine ambulacra. There is an aperture placed in 

 the centre of the calyx at the point of convergence of the 

 ambulacra ; another small one on one side of this ; and, third- 

 lv, the aperture of the pyramid. The first of these is com- 

 monly regarded as the mouth, the second as the anus, the 

 third as the reproductive aperture. 



The Cystidea would, on this interpretation, differ from all 

 other Echinodermata, except the Edrioasterida and Holo- 

 tkuridea, in the genital outlet being single ; but around the 

 central aperture five pores are seen, in some species at least, 

 to which a genital function has been ascribed. In any case, 

 the Cystidea would appear to come very close to the Cri- 

 noidea. 



The Edrioasterida. — This group contains several genera 



