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 
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 liolothurians, 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 Echi/nosphcera, 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 
