THE SCYPHOMEDUSAE 61 



The Scyphomedusan hydroid or scyphistoma (Fig. 6) is, com- 

 paratively speaking, insignificant in size and monotonous in 

 structure ; it is known only among Ephyroniae (Discomedusae), and 

 will be described under that group. 



The medusoid (Figs. 4, 8) is, roughly speaking, of the same 

 type as that of the Hydromedusae manubrium, tentacles, ex- 

 umbral and subumbral surfaces are of the same general character ; 

 but the velum is absent, its place being sometimes taken by the 

 velarium; the latter may be either the inflected edge of the bell 

 (Aurelia), or a definite subumbral outgrowth containing coelenteric 

 canals (Charybdaea), but in neither case agrees with the Hydro- 

 medusan velum in position or in structure. The gastric cavity 

 exhibits four pouches, from which or from between which lead the 

 radial canals ; the latter are separated by an endoderm lamella in 

 the essentially medusoid forms. 



In the more scyphistomoid forms (Fig. 2 x ) strong plates or 

 pillars of mesogloea run from body wall to stomodaeum, forming 

 the taeniolae or mesenteries, into which ectodermal pits (subumbral 

 funnels, subgenital pits) of varying depth penetrate from the oral 

 surface. The mesenteries do not appear in all cases to be formed 

 by endodermal concrescence. 



The canals are often numerous ; they frequently branch, and 

 sometimes anastomose ; they open into a circular canal at the edge 

 of the bell. Gastric filaments (phacellae), interradially placed, 

 are characteristic of this group of organisms. The generative 

 organs are interradial or adradial in position, and are derived from 

 endoderm cells. 



ORDER 1. Stauromedusae. 



DEFINITION. Scyphomedusae which are devoid of tentaculo- 

 cysts, but in some cases have in their place marginal anchors. The 

 tentacles are perradial and interradial in position. The body is 

 more scyphistomoid than medusoid, exhibiting a stomodaeum 

 suspended by four mesenteries, between which lie the four broad 

 perradial pouches. There is no alternation of generations. 



The Stauromedusae (Figs. 2, 3) are hypogenetic ; the single 

 form of individual presents features intermediate between those 

 of hydroid (scyphistomoid) and medusoid forms. It is either 

 purely free -swimming (Tessera), or has the power of temporary 

 fixation (Haliclystus) by the aboral pole. 



The organism is goblet-shaped, with a narrow stem which ends 

 conically (Tessera), or in a disc (Haliclystus) which can be used 

 for adherence to a solid object. The manubrium is well developed, 

 but no velum is present. The edge of the bell is either (1) simple, 

 and provided with four perradial and four interradial tentacles 



