DEKMAL SKELETON OF ECHIXODEKMA. 



201 



d d' c). The whole body is covered with cilia at a period before 

 that when they are arranged on the ridge-like projections from the 

 ciliated band ; bnt this general ciliation is only found during the 

 most indifferent condition of the larva. 



The cilia are retained even later on, on many parts of the soft 

 dermal layer which invests the calcareous skeleton : as, for example, 

 in the ciliated tracts, which reach to the mouth in the Spatangidre 

 (semitos). On other parts, such as the dermal branchias (cf. supra), 

 ciliation appears to be correlated with the respiratory function 

 of the integument, in which the ambulacral feet may also have 

 a share. 



The extent to which the integument is calcified varies greatly. 

 Sometimes the calcareous particles are united with one another into 

 larger pieces, and form plates which are movably or immovably 

 connected together : this arrangement either extends over the whole 

 body, or is confined to definite tracts of its surface. In other cases 

 the calcareous particles are scattered, and allow of great variations 

 in the form of the body. In this case a large number of the 

 characteristic features of the Echinoderm disappear from other parts 

 of its organisation ; so that the disappearance of a calcified integu- 

 ment is a departure from the type, and the general phenomenon of 

 a scanty deposit of calcareous matters is not to be regarded as an 

 early, but as a final stage in the series of forms. 



Calcification converts the integument into an organ of support 

 for the body, or dermal skeleton ; 

 in many cases this sends out pro- J v 



cesses into the interior of the body. 

 These give rise to calcified struc- 

 tures, which form an internal skele- 

 ton and combine with the external. 

 The whole thickness of the peri- 

 some is not affected by the process 

 of calcification. A thin non-calci- 

 fied layer of tissue is always found 

 on the inner, as well as on the outer 

 surface ; on the latter, however, this 

 layer disappears at an early period 

 from some parts, so that tlie calcified 

 parts are exposed ; this happens, 

 for instance, on the spine-like struc- 

 tures, as well as on other processes 

 of the calcareous skeleton. 



The lime-salts are always de- 

 posited in a regular manner in the 

 integumentary layer. They form 

 delicate frameworks or retiform 

 structures (Fig. 101), in the spaces 

 of which soft organic substance persists. The most solid skeletal 

 parts are thus traversed by soft structures ; when the calcareous 



Fig. 101. View of a calcareous net- 

 work of a plate of the dermal skeleton 

 of an Echinid (Cidaris). b Trabecular 

 cut throiigh. These were directly per- 

 pendicular to the horizontal network 

 (somewhat highly magnified) . 



