34 



THE HYDROMEDUSAE 



cavity, and an ectodermal connection of cnidoblasts and sense 

 cells with the two marginal rings, thus forming the characteristic 

 peronia. The sense organs are always free tentaculocysts at the 

 margin of the bell ; they are never closed as in the Trachomedusae ; 

 originally they are always four in number and interradial in 

 position ; this number may be retained throughout life (Cunantha), 

 but by later additions they may become extremely numerous 

 (Cunina). The otoliths are secreted by one or more endoderm 

 cells in each tentaculocyst ; they are generally crystalline, 

 occasionally spherical. Stripes of cnidoblasts, like that of the 

 peronium, which run from their bases up on to the exumbrella, 

 form the characteristic otoporpae. The cavity of the subumbrella is 

 small, when compared with that of previously described groups, 



PC.. 



Fio. 40. 



Fio. 41. 



40. Aeginura myosura, a species with eight tentacles and sixteen tentaculocysts ; letters 

 as in Fig. 41. (After Haeckel.) 



41. Half-section of Cunina. CC, circular canal ; 0, gonacl ; L, dotted outline of lappet ; 

 between each pair of lappets lies a peronium and a tentacle ; M, mouth ; XR, nerve ring ; r<\ 

 peronial canal ; EC, radial canal ; T, tentacle, the root of which penetrates to the radial canal ; 

 V, velum. (After Haeckel.) 



owing to the great development of the gastric cavity ; and its 

 musculature is far weaker. 



The COELENTERON in most members of this group differs some- 

 what from the type already familiar. The manubrium is generally 

 absent, the mouth opening directly into the gastric cavity ; it is, 

 however, sometimes present, though short (Cunina). The gastric 

 cavity is large, and occupies almost the whole of the subumbral 

 aspect. In the Cunanthidae, the radial canals are short, broad, and 

 shallow pouches, extending as far only as the base of each tentacle ; 

 at this point each canal is split by the peronium into two peronial 

 canals which, after a short radial course, turn round the edge of the 

 bell in festoons to form the circular or festoon canal. In the Pegan- 

 thidae and Aeginidae the conditions are much the same as in the 

 Cunanthidae, but the radial canals are practically suppressed ; the 

 peronial and festoon canals remain. In most members of the Sol- 



