3262 ECHINODERMS 



name is applied differ greatly from one another in structure. Echinoderms are built 

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

 formed upon any 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 

 1 e 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 starfish (Asteracanthium] and a brit- 

 tle 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 icebound 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 distribu- 

 tion renders their study one of importance for the geologist, especially as their cal- 

 careous 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 dur- 

 ing which any one species was in existence, combined with the wide area of dis- 

 tribution possessed by many species, render these fossils of great value for the 

 correlation of strata in different countries. 



THE CYSTIDS Class Oystidea 



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 characterizes later 

 Echinoderms. Such forms as Echino splicer a, commonly called the crystal apple, are 

 mere round balls composed of a number of plates in which it is hard to see any ar- 

 rangement. 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 rela- 

 tion to the plates of the test. In some, however, such as Glyptosph&ra, the ambu- 

 lacral 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 sur- 

 face. 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 



