132 COLLEGE ZOOLOGY 



An alternation of generations occurs in Aurelia, but the hydroid 

 stage is subordinate. The eggs develop into free-swimming 

 planulae which become attached to some object and produce 

 hydra-like structures, each of which is called a hydra-tuba 

 (Fig. 81, A). This buds like Hydra during most of the year, 

 but finally a peculiar process called strobilization takes place. 

 The hydra-tuba divides into discs which cause it to resemble 

 a pile of saucers (B); at this stage it is known as a strobila. 

 Each disc develops tentacles (C), and, separating from those 

 below it, swims away as a minute medusa called an ephyra 

 (D, E). The ephyra gradually develops into an adult jellyfish. 



b. Classification of the Scyphozoa 



Four orders of Scyphozoa are usually recognized. The most 

 obvious ordinal characteristics are the presence or absence of 

 stomodaeum and mesenteries, and the position of the tentacles 

 and tentaculocysts. The slomodceum or gullet is a passageway 

 between the mouth and the gastro vascular cavity or " stomach"; 

 it is often held in place by membranes called mesenteries. The 

 position of the tentacles and tentaculocysts is described with 

 regard to their relation to the four radial canals. Those at the 

 ends of the radial canals are said to be perradial (Fig. 79, p.r.c) ; 

 those halfway between two perradii are called interradial (i.r.c) ; 

 and those halfway between a perradius and an interradius are 

 termed adradial (a.r.c). 



Order 1. Stauromedusae. — Scyphozoa without tentacu- 

 locysts; tentacles perradial and interradial; umbrella goblet- 

 shaped; sometimes attached by the aboral pole; a stomodaeum 

 is present, suspended by four mesenteries; no alternation of 

 generations. Examples: Tessera (Fig. 82, A), Lucernaria. 



Order 2. Peromedusse. — Scyphozoa with four interradial 

 tentaculocysts; tentacles perradial and adradial; umbrella 

 conical, with transverse constriction; a stomodaeum is present 

 suspended by four mesenteries; no alternation of generations. 

 Example: Periphylla (Fig. 82, B). 



