262 PHYLUM ECHINODERMA. 
arms. They agree with starfishes in being free, in having 
radially disposed gonads, in having the tube-feet restricted to 
the under surface, and in other features. 
The body of a brittle-star differs from that of a star- 
fish in the abruptness with which the arms spring from 
the central disc (cf. Brisinga). These arms are muscular, 
and useful in wriggling and clambering; they do not con- 
tain outgrowths of the gut, nor reproductive organs. 
Moreover, there is no ambulacral groove, and the tube-feet 
which project on the sides are usually very small. They 
are often of locomotor service, adhering even to vertical 
surfaces, but in many cases they seem to be only sensory. 
Each segment of the arm includes a central “vertebral 
ossicle,” with four plates forming a tube round about it. 
There is a complete oral skeleton. .The madreporic plate 
is situated on the ventral surface, usually on one of the 
plates around the mouth. The food canal ends blindly. 
Some brittle-stars have small luminescent glands, e.g. 
Amphiura squamata. The reproductive organs lie in 
pairs between the arms, and open into pockets or bursze 
formed from inturnings of the skin, which communicate 
with the exterior by slits opening’ at the bases of the arms. 
Water currents pass in and out of these pockets, which 
probably have both respiratory and excretory functions. 
The free-swimming larva is a luteus, very like that of 
Echinoids (see Fig. 131). 
Ophiuroids are first found in Silurian strata. 
The Ophiuroids are usually classified according to the characters of 
their ossicles and covering plates. Some common genera are Ophiothrix, 
Ophiocoma, Ophiopholis, Ophiura. In the deep-water Astrophyton and 
Gorgonocephalus the arms are repeatedly branched. In <Astronyx 
Jovenz, often caught in the trawl off the north coast of Britain, the disc 
is relatively large and soft and the arms very long. In the extinct 
Lysophiuree there is an ambulacral groove. 
Class EcuinoipEA. Sea-Urchins, e.g. the common 
Lchinus esculentus 
Lchinoderms with the body covered by rows of plates, 
usually in vertical series and forming an inflexible test; the 
shape of the majority approaches a sphere, but some are pin- 
