INVERTEBRATE ANIMALS. 



CHAPTEK Yl. 

 DIVISIONS OF THE HYDROZOA. 



The Ifijdro:.oa are divided into five great divisions, each of which 

 requires some notice, as presenting points of special interest. These 

 divisions or suli-classes are known by tlie names of Ifijdroida, Sijjho- 

 nophora, Luccmarida, (j raptolitidce, and Uydrocorallinw. 



SUB-CLASS HYDROIDA. 



Tliis sub-cl;i.ss compiises all the Sea-firs and their allies, commonly 

 known to naturalists as the "Hydroid Zoophytes," from their resem- 

 blance to the Fresh-water Polype {lli/dru), which is also a member of 

 this division. The lii/droida are defined by the fact that they co/isiit 

 vf (III (dimentary region or ^^ polypite" which is furnished with a mouth 

 and prcJiciisile tentiicles at its distal end, and %oith an adherent disc at 

 its prii.viiind c.rtreniit//. In some few cases the hydrosoraa consists of 

 Ijut one sucli jjolypite (as in the Ilydrida and some of the VOrynida) ; 

 but generally the hydrosoraa is composed of a greater or less number 

 of similar polypites all united by a ctenosarc or common trunk (as in 

 the majority of the Curynidn, and in the !<ertidarida and Campanu- 

 laridii). In the great majority of cases, also, the hydrosoma is not 

 unattacheil, but is fixed to some solid object by its jiroximal extrem- 

 ity or " hydrorliiza." The Ilydrriid Zoophytes exliiliit four principal 

 ty i ;es of structure, which cc )nstitute so many oidcrs. 



Oiiijr.:R I. IIydhida. 



Ill the first order we have only the well-known Fresh -water 

 rolypcH or Jlydni; of which we may take the common green 

 Ifi/ilni {If. ririi/is) as the tyjut. When unconti-acted, the body of 

 the J/ydrii is in the for-m of a cyliuilrical tube (tig. 30), composed 

 of the two fnudamciital layers, the ectoderm anil endoderni, of 

 wliich (lie former contains many thread-cells. The integument 



