n6 ELEMENTARY ZOOLOGY 



for the most part of green algae and the red sea-weeds. 

 Correlated with this difference in food-habits there are 

 certain differences in the structure of the internal organs. 

 For example, the alimentary canal in the sea-urchin winds 

 in about two and one-half turns within the body-cavity 

 before it reaches the anus. 



OTHER STARFISHES, SEA-URCHINS, SEA-CUCUMBERS, 



ETC. 



Without exception all the Echinoderms, under which 

 term are included the starfishes, sea-urchins, brittle-stars, 

 feather-stars, and sea-cucumbers, live in the ocean. Some 

 of them, the starfishes and sea-urchins, are among the 

 most common and familiar animals of the seashore. Most 

 of them are not fixed, but can move about freely, though 

 slowly. Some of the feather-stars are fixed, as the 

 sponges and polyps are. 



Shape and organization of body. The body-shape of 

 the Echinoderm varies from the flat, rayed body of the 

 starfish to the thick, flattened egg-shape of the sea-urchin, 

 the melon-like sac of the sea-cucumber and the delicate 

 many-branched head of the sea-lily sometimes borne on 

 a slender stalk* But in all these shapes can be seen more 

 or less plainly a symmetrical, radiate arrangement of the 

 parts of the body. The Echinoderm body has a central 

 portion from which radiate separate arm or branch-like 

 parts, as in the starfishes and sea-lilies, or about which 

 are arranged radiately the internal body-parts, although 

 the external appearance may at first sight give no plain 

 indication of the radiate arrangement. This is the case 

 with the sea-urchins and sea-cucumbers, yet, as has been 

 seen in the sea-urchin, the radiate arrangement can be 

 readily perceived by closer exanrnation of the surface of 

 the egg- or sac-like body. The radiating parts of the 



