CHAPTER IV. 



BEANCH IV.— ECHINODERMATA (Star-fish, Sea- 

 UECHiNS, Sea-cucumbers, etc.) 



General Characters of Eohinoderms.— We now come to 

 animals of much more complicated structure than any of 

 the foregoing branches, and in which the radiated arrange- 

 ment 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), di- 

 vided into five wedge-shaped portions (sph^omeres), or pro- 

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

 finger, but the nervous system, the reproductive organs, 

 the blood and water-vascular systems, and the locomotive 

 appendages of the latter, are usually arranged in accordance 

 with the externally radiated form of the body. Still these 

 animals are in many cases, as in the higher sea-urchins, 

 plainly bilateral, while in the larval forms of all Echino- 

 derms whose development is known the young are not 

 radiated, but more or less bilateral, as in the larvae of worms 

 and moUusks. The most trenchant character, however, 

 separating the Echinoderms from the Ccelenterates, and ally- 

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

 canal which lies free in the body-cavity (perivisceral cavity), 

 and may be several or many times the length of the body. 



The student can gain a correct idea of the general struc- 

 ture of the Echinoderms from a careful examination of the 

 common star-fish {Asterias vulgaris Stimpson), which is the 

 most common and accessible Echinoderm to be found on the 

 New England shores. After placing a star-fish in some sea- 

 water and noticing its motions, the thrusting out of the am- 

 bulacral feet or suckers by which it pulls or warps its clumsy 



