GENERAL CHARACTERISTICS 



3257 



by a deposit of similar structure. Although, as has been said, each element of the 

 skeleton follows the laws of the typical crystallization of carbonate of lime, yet the 

 structure of the trellis work varies greatly, and is often characteristic 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 ra- 

 diate structure in many cases giving to the 

 animal a star shape, to which the common 

 names starfish, brittle star, and the like are 

 due. The ordinary red starfish 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 arrange- 

 ment of the organs in the majority of the 

 Echiuoderms. It can be detected even in 

 a sea cucumber or holothurian, where, be- 

 side 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. The internal organs, 

 as will be seen later on, are variously 

 affected in the various classes of the Echino- 

 derms by this five-rayed symmetry. A 

 radiate arrangement is not, however, con- 

 fined to Echinoderms, as it also occurs in 



jellyfish 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 jellyfish, 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 

 \vith in the preceding pages, whereas the jellyfish and their allies differ from .them 

 in having no body cavity separated off from the stomach and its processes. More- 

 over, Echinoderms resemble the higher animals in the possession of a system of 

 branched tubes conveying blood through the body. 



Examining a starfish 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 

 surrounding objects; and it is thereby able either to hold itself firm against waves 



ANCHOR SEA CUCUMBER (Synapto). 

 Tentacles round the mouth ; e. Anchor and plate- 

 shaped spicules ; b, c, d. Similar spicules of an 

 allied form. 



