290 
E CHINODERMS. 
internal organs being often strengthened by a deposit of similar structure. 
Although, as has been said, each element of the skeleton follows the laws of 
the typical crystallisation of carbonate of lime, yet the structure of the trellis- 
work varies greatly, and is often character¬ 
istic of the species in which it occurs. 
Thus, the species of sea-cucumber can be 
distinguished by the shape of their spicules; 
and the same is said to be the case with 
those sea - urchins that 
amono- their viscera. 
The next feature noticeable is the 
radiate structure, in many cases giving to 
the animal a star - shape, to which the 
common names star-fish, brittle - star, and 
the like are due. The ordinary red star¬ 
fish, or cross-fish, of the English coasts has 
five distinct rays, or arms; and this number 
five, to a greater or less extent, controls 
the arrangement of the organs in the 
majority of the Echinoderms. It can be 
detected even in a sea-cucumber or holo- 
thurian, where, beside the feathery tentacles 
of the head, are rows of shorter sucker-like 
processes, which extend the length of the 
body ; these rows being five in number. 
a, Tentacles round the mouth ; e. Anchor- and plate- The internal organs, as will be seen later 
shaped spicules; b, c, d, Similar spicules of an , ™ . ,, 
allied form. on, are variously affected m the various 
classes of the Echinoderms by this five- 
rayed symmetry. A radiate arrangement is not, however, confined to Echino¬ 
derms, as it also occurs in jelly-fish and sea-anemones. Hence those animals 
were once grouped with the Echinoderms, under the title of Radiata. But, if a 
sea-cucumber or a sea-urchin be opened, there is a marked distinction between 
it and a jelly-fish, in the presence of an intestine, shut off from the rest of the 
body-cavity, and often coiling round inside it. In this respect the Echinoderms 
resemble all the animals that have been dealt with in the preceding pages, 
whereas the jelly-fish and their allies differ from them in having no body-cavity 
separated off from the stomach and its processes. Moreover, Echinoderms resemble 
the higher animals in the possession of a system of branched tubes conveying blood 
through the body. 
Examining a star-fish or a sea-urchin, one sees, on the under surface of the rays 
in the former, and passing in five bands from top to bottom of the latter, a number 
of small cylindrical processes, which are usually gently waving about like trees in 
a wind. They lie in each band, or in each ray, along two rows, with a clear space 
between, like trees on either side of an avenue; hence the whole band of them in 
each ray is called an ambulacrum (garden-walk). Most of these little processes 
end in sucker-like discs, which the animal can stretch out and attach to smooth 
deposit spicules 
anchor sea-cucumber ( Synapta ). 
