52 



ZOOLOGY 



edge of the umbrella, and the radial canals, which communicate 

 with the cavity of the oral cone on the one hand and the 

 circular canal on the other. On the aboral surface of the 

 endodermal lamella the middle layer usually becomes much 



Fro. 39, 



thickened, its gelatinous substance may remain structureless, 

 or cells may invade it. 



Tubularia is one of those Rydromedusae which do not 

 give rise to free medusae. The medusa is produced, but does 

 not break away, it remains connected with the hydranth by 

 the aboral pole. In a mature individual a racemose extension 

 of the body-waU is formed between the two rows of tentacles. 



