ECHINODERMATA. 10g 
in the sea, especially in the neighbourhood of oyster-banks. 
Others spend their existence buried in the sand; and one 
species excavates holes for itself in the solid rock, apparently 
by some mechanical action. 
OrDER IJ, ASTEROIDEA.—As the structure of the sea-urchins 
may be taken as embodying the most important anatomical 
peculiarities of the Echinodermata, and as this has been de- 
scribed at some length, it will not be necessary to do more than 
briefly indicate the more important characteristics of the remain- 
ing orders. In the present order are included all the true star- 
fishes, the sand-stars and brittle-stars being generally regarded 
as a distinct group. The body in all the Aszerozdea is more 
Fig. 44.—Cribella oculata (after Forbes). 
or less obviously star-shaped (fig. 44), consisting of a central 
disc surrounded by five or more lobes or arms, which radiate 
from the body, are hollow, and contain prolongations from the 
stomach. The body is not enclosed in an immovable box or test, 
as in the sea-urchins, but the integument is of a leathery nature, 
and is richly furnished with calcareous plates, tubercles, and 
spines. The true star-fishes are distinguished from the nearly 
allied brittle-stars (Ophiurozdea) by the fact that the arms are 
direct prolongations of the body, that they contain prolonga- 
tions of the stomach, and that they are deeply grooved on their 
under surfaces for ‘the radiating vessels of the water-vascular 
