108 



ACALEPIIS IN GENERAL. 



Part I. 



Hydroids ; and al80 iiudor what common name they .should l^e designated. The 

 answer to these two questions is not difficult. 



Since the free Medusoi known to originate from Hydroids all belong to the 

 type of the Dmopliorcv. Cn/jjtocarjxc of Eschsoholtz, the Gi/mnophtliahnuta of Forbes, or 

 CraspcJota of Gegenljain-, there is presumptive evidence that the final investigation 

 of the true affinities of tlie.se Medusai will lead to a natural association of all those 

 which are really and closely related to one another, to the exclusion of the possible 

 foreign admixtures now left in this group, and that such a natural group will in 

 the end embrace all the Medusa3 originating from Hydroids. It is also possible, 

 however, that such a natural group of Medusa? may embrace genera undergoing 

 a direct metamorphosis from the egg to the perfect Medusa without intervening 

 Hydra stock, as we already know that there are higher Discophorte, such as 

 Pelagia, which reproduce themselves without passing through the Strobila .state. 

 But this would not alter the case of the affinity of such Medusa? : it would only 

 .show that the natural grou]) to which they belong exhiljits a Avider range in its 

 modes of development. The .systematic position of any Medusa must be determined 

 by an investigation of its special structure, and if tliere are any Medusa^, not 

 arising from Hydroids, Ijut growing up directly from eggs to their jiermanent form, 

 and 2^i"e.senting the same special structure as those that arise from Hydroids, there 

 is no reason why they should Ijc separated. Upon this view we shall hereafter 

 consider the affinities of the iEqn()rida\ the mode of development of which is not 

 yet fully ascertained, and those of the iEginida?, some of which are known to 

 undergo a direct metamorphosis. As to the Polyp-like Acalephs already known 

 to produce free Medusa^, they have all been united liy Johnston into one natural 

 /■■/-/. Gi. division, which he has called ///- 



droideu. But among these Hydroi- 



dea there are those which produce 



no free Medusa^, and yet as H}^- 



droids in no way differ from those 



that produce them. There is, 



therefore, no reason Avhy they 



.should be .separated : the less since, 



instead of free Medusa^, they pro- 

 duce sessile Medusa? buds identical 



in their structure with the free 



Medusa^ originating from the other 



Hydroids. On account of its re- 

 semblance to Sii^honoiihora^, Hydractinia {Fhjs. Gl and 

 62) affords an excellent example of this type. 



Fhj. 62. 



Hvl>ItACTINIA rOLVCLISA, Ag. 



o rt sterile iudividuais. — b Fertile 

 indiviilual, produciDg male Me- 

 dusae. — d Clusters of male Me- 

 dusae. — Proboscis, with the 

 mouth at the apex. — t Elongated 

 tentacles of the sterile individu- 

 als ; in the fertile one 6, they are 

 simple knobs o. 



Hvnr.ACTIXIA POLYCLIXA, Ag. 



t .Sterile individual. —6 Fertile individual pro- 

 ducing female Medusa?. — d e Female Medusa?, 

 containing advanced eggs. — fghi Cluster 

 of female Meduste with less advanced eggs. — 

 o Peduncle of the mouth with short globular 

 tentacles. — f Intlividual, with globular ten- 

 tacles, upon which no Meilus.-e have as yet 

 appeared, or from which they have already 

 droj.ped- 



