I'HYSALIA. 



83 



Fig. 117. 



duction, <tc,, but that it is indeed a community of distinct indi- 

 viduals corresponding- exactly to the polymorjihous Ilydroids, 

 whose stocks are attached, such as Hydractinia, and dilTcring from 

 them only in being free and floating. 



The homologies of the Siphonophorae or floating Hydroids, 

 with many of the fixed Hydroids, is perhaps more striking 

 when we compare the earlier stages of 

 their growth. Suppose, for instance, that 

 the planula of our Molicertum (See Fig. 

 81) should undergo its development with- 

 out becoming attached to the ground, — 

 what should we then have ? A floating 

 community (Fig. 83), includhig on the 

 same stock like the Nanomia, both sterile 

 and fertile Hydras, from the latter of which 

 Medusae bells are developed. The little 

 Hydractinia community (Fig. 100), in 

 which we have no less than four distinct 

 kinds of individuals, each performing a 

 definite distinct function, affords a still 

 better comparison. 



Physalia. {Phymlia Arethusa Til.) 



Among the most beautiful of the Sipho- 

 nophores, is the well-known Physalia or 

 Portuguese man-of-war, represented in Fig. 

 117). The float above is a sort of crested 

 sac or bladder, wiiile the long streamers 

 below consist of a number of individuals 

 corresponding in their nature and functions to those composing a 

 Hydroid commiuiity. Among them are the fertile and sterile 

 Hydrae (Fig. 118), the feeders and Medusae bells (Fig. 119). 

 The Physalia properly belongs to tropical waters, but sometimes 

 floats northward, in the warm current of the Gulf Stream, and 

 is stranded on Cape Cod. When found so liir from their home, 

 however, they have usually lost much of their vividness of color; 



Fig. 117. Physiilia ; ab air sac with crest c, m bunches of individuals, n central tenta-.-lfS, t I ex- 

 panded teotacles. {Jya.ssix.) 



