104 



HYDROZOA. 



Far different is the physiognomy of Velella, 

 whose coenosarc appears almost wholly lost in the 

 horizontal, or slightly convex, rhomboidal pneu- 

 matophore, which distinguishes this singular genus. 

 The proximal surface of the pneumatophore is 

 traversed diagonally, from one of its angles to the 

 other, by an upright, triangular crest, which, in 

 common with the horizontal disc, consists of a soft 



Figf. 21. 



5 



Morphology of Yelella : — a, Velella spirans, somewhat en- 

 larged ; b, one of its smaller polypites, much magnified ; — v, 

 crest; A, liver; o, mouth of polypite ; S, its digestive cavity; 

 (p, rounded elevations, containing thread-cells; p, medusiform 

 zooids. 



marginal membrane, or " limb," bounding the 

 "firm part," or central portion {fig. 2i, a). To the 

 distal surface of the firm part of the disc are 

 attached the several appendages; including (i) a 



