CHAPTER IV. 
Branch IV.— Echi]^odermata {Star-fish, Sea-urchins, 
Sea- cucumbers, etc. 
General Characters of Echinoderms.— We now come 
to animals of much more complicated structure than any 
of the foregoing branches, and in which 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, 
the 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-fisli 
{Aste7'ias vulgaris), which is the most common and accessible Echino- 
derm 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 ambulacral feet or suckers 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 observing the red eye-spot at the end 
of each arm or ray, and the movements of the numerous spines which 
are attached by a sort of ball-and-socket joint to the calcareous frame- 
work of the body-walls, and examining the movements of certain 
modified spines csMed pedicellarice, which are pincer-like bodies situ- 
