32 HYDEOZOA. 



nutrient system of the Medusidce and Lucernaridce 



{fig- 23). 



The cavity of the coenosarc, or of the peduncles 

 of the polypites in connection therewith, may be- 

 come, as in Tuhularia, 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 Hydvozoa. 



4. Preh ensile 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 Hydridw, Sertularidce, and some genera 

 of Corynidce, 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 Coiy- 

 nidce, they spring from various parts of the walls 

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

 of this group, Hydractinia, possesses, in addition, 

 long isolated tentacles, having an independent 

 oriofin of their own from the coenosarc. Sinsfle 

 tentacles of this kind are the only ones which 

 occur in the genera Physcdia, Velella, and For- 

 pita, among the Physophoridce (fig. 11, c). In 

 other Physophoridce, and all Calycophoridce, the 

 tentacles are inserted on the sides of the polypites, 

 about the junction of the gastric and proximal 

 divisions {figs. 20 and 22). In the Medusidce 

 and Lucernaridce y the tentacles usually surround 

 the open border of the bell-shaped swimming 

 organ {figs. 7, 23-25). 



