PHYSALIA. 



83 



Fig. 117. 



duction, &c., but that it is indeed a community of distinct indi 

 viduals corresponding exactly to the polymorphous Hydroids, 

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

 them only in being free and floating. 



The homologies of the Siphonophoras 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 Melicertum (See Fig. 

 81) should undergo its development with 

 out becoming attached to the ground, 

 what should we then have ? A floating 

 community (Fig. 83), including on the 

 same stock like the Nanomia, both sterile 

 and fertile Hydrae, 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. (Physalia 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, while the long streamers 

 below consist of a number of individuals 

 corresponding in their nature and functions to those composing a 

 Hydroid community. Among them are the fertile and sterile 

 Hydras (Fig. 118), the feeders and Medusas 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 far from their home, 

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



Tig. 117. Physalia ; ab air sac with crest c, m bunches of individuals, n central tentacles, 1 1 ex 

 panded tentacles. (Agassiz.) 



