HYDROZOA. 



Ill 



Attention has, in a previous section, been 

 directed to some of the modifications which the 

 tentacles of the Physophoridce present. They 

 appear in Apolemia as simple tubular processes, 

 with numerous large thread-cells on one side : in 

 Velella, they are equally simple, but much shorter, 

 and slightly 



enlarged 



at their free ends. 



In 



Porpita there is, in addition, a series of longei 

 prehensile appendages, having their distal 



ex- 



tremities clavate and beset with stalked knobs, or 

 capitula, containing urticating organs, which are 

 wanting in the smaller marginal " cirrhi." In 

 Physalia, as above described, each tentacle con- 

 sists of a broad conical basal sac, and a long 

 simple ribbon-like process, having transverse reni- 

 form enlargements (fig. u, d). In all other 

 Physophoridce the tentacula are furnished with 

 lateral branches, which in Rhizophysa alone 

 appear to be destitute of sacculi. These may 

 want involucra, as in Halistemma or Forskalia, 

 or possess these structures, and become further 

 modified, as in all the remaining genera. 



The structure of the gonophores, though always 

 medusoidal, is, in other respects, liable to much 



many Physophoridce well- 

 of size, aspect, or relative 



variation. Among 

 marked differences 

 number, distinguish the male and female reproduc- 

 tive bodies in the same species. Thus in Agalraa 

 and Physophora the gynophores are only half the 

 length of the androphores, than which, however, 

 they are more numerous. In Velella and Porpita 

 both androphores and gynophores become com- 

 pletely detached, and the same is probably true of 

 the female organs in Physalia. But the andro- 

 phores of this genus, and the two kinds of gene- 



