328 COMPARATIVE ANATOMY CHAP. 



anus. The apertures of the stone canal are not found on any of the 

 apical plates, but ventrally, on one of the oral shields. 



IV. Pelmatozoa. 



In no other class of the Echinodermata do the plates of the apical 

 system form so large a part of the skeleton of the body wall (apart 

 from the arms) as in the Pelmatozoa, The body of these Echinoderms 

 consists of a central calyx, which contains the viscera, and usually 

 carries jointed appendages, radially arranged at its edge ; these are 

 the arms and pinnulse. Typically the Pelmatozoa are attached to 

 the sea-floor by their apical poles, with or without the intervention 

 of a stem ; in some the stem becomes separated from its attachment 

 (Pentacrinus), and may dwindle in size (Millericrinus), or may be present 

 only in the embryonic stages (Antedon), or there may be no trace of 

 either stem or attachment (Marsupites). The oral side of the calyx 

 (and also of the arms) is thus turned upwards, while the apical side 

 of the calyx (the dorsal cup) is turned downwards and either 

 surrounds the viscera like a bowl or carries them like a dish. The 

 plated test of this bowl or dish consists exclusively, or for the greater 

 part, of the plates of the apical system : the basals and the radials, 

 to which infrabasals may be added. The anal aperture always lies 

 interradially, usually on the oral side of the body and not con- 

 nected with the apical system. 



Sub-Class 1. Crinoidea. 



There are a good many Crinoids in which the apical system is 

 completely developed. The five radials and the basals are constant, 

 although the latter may be hidden. The infrabasals are inconstant. 

 The Crinoids in which the latter are present are said to have a 

 dicyclie base, those in which they are absent have a mono-cyclic 

 base. 



A central plate has been observed in the larva of A)if<.'Jti. It 

 occurs at the distal or root end of the larval stem, and ultimately 

 becomes severed from the animal. 



The part taken by the plates of the apical system in the construc- 

 tion of the apical capsule varies greatly. In the stalked larva of 

 Antedon they alone form the skeleton of the apical side of the calyx ; 

 although an anal interradial has a transitory existence. The same is 

 the case also in many other adult Crinoids, which in this respect show 

 a primitive or an embryonic character (many Ii'mdniuifn. larrifonn'm 

 and many Inadunata fistulata, Encrinux, Marsupites, ll<>l<>pus, Hyocrinus, 

 Bathycrinus, and a few Canaliculata : Rhizocrinus, Pentacrinus). 



In most Crinoids, on the other hand, the plates of the typical apical 

 system, i.e. the infrabasals (where these occur), basals and radials do 

 not form the whole skeleton of the apical capsule, but only a certain 



