iv PHYLUM CCELENTERATA in 



Each gonad(W*.)is a horseshoe-shaped frill-like structure 

 situated on the floor of the gastric pouch. When mature, its 

 products ova or sperms are discharged into the stomach, 

 and pass out by the mouth. The sexes are lodged in 

 distinct individuals. 



Lying parallel with the inner or concave border of each 

 gonad is a row of delicate filaments supplied with stinging- 

 capsules. These are the gastric filaments : their function 

 is to kill or paralyse the prey taken alive into the stomach 

 (compare Fig. 5i,./). 



The development and life-history of Aurelia present 

 several striking and characteristic features. The impreg- 

 nated egg-cell or oosperm becomes converted into a closed 

 two-layered sac or planula (Fig. 50, A), similar to that of 

 a Hydrozoon. The planula swims about by means of the 

 cilia with which its ectodermal cells are provided, and, after 

 a brief free existence, settles down, loses it cilia, and be- 

 comes attached by one pole. At the opposite pole a mouth 

 is formed. On two opposite sides of the mouth hollow 

 processes grow out, forming the first two tentacles ; soon 

 two others appear at right angles to these. Subsequently 

 other tentacles appear. At the same time the attached or 

 proximal end is narrowed into a stalk-like organ of attach- 

 ment (E). 



The outcome of all these changes is the metamorphosis 

 of the planula into a polype (F), not unlike a Hydra. The 

 Scyphozoon-polype is called a Scyphnla. The Scyphula some- 

 times multiplies by budding. After a time it undergoes a 

 process of transverse fission (G), becoming divided by a series 

 of constrictions which deepen until the polype assumes the 

 appearance of a pile of saucers, each with its edge produced 

 into eight bifid lobes. Soon the process of constriction is 

 completed, the saucer-like bodies separate from one an- 



