CYSTIDS. 



295 



one or other of the plans of structure just described. Moreover, the animals 



formed upon any one of these plans are found to agree with one another and to 



differ from the rest in yet other features. Hence zoologists have divided the 



Echinoderms into seven classes, each of which is again divided into orders. 



„ M „ - All Echinoderms live in the sea, where they find in solution the 



Mode of Life. . J 



lime-salts from which their skeletons are built. None have become 



modified for a truly fresh-water existence, and in this respect they are peculiar 



among animals; a few holothurians, however, are found in the mud of some 



estuaries and brackish- water lagoons, while a star-fish (Asteracanthium) and a 



brittle-star (Ophioglypha) occur in the brackish waters of the Eastern Baltic. 



Neither can Echinoderms live on land, and though they may exist for a short time 



out of the water when left by tides, still it is only in the water that they can 



breathe or feed. In the sea, however, they have a universal distribution ; from 



ice-bound seas to the Equator ; from shallow shore-pools to mid-ocean ; from the 



surface to the abyss ; on rocky shores, sandy beaches, muddy shoals, and bottom 



oozes, among the roots of the mangrove, or in the meadows of seaweed. This 



universal distribution renders their study one of importance for the geologist, 



especially as their calcareous skeletons are readily preserved as fossils. Their 



remains are known from rocks of every age in which animals are known to have 



existed, and even the spicules of sea-cucumbers have been found as far back as the 



Carboniferous period. Moreover, the rapidity of evolution in the group, and the 



short period of time during which any one species was in existence, combined with 



the wide area of distribution possessed by many species, render these fossils of 



great value for the correlation of strata in different countries. 



The Cystids, — Class Cystidea. 



The Cystidea have been extinct since the Carboniferous period. Not only 

 are they among the oldest animals, but there is reason to suppose that they 

 approach more nearly the primitive forms from which all the classes of the 

 Echinoderms were derived. Many have not that regularity of symmetry which 

 characterises later Echinoderms. Such forms as Echin o*j >h cera, commonly called 

 the crystal-apple, are mere round balls composed of a number of plates in which 

 it is hard to see any arrangement. Some of them seem to have been unstalked, 

 while in others the stalk is quite short. The arms are short, and vary in number, 

 bearing but slight relation to the plates of the test. In some, however, such as 

 Glyjytosphcera, the ambulacral grooves, though rather irregular, are five in number 

 and lie on the surface of the test, all meeting at the mouth, which is placed in the 

 centre of the upper surface. Other cystids seem to be composed of an irregular 

 number of plates; but they have become more definitely radiate in structure. 

 Some, like Agelecrinus, are fiat circular forms, which live attached by their under 

 side to the fiat surfaces of shells, and which have five distinct ambulacral grooves 

 radiating from the central mouth on the upper side ; while others, like Mesites,- 

 which resembles Agelecrinus in the arrangement of its grooves, — were attached, if 

 at all, by only a small part of the under side. Yet other cystids are definitely 

 attached by well-developed stalks, and have their bodies enclosed by a limited 



