THE SKELETON OF MOLLUSCA. 



103 



treniity of which the eyes are situated, corresponding to their 

 position in the echinidae ; the summits of the areas being ana- 

 logous to the extremities of the rays bent up towards the 

 anal pole. 



In the CRIXOIDEJE, or sea lilies, which may be likened to 

 sea-stars supported upon many jointed columns, the skeleton is 

 very complicated, being composed of many thousand separate 

 pieces, beautifully and nicely fitted to each other. Fig. 72 

 represents the pear encrinite (Apiocrinus rotunda), from the 

 Bradford clay ; and fig. 73, the lily encrinite (Encrinus mo- 

 niliformis\ from the Muschelkalk. These stalked echinoderms 

 attained a great generic development in the palaeozoic rocks, 

 entire strata being sometimes composed of their broken 

 skeletons ; their forms are less numerous in the triasic and 

 oolitic periods ; a few only are found in the chalk, and one 

 rare species lives in the warm regions of our present seas. 

 T. W.] 



Fig. 74. Cyprtsacdssis rufa; a, mature, b, immature state of the same 



shell. 



220. In the mollusca, the solid parts are secreted by the 

 skin, most frequently in the form of a calcareous shell of one, 

 two, or many pieces, serving for the protection of the soft 



