Io4 INVERTEBRATE ANIMALS. 
CLass I. ECHINODERMATA. 
The members of this class are popularly known as sea-urchins, 
star-fishes, brittle-stars, feather-stars, sea-cucumbers, &c., and 
derive their name of Echinodermata (Gr. echinos, a hedgehog ; 
and derma, skin) from the generally prickly nature of their in- 
teguments. In all, the skin is possessed of the power of secret- 
ing carbonate of lime, but in very different degrees. In the 
. sea-urchins this goes so far that the body becomes enclosed in 
an immovable box, composed of numerous calcareous plates 
firmly jointed together. In the star-fishes and their allies the 
skin is rendered prickly by grains, tubercules, or spines of cal- 
careous matter, and the body is either destitute of regular plates 
or is only partially enclosed by them. In the sea-cucumbers, 
again, the calcareous matter is only present in the form of 
minute grains scattered in the skin. When adult, they all show 
amore or less distinctly radiate structure, which is most con- 
spicuous in the star-shaped star-fishes and sand-stars, but can 
be detected in all the members of the class. When young, 
however, they almost always exhibit what is called “bilateral 
symmetry "—that is to say, they show similar parts on the two 
sides of thebody. In all Echinoderms there is a water-vascular 
system of tubes, which is termed the “ ambulacral system,” 
which generally communicates with the exterior, and which in 
most cases is used in locomotion. An alimentary canal is 
always present, and is always completely shut off from the 
general cavity of the body. A vascular or circulatory system 
is sometimes present. There are always distinct organs of re- 
production, which are almost always placed in different indivi- 
duals, so that the sexes are distinct. The nervous system is in 
the form of a ring surrounding the gullet and sending branches 
in a radiating manner to different parts of the body. 
The Echinodermata are divided into seven orders, as follows: 
. Echinoidea (Sea-urchins). 
. Asterotdea (Star-fishes). 
. Ophiurotdea (Sand-stars and Brittle-stars), 
. Crinoidea (Feather-stars). 
. Cystotdea (extinct). 
. Blastoidea (extinct). 
- ffolothuroidea (Sea-cucumbers). | 
NM OAmBWN & 
