362 COMPARATIVE ANAToMY chap. 



tliat the apical system, whether complete or iiicoinplete, forms, in many Opliiuroidea, 

 even in adults, the greatest, or at any rate a considerable, part of the dorsal carapace 

 of the disc. Those regions which are not covered by the apical system are plated 

 by the perisomatic skeleton. The plates of this skeleton vary much in size, form, 

 number, and arrangement, and not infrequently, esi)ecially in cases where the apical 

 system does not consist of large distinct plates, the dorsal integument of the disc is 

 soft, and only provided with scattered skeletal pieces, which are sometimes micro- 

 scopically small. 



Ten large perisomatic. plates appear most constantly (even more constantly than 

 any of the circle of plates of the apical system) ; one pair of these lies near the base 

 of each arm. These are called the radial shields (Figs. 244, p. 299, and 314 rs), 

 and are often present even when there are no large plates in the rest of the dorsal 

 carapace of tlie disc. Sometimes the radial shields, covered with a soft integument, 

 reach from the base of the arm to near the centre of the disc, their presence being 

 then outwardly marked by a graceful rosette formed of five pairs of radial ridges. 



V. Crinoidea. 



(Cf. the apical and oral systems of this class, pp. 328-333). 



The perisomatic skeleton of the Crinoidea consists of: (1) The 

 perisomatic skeleton of the calyx ; (2) the skeleton of the arms and 

 pinnulae ; (3) the skeleton of the stem. 



{(i) The Perisomatic ^ Skeleton of the Calyx. 



In this are included all the skeletal pieces of the calyx, Avhich do 

 not belong either to the apical system (central, infrabasals, basals, and 

 radials) or to the oral system (orals). 



In the young stalked larva of Antedon the skeleton of the calyx 

 has no perisomatic pieces ; it consists exclusively of the typical plates 

 of the oral and apical systems (Fig. 270, p. 318). 



The only forms in which this stage persists throughout life are 

 those of the type Inadunata larviformia, e.r/. genus Ilaplocrhnifi 

 (Fig. 297, p. 334). 



In all other living and extinct Crinoidea a perisomatic skeleton is 

 developed, although it varies in extent to an extraordinary degree. 



This skeleton may consist of very various systems, and may be 

 developed both in the dorsal cuj^ and the tegmen. 



a. One, or several, or even many, pieces may appear only in the 

 posterior or anal interradius, especially in the dorsal cup supporting 

 or bordering the anus. These anals, which characterise the posterior 

 interradius, more or less markedly disturb the regularly radial 

 structure of the calyx. 



h. In all the five interradii one or many pieces may occur, both 



' Mr. F. A. Bather [Xatural Science, vol. vi. pp. 418, 419 : 1895), iu reviewing the 

 original German edition of this work, adduces strong reasons against the rise of the term 

 " perisomatic " as here employed by the author. The term is, nevertheless, retained in 

 tliis English version liecause its exclusion seemed to necessitate the entire rearrangement 

 of this Section V. — Tr. 



