74 



ANIMAL BIOLOGY 



derm and endoderm so that the body wall is relatively thick. The 

 mouth is in the center of the under (oral) surface, opening into 

 a large enteric cavity which branches into radial canals that run 

 to the circumference, or 'rim,' where they merge into a circular 

 canal. Thus digested food is carried directly to all parts of the 

 animal without the necessity of a special circulatory system. Sur- 

 rounding the mouth are four ribbon-like oral arms which, to- 



Gonad 



Enteric 

 cavity 



Gonad 



Radial canal 



Circular canal 



Tentacles 



Sense organs 



Fig. 39. — Aurelia, lateral view; one quadrant shown in section. 



Parker and Haswell.) 



(From 



gether with the fringe of tentacles about the rim of the animal and 

 certain filaments in the radial canals, are well supplied with nema- 

 tocysts for stinging the prey. (Fig. 39.) 



Among the tentacles on the rim are small rounded sense organs 

 which control, through the underlying nerve cells and muscle 

 bands, the movements of the animal. Contraction of the rim of 

 the bell, slowly and rhythmically, presses the water within the 

 bell against that without and so forces the animal along. But 

 Aurelia is largely at the mercy of wind and wave — few being 

 destined to reproduce before dissolution, and all being destroyed 

 by winter. 



The reproductive organs of Aurelia consist of four large U- 

 shaped gonads which shed their products, either eggs or sperm, 

 into the radial canals and so to the outer world through the mouth. 

 Fertilization takes place in the open water to form a zygote that 

 develops into a free-swimming larva. This soon settles to the 

 bottom, becomes attached, and assumes a polyp-like form which 

 is comparable to the hydroid generation of the Hydrozoa. After 



