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. 



All Echinoderms live in the sea, where they find in solution the 

 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 Echinosphoera, 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 

 Glyptosphcera, 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 flat circular forms, which live attached by their under 

 side to the flat 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 



