38 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



In its simplest known condition, e.g. Hydra 

 (Fig. 8), a Ccelenterate has a terminal mouth (m) 

 which leads into a digestive cavity (be), and around 



which ten- 

 tacles (t) are 

 developed; 

 these tentacles, 

 which serve as 

 organs of pre- 

 hension, sen- 

 sation, and 

 offence, are 

 hollow, con- 

 tinuations of 

 the enteric 

 cavity passing 

 into them. 

 There is no 

 second orifice 

 to the enteron, 

 and reproduc- 

 tion is effected 

 either by gem- 

 mation, or by 

 the formation 

 of ova and 

 spermatozoa. 



In the more 

 complicate d 

 members of 



the group the hydriform body gives off buds, and be- 

 comes one of a colony (Fig. 9) ; and the separate 

 " persons " of this colony are connected together by a 

 common trunk, which is hollow within, and continuous 

 with the enteric cavity of each person ; in the simplest 

 stage of these colonial formations each person performs 

 the same duties, but in the more complex different 



Fig. 10. Figure of the Medusa of a Hydroid 

 (After Hiucks.) 



