THE COELENTERATES AND CTENOPHORES 95 



Medusae of the Scyphozoa are often less that an inch in diameter, 

 though one species of Cyanea on the Atlantic coast is said to 

 reach a diameter of 7 feet across the bell and to have tentacles 

 120 feet long. The scyphopolyp or scyphistoma when present is 

 distinguishable from the hydropolyp through the presence of 

 four longitudinal folds of the entoderm called the taeniolae. In 

 the development of the medusae, these form the gastral tentacles 

 or phacellae. When the germ cells leave the gonade they are 

 liberated into the gastrovascular cavity, not directly to the 

 exterior as in the Hydrozoa. The scyphomedusa lacks a velum, 

 has a more or less notched margin, and bears entodermal gonads. 

 While Scyphozoa are usually dioecious, some genera are herma- 

 phoroditic. Form and structure of the medusa are more readily 

 understood when the development has been outlined. 



Fig. 54. — Larval stages in the development of Aurelia. A, scyphistoma; B, 

 strobila; C, ephyra. (Orig.). 



Life History. — The medusae of Aurelia produce germ cells, 

 which, upon fertilization, undergo cleavage and develop into a 

 ciliated larva called a planula. After a brief free existence, the 

 planula becomes attached to some object and transforms into an 

 individual superficially like a Hydra, to which the names scyphi- 

 stoma (Fig. 54 A) or scyphopolyp have been apphed. By a 

 series of transverse constrictions, the scyphistoma becomes 

 separated into a pile of saucerlike structures (Fig. 54 B) and in 

 this stage is called a strobila. As the constrictions become 

 deeper, a series of ephyrae become detached from the strobila. 

 At the time of their liberation, each ephyra (Fig. 54 C) has eight 

 prominent armlike projections arranged radially about the mar- 

 gin of the body. Growth of the medusa from this ephyra 



