HYDROZOA 



23 



derm, which is ciliated. The endoderm also forms the axes 

 of the tentacles. There is also a layer of endoderm between the 

 radial canals extending from the circular canal to the gastric 

 cavity. Between the endoderm and the ectoderm, which 

 covers the convex surface or ex-umbrella, is a much-thickened 

 jelly-like mass of the mesoglea, while between the endoderm 

 and the ectoderm covering the sub-umbrella there is a thin 

 layer of mesoglea. The ectoderm, of course, covers the tenta- 

 cles, where it is well supplied with stinging cells. At the margin 

 the ectoderm of both the sub- and ex-umbrellas forms a narrow 





Fig. 14. — 1, Pela'gia panopy'ra, oral view of mature medusa. 2. The same, 

 side view. (Mayer, in Bull. U. S. F. C, 1903.) 



fold or shelf, the velum, which hangs down when at rest, but 

 draws up like a diaphragm across the bottom of the umbrella 

 when the bell contracts. By the forcing out of the water the 

 animal is forced forward, and so locomotion is effected. Around 

 the outside of the velum is a row of tentacles, usually four or 

 some multiple of four in number. 



Muscles of a longitudinal character control the tentacles, 

 while circular striped muscles surround the sub-umbrella and 

 velum, and, by contracting the umbrella and velum, produce 

 locomotion. 



