HTDROZOA. 



235 



vex or exumbrella with ectodermal epithelium. At the margin 

 of the bell the sub- and exumbrellar ectoderm is produced into a 

 two-layered sheet with a central opening, the velum or craspedon 

 (fig. 178, B, v) of systematic importance, since these medusae are 

 often spoken of as Craspedota. Tentacles (usually 4, 8, or 

 multiples in number) also arise from the edge of the bell just 

 outside the velum. 



FIG. 179. Tiara pileata. (After Haeckel, from Hatschek.) 



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

 umbrella is the manufrrium, 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 sub- 

 umbrellar 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. 



