168 



INVERTEBRATE ZOOLOGY 



sea urchins, Frank Lillie developed an explanation of why the 

 sperm cell moves toward the egg cell. A specific substance 

 called fertilizin is given off by the eggs into the water and serves 

 as a chemical stimulus to direct the sperm cells to the eggs. The 

 nature of the ultimate structure of protoplasm and the role of 

 membranes in fertilization and in other life processes have been 

 largely conducted upon echinoderm eggs. Jacques Loeb used 

 sea urchin eggs in his carefully devised experiments on partheno- 

 genesis to determine the factors that induce eggs to undergo 

 cleavage without fertilization. These are only samples of the 

 hundreds of researches in which the germ cells of echinoderms 

 have played an important part in developing our ideas of living 

 matter. 



Orientation. — The main axis of the body extends between the 

 oral and aboral poles. This arbitrary morphological orientation 

 is frequently not in accord with the natural or physiological orien- 

 tation as determined by the natural position of the body of the 

 living animal. In Asteroidea, Ophiuroidea, and Echinoidea 



the oral surface is ventral 

 in position, while in the 

 Crinoidea the mouth is 

 directed upward, and in 

 the Holothuroidea the 

 chief axis is parallel to 

 the surface on which the 

 animal rests. Typically, 

 the parts of the body are 

 arranged in radial manner 

 about the main axis. In 

 most echinoderms, the 

 ambulacral system occu- 

 pies certain radial regions 

 of the body where the 

 skeletal plates are perforated for the tube feet. These regions 

 are designated the ambulacral areas, while the plates between 

 the ambulacral areas constitute the interambulacral areas. 

 Many of the echinoderms have on the oral and aboral surfaces 

 series of plates which seem to be constant enough in their 

 appearance in the various groups to be considered homologous. 

 The crinoids and the extinct Mastoids and cystoids are seden- 

 tary. Coi related with this habit, they possess a stalk of attach- 



FiG. 83.— Starfish devouring an oyster. 

 (From Linville, Kelly, and Van Cleave, 

 General Zoology). 



