GENERAL CHARACTERS. 
2 9 3 
what branched and knotty, which communicate with the exterior at the angles 
between the rays. These are the generative glands. In all Echinoderms, except 
sea-cucumbers, these glands are affected by the radiate structure of the animals; in 
crinoids the generative products are even produced in the extremities of the arms. 
Distinction of Having glanced at those points of structure in which Echinoderms 
the Classes, resemble one another and differ from the rest of the animal kingdom, 
we may shortly examine the main characters in which a sea-urchin, a star-fish, a 
crinoid, a brittle-star, and a sea-cucumber differ from one another. First may be 
noted obvious differences in form and in position in the living state. In an ordinary 
sea-cucumber (as shown in the illustration on p. 291) the body is cucumber¬ 
shaped, with the mouth at one end and the vent at the other; between these run 
the five ambulacra, one or two of which are often more developed than the others, 
so that the animal crawls along on that side of its body, with its mouth foremost. 
A sea-cucumber has no arms or projecting rays, but its mouth is surrounded by a 
circlet of tentacles, often branched, which can be retracted at will. A regular 
sea-urchin, such as the sea-egg {Echinus), shown in a later figure, resembles 
a sea-cucumber in being without projecting rays; but it is more spherical in shape, 
and moves with its mouth towards the sea-floor. On the other hand, in a heart- 
urchin (Spatangus), which moves through and swallows mud and sand, the body 
has become transversely elongate; that is to say, the long axis is at right angles 
to the position it occupies in a sea-cucumber; the mouth having moved a little 
forward, and the vent being transferred from the top of the body to its lower 
surface, so that both the mouth and vent lie on the under surface, at either end 
of the long axis. In a star-fish, as in a regular sea-urchin, the mouth is in the 
centre of the under surface, while the vent is almost in the centre of the upper 
surface, although absent in a few forms. The body is either markedly pentagonal 
in outline, or more or less star-shaped. In the latter case it is said to consist 
of a central disc extended into arms, as in the illustration on p. 304. The number 
of these arms varies from five (Aster ias) to over forty ( Heliaster ); but in each 
species with more than six arms the number may vary slightly, although constant 
during the life of the individual; in Lcihicliaster, however, fresh arms grow 
out even in the adult. A brittle-star (illustrated on p! 291) resembles a star-fish 
in which there is a sharp distinction between arms and disc; the mouth being 
on the under surface, but the vent wanting. And whereas the arms of a star-fish 
are simply extensions of the body, containing the generative glands and processes 
from the stomach, those of a brittle-star are mere appendages to the body, with a 
stout internal skeleton of separate ossicles, working on one another by well- 
developed muscles, and containing only blood-vessels, water-vessels, and nerves. 
The arms of the brittle-stars are nearly always five in number, though sometimes 
there may be from six to eight. As in the star-fish, the arms are unbranched, 
except in the family Astrophytidce, where they fork ten or twelve times, and 
where the numerous branches interlace so as to form a kind of basket-work all 
round the disc, whence these animals are called basket-fish, or medusa-head star¬ 
fish. A crinoid (illustrated on p. 297) differs markedly from a sea-urchin, star-fish, 
or brittle-star, in that the mouth faces upwards; the vent being also on the upper 
surface. This position is due to the fact that, so far as we know, all crinoids are 
