88 Guide to the Invertehrata. 



III.— ECHINODERMA. 



GALLEEY The Starfish, the Sea-rrchin, the Brittle-Star, the Feather- Star, 

 ^^^^- and the Sea-Cucumber, all of which live in modern seas, are examples 

 WalUases ^^ ^|^g subkingdom Echinoderma. Though differing from one another 

 Table' in outward appearance, they resemble one another and differ from 

 *J"®' l'^" the rest of the animal kingdom in the following characters. 



The soft tissues of the animal have the power of depositing 

 crystalline carbonate of lime. This may remain in the shape of 

 minute separate spicules ; or the spicules may grow together into 

 a trellis-work, which forms rods and plates. The deposit is usually 

 most abundant in the skin, Avhere it may be built into a continuous 

 skeleton, such as the test of the sea-urchins. Often, too, spines of 

 the same carbonate of lime are borne outside the test. This feature, 

 so characteristic of the sea-urchin, has given to the subkingdom 

 the name Echinoderma {ix^vo^ ' an urchin,' and Bdpfia ' skin '). 



The next feature noticeable in an echinoderra is its radiate 

 structure, in many cases giving the animal a star-shape, to which 

 the common names starfish, brittle-star, and the like, are due. 

 The number of rays is usually five. The radiate arrangement 

 always affects the skin and test and the organs connected there- 

 with ; but it may also affect the internal organs. In a starfish, 

 for instance, nearly all the organs of the body are arranged in 

 a radiating manner around a central axis in such a way that the 

 animal can be divided into five similar portions. This radiate 

 symmetry must not be confused with that seen in the sub- 

 kingdom Coelentera (which includes corals and jelly-fish), since 

 the Echinoderma differ from those animals in having, as in all 

 liigher subkingdoms, their digestive apparatus in the form of a 

 gut entirely shut off from the main body-cavity, while a system 

 of branched tubes or spaces carries hlood through the body. 



The third feature characteristic of the Echinoderma is a system 

 of sacs, canals, and tubes, that carry water through the body. 

 Small hollow processes, called podia, are given off from this 

 system, and pass through holes in the test. These holes are 

 arranged in five double rows, called avenues or ambulacra, which 

 definitely mark the five rays of the animal. The processes may 

 be retracted, or may be stretched out by the squeezing of water 

 into them. Some of the processes, or podia, end in small sucker- 

 like discs, which the animal can attach to smooth surrounding 



