290 



ECHINODERMS. 



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 deposit spicules 

 among 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 attected in 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 



anchor sea-cucumber (Synapta), 



