THE DIVISIONS OF THE HYDROZOA. 59 



other. One usually sinks into insignificance, slurred over and fre- 

 quently even entirely suppressed. And according to which phase is 

 magnified, have we one of the principal points characteristic of one 

 or the other of the two sub-classes of the Hydrozoa. If the free- 

 swimming sexual or Medusid-stage be the chief life-form, and the 

 Hydroid be all but suppressed, then we have the division of the 

 Scyphomedusae the Acelephse of some writers. Herein are in- 

 cluded all the large Jelly-fishes so familiar in summer time on our 

 coast, during warm weather, and moving at times in shoals of 

 immense numbers. The other division is that where the Hydroid- 

 stage is exaggerated and usually colonial, and where the Medusid or 

 swimming stage is tiny or even eliminated. Such constitute the 

 Hydromedusae which are further distinguished from the Scypho- 

 medusa3, in that the Medusae of the former have the edge of the bell 

 turned horizontally inwards forming a delicate veil, the velum, 

 partially closing the mouth of the bell. This character constitutes a 

 Craspedote Medusa, as contrasted with the Acraspedote form of the 

 Scyphomedusaa, where the velum is absent. 



The Hydroid Zoophytes, or Hydroidea, constitute the central 

 order of the Hydromedusse, and possess the typical life-history. In 

 passing it is well to remember that besides the Hydroidea, there are 

 two other orders, viz., the Trachymedusaa and the Siphonophora, both 

 characterised by pelagic habit, and the fact that they never have a 

 fixed or stationary hydroid phase. 



The Hydroidea show much variation in the form of their Hydroid 

 stage and in the completeness of their life-history. Obelia is 

 very typical of unabbreviated life-cycle, so is specially useful to 

 use as an introductory type for study. The colonial stage is 

 remarkably beautiful, its delicate zig-zag branches forming dense 

 miniature forests on the broad brown oar-weed. An old writer 

 speaking of a nearly related and not more beautiful species, says, 

 " Delicacy, transparency, and grace pervade the entire structure ; the 

 spirit of beauty has thrown itself into every curve and line : the eye 

 rests with full satisfaction on the little cups, so perfect in their form ; 

 and hardly less beautiful are the ringed and twisted pedicles that 

 support them." 



The colony is protected in all its parts by a delicate transparent 

 horny substance the perlsarc, and consists of a zig-zag stem, jointed 

 at each curve, and giving off on alternate sides, a short ringed pinna, 

 bearing aloft a tiny crystal cup (hydrotheca or calcycle), wherein 

 lodges an equally tiny polypite or hydranth (PL ix, Fig. 2, p.), armed 

 with a well-marked proboscis and numerous tentacles. To the polypite 

 is delegated the sole duty of capturing prey. The whole organism is 

 the truest, of commonwealths ; each polvpite captures not for its own 



