ECHINODEEMA — CYSTIDS. 
65 
special o])eniu^ found in the hard body-wall ot tliese forms 
served for the extrusion of the eggs ; and we infer that the 
necessary process of respiration was effected by means ot 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 ot the 
Cystidea, as of most groups of animals, is the history ot 
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 
common in their country and found also in our Welsh Ordo- 
vician rocks, namely Ecliinosioliaera (Fig. 29 5) and Sphacronis, 
or the more advanced Frotocrimis (Fig. 29 c). 
The numerous plates of Ecliinosphacra appear to be joined 
by fine lines, whicli represent canals in their substance and 
are arranged in rhombs. In Sphaeronis and rrotoo’imis 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 Frotocrinus they pass over the surface of the plates 
before reaching the brachioles. Turn next to the Wenlockian 
Lcpadocrinus, of which a reconstruction is here drawn 
(Fig. 29 d), and note that the canals joining the plates are 
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 Froteroblasius, in which there are five 
food-grooves passing riglit 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 Flacocystis respectively represent the Cam- 
brian, Ordovician, and Silurian stages of evolution. The two 
F 
Gallery 
VIII. 
Wall-case 
18 . 
Table-case 
30 . 
Table-case 
30 . 
