32 HTDROZOA. 

















nutrient system of the Medusidce and Lucernarid<% 



The cavity of the coenosarc, or of the peduncles 

 of the polypites in connection therewith, may be- 

 come, as in Tubular ia, partially obliterated by 

 vacuolation, a process, however, which does not 

 seem to impair its vital efficiency. 



Some have conjectured that the short canal 

 which penetrates the attached extremity of the 

 Hydra represents the coenosarcal cavity of the 

 higher Hydrozoa. 



- 



4, Prehensile apparatus. — The tentacles, or 

 prehensile organs, which present so striking a 

 feature in the physiognomy of the Hydrozoa, vary 

 exceedingly, both in position and structure. 



In the Hydridce, Sertularidce, and some genera 

 of Corynida3, they arise, in one or more circles, 

 either immediately around the mouth or at a short 

 distance below it {fig. 3, d). But, in other Cory- 

 nidce, they spring from various parts of the wall 

 of the polypite {figs. 16 and 17), and one genus 

 of this group, Hydractinia, possesses, in addition, 

 long isolated tentacles, having an independent 

 origin of their own from the ccenosarc. Single 

 tentacles of this kind are the only ones which 

 occur in the genera Physalia, Velella, and For* 

 pita, among the Phy sophoridce (fig. 1 1, c )> * D 

 other Physophoridw, and all Calycophoridcv, the 

 tentacles are inserted on the sides of the polypiteSj 

 about the junction of the gastric and proxiin al 

 divisions (figs. 20 and 22). In the Mediisida 

 and Lucernaridce, the tentacles usually surround a 

 the open border of the bell-shaped swimming 

 organ (figs. 7, 23-25). 



8 





