ECHINODEEMA — CYSTIDS. 



65 



special opening found in the hard . body-wall of these forms Gallery- 

 served for the extrusion of the eggs ; and we infer that the VIII. 

 necessary process of respiration was effected by means of the 

 pores which penetrate the plating of the sack or theca. 

 Feeding and breathing are the two processes without which 

 the life of an animal must stop, and the history of the 

 Cystidea, as of most groups of animals, is the history of 

 changes by which these processes were ever better and better 

 carried out. Let us briefly contrast in these respects the two 

 forms of the Crystal-apple, as the Swedes call a fossil so Wall-ease 

 common in their country and found also in our Welsh Ordo- rpg^^Jg Qg^gg 

 vician rocks, namely Echinospliaera (Fig. 29 &) and Splmeronis, 39^ 

 or the more advanced Protocrinus (Fig. 29 c). 



The numerous plates oi Echinosphaera appear to be joined 

 by fine lines, which represent canals in their substance and 

 are arranged in rhombs. In Sphaeronis and Frotocrinus a 

 number of small ovals are scattered over the plates, and each 

 of these contains two vertical canals meeting near the inner 

 surface. Both of these structures appear to have contained 

 spaces that brought the soft tissues and interior fluids of the 

 animal close to the outer aerated sea-water. In Echinosphaera 

 the food-grooves were borne entirely on separate skeletal 

 pieces ; in Protocrinus they pass over the surface of the plates 

 before reaching the brachioles. Turn next to the Wenlockian 

 Lepadocrinits, of which a reconstruction is here drawn Table-case 

 (Fig. 29 d), and note that the canals joining the plates are 30. 

 now visible only in three places, where they are intensified 

 as deep folds ; here too the food-grooves are carried far over 

 the surface on a series of special pieces from which the 

 brachioles arise ; to keep these away from the mud, the 

 whole body is now raised on a stem. This then is a develop- 

 ment of the Echinosphaera type. The other line of evolution 

 leads to a form like ProteroUastus, in which there are five 

 food-grooves passing right down the actual surface of the 

 plated sack or theca, and bordered regularly by brachioles ; 

 the double canals are here concentrated on the plates that 

 bear the brachioles. On these two distinct lines of evolution 

 are based the two Orders : Rhombifera (with canals or folds 

 in rhombic pattern) and Diploporita (with canals opening 

 in double pores). 



Notable Cystidea, perhaps to be regarded as a distinct 

 Order, are the Anomalocystidae^ of which Trochocystis, 

 Mitrocystis, and Placocystis respectively represent the Cam- 

 brian, Ordovician, and Silurian stages of evolution. The two 



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