CHAPTER V. 



Bkanch V. — EcHiNODEKJrATA {Star-fisfi, Sea-nrchins^ 

 Sea-cucuniljers, etc.). 



General Chaeactees of Echinoderms. — We now come 

 to uiiimals of mncli more complicated structure than any 

 of the foregoing branches, and in whicli the radiated ar- 

 rangement of the parts of the body is in most cases as 

 marked as the jointed or ringed structure of worms or 

 insects; for not only are the body-walls of the star fish or 

 sea-urchin, or even many of the Holothurians (though less 

 plainly), divided into five wedge-shaped portions, or pro- 

 duced into five arms as in the common star-fish or five- 

 finger, but the nervous system, the reproductive organs, 

 tlie blood and water-vascular systems, and the locomotive 

 organs, are usually arranged in accordance with the star- 

 like form of the body. The most trenchant character which 

 separates the Echinoderms from the Coelenterates, and 

 allies them to the worms, is the genuine tube-like digestive 

 canal which lies free in the body-cavity, and may be sev- 

 eral or many times the length of the body. 



The student can gain a correct idea of the general structure of the 

 Echinoderms from a careful examination of the common star-fish 

 {AxteriiiH vulgaris), wliich is the most conunou and accessible Echiuo- 

 derm to be found on the New Eughuid sliores. After phicing a star- 

 fish in some sea-wuler and noticing its motions, the thrusting out of 

 the ambulacra] feet or sucliers by which it pulls or warps its clumsy 

 body over the mussel-beds, or rocks, or weeds, the arms being capa- 

 ble of slightly bending; after ot)serving the red eye spot at tlie end 

 of each arm or ray, and the movements of tlie numerous spines whicli 

 are attached by a sort of ball-and-socket joint to the calcareous frame- 

 work of tlie body-walls, and examinuig the movements of certain 

 modified spines coMgA pediceUarim, whicli are pincer-likc bodies situ- 



