HYDROZOA 



211 



The lack of sexual organs, which distinguishes most marine species 

 from Hydra, is due to the fact that sexual individuals of special form are 

 produced from the colony by budding. These, the meduste, may separate 

 early from the colony and swim freely. A medusa (figs. 175, 176) has the 

 form of a dome-like or disc-like bell and consists chiefly of very watery 

 jelly. The bell or umbrella of the medusa is covered on both its surfaces — 

 the concave or subumbrella, the convex or exumbrella — with ectodermal 

 epithehum. At the margin of the bell the ectoderm is produced into a 



Fig. 174. 



Fig. 173. — Campanularia geniculata. eh, ectoderm; en, entoderm; p, perisarc, ex- 

 panded around hydranth to a hydro theca; s, supporting layer. 



Fig. 174. — A bit of ilillepora alcicornis*, enlarged (after Agassiz). 



two-layered sheet with a central opening, the velum or craspedon (fig. 175, 

 B, v) of systematic importance, since these medusas are often called Cras- 

 pedota. Tentacles (usually 4, 8, or multiples in number) also arise from 

 the edge of the bell just outside the velum. 



Comparable to the tongue of the bell or the handle of the umbrella 

 is the 7naivubrium, hanging from the highest point of the subumbrella and 

 bearing the mouth at its tip. It contains the chief digestive space, from 

 which radial canals run on the subumbrellar surface to a ring canal in 

 the margin of the umbrella. The radial canals are usually four in number, 

 but in some species the number is increased during growth even to a 

 hundred or more. Manubrium and canals are lined by entoderm, which 

 also extends into the tentacles and forms their axes. 



