JELLY-FISHES. 



105 



Agalma stripped of all its appendages, except feeding polyps and sexual bodies. There 

 is in it no distinction between nectostem and poljpstem, and no means pf voluntary- 

 motion. The float is particularly large 

 and has an apical opening through 

 which its contents communicate with 

 the surrounding water. The feeding 

 polyps hang from the stern at regular 

 intervals when extended, and midway 

 between them appear on the same axis 

 botryoidal clusters which are called. the 

 sexual organs. Tentacles hang from 

 the bases of the polypites as in other 

 related siphonophores. The tentacular 

 knobs have, however, a highly charac- 

 teristic form which varies with different 

 species in number and general anatomy. 



Oedee III. — DIPHY^. 



In all the genera thus far studied, 

 there is always a float at one extremity 

 of an axis when such was present. In 

 no case is a float missing, although often- 

 times it is functionally unimportant. 

 In none of the remaining Siphonophora, 

 on the other hand, is a float present. 

 These last medusae may conveniently 

 be divided into the Diphyse, in which 



Fig. 97. — P7j«saZia aretfmsa, Por- , t t 



tuguese man-of-war, one-tifth there are One or two nectocalices, and 

 size. ^^ Hippopodiae, floatless forms in which 



there are several or more than two swimming-bells. 



One of the best marked families of the Diphyse is the Diphy- 

 VDM, of which JDiphyes is a typical genus. This genus and most of 

 its relatives is smaller than the majority of those already studied, 

 and are easily distinguished from the former by the absence of a 

 float, and the presence of but two nectocalices. The two swim- 

 ming-bells which are possessed by Diphyes are of somewhat differ- 

 ent form. The anterior is conical in shape in order to facilitate 

 rapid progression through the water, while the posterior which 

 lies behind it, seems to perform the greater part of the work in ^^^ ^ 

 the progression of the medusa. As in Agalma, onward motion 

 is caused by the resistance of the water as it leaves the bells on the surrounding 

 •medium in which the animal swims. The motions of the nectocalices are spasniodic 

 and not long continued as in the Agalma and other Physophorje. The axis of 

 Diphyes hanging from the interval between the two bells, is a long, filamentous, 

 flexible structure not unUke that of Agalma. It is highly contractile, and has a cavity 

 throughout its entire length. The polypites arise at intervals along the length of the 

 stem, and are in no respect peculiar. Each polypite bears a tentacle and tentacular 



