208 ELEMENTARY ZOOLOGY. 



between the surfaces. How many different types of spines 

 do you find? Are the spines arranged in the same way 

 everywhere? What portions of the body have no spines? 

 What takes the place of the spines here? Are these struc- 

 tures hard or soft in life? What is the appearance of the 

 madreporic body under a lens? Are there any smaller 

 structures among the spines on the aboral surface? Ex- 

 amine with a lens. 



Make a sketch from the aboral view, drawing the 

 spines on the disc and one arm. Make a similar diagram 

 of the oral surface showing the mouth, the ambulacral 

 groove, the ambulacral feet, etc. (See Fig. 62.) 



225. A Library Exercise. A comparative study of the general 

 form and external features of the five representatives. 



Rule a sheet of paper with five vertical columns. At the head of 

 these columns put the words starfish, sea-urchins, brittle-stars, 

 sea-lilies, sea-cucumbers. By reference to figures and descriptions 

 in the texts, determine the condition in each of these groups with 

 respect to the following features, and insert the facts under their 

 appropriate columns: 



1. The symmetry and general form of the body. Length and 

 position in nature of the oral-aboral axis. 



2. The degree of development of the arms, their form, size, and 

 relation to the disc. 



5. Degree of development and character of the skeleton and spines. 



4. Position of the mouth and anus. 



5. Position of the madreporic body. 



6. Character, position, and use of the ambulacral feet. 



226. The Interior of the Body. As in the earthworm, if one cuts 

 through the body wall (which is made up of the skin and the muscular 

 elements among which bony plates may be formed) he finds a large 

 body cavity in which the internal organs lie. This body cavity 

 is largest in the disc; it may or may not pass out into the arms. It 

 contains a fluid similar to that found in the poorly developed blood 

 vessels. The principal organs found in the body cavity are the 

 digestive organs and the reproductive organs (Figs. 63 and 64). 



227. The Digestive System. The nature of. the digestive tube in 

 animals is always adjusted to the nature of the food used. In the 



