50 



ACALEPIIS IN GENERAL. 



Part I. 



Fhj. 2C. 



compound communities of Ilydioids arc attached to the ground, those of Siphono- 

 phorai are free ; but this is not a. character exclusively peculiar to them, for among 

 the Polyps we have also free communities belonging to the same order as others 

 that are immovaljly attached to the ground. Such are the genera Renilla, Penna- 

 tula, Virgularia, Veretillum, etc., which are inseparable from the genera Gorgonia, 

 Alcyonium, Xenia, Tubipora, etc., or at least Ijelong to one and the same order. 

 In these locomotive Halcyonoids the individual Polyps are identical among them- 

 selves, but grouped together in the most diversified ways, varying in that I'espect 

 quite as much among themselves as the fixed Halcyonoids. In Pennatula and 

 Virgularia they form i-egular rows upon the two sides of a feather-like stem, in 

 Veretillum they are scattered around a cylindrical stem, in Eenilla they are 

 arranged in symmetrical lines upon the surface of a kidney-shaped disk. And yet 

 these connnunities move and act as one individual. I have frequently seen Eenilla, 

 which is our only genus of free Halcyonoid Polyps, move slowly about in tlie sand, 

 its stem Ijuried in a vertical position with the disk spread horizontally. 



Now, if I have succeeded in showing that, l>y their structure, the so-called 

 Hydroid Polyps are not Polyps, but Acalephs, and if I should also succeed in 

 showing that the different kinds of individuals forming the 

 communities of 8i[)lionophoraj have the same structure as 

 the Hydroids, and jn-esent everywhere, in all their parts, 

 special homologies with the Hydroid Polyps and naked-eyed 

 Meduste, without even exhiluting one of the peculiar char- 

 acteristics which distinguish the true Polyps from the Hy- 

 droids, I should then have proved that the Siphonophora) are 

 reall)'^ Hydroid Acalephs, and not Polyps, as Kijlliker believes 

 them to be. The evidence thus adduced would be an 

 additional reason for keeping the true Polyj^s, the so-called 

 Anthozoa, liy themselves, in a distinct class. 



Let us therefore compare more in detail the different 

 kinds of Siphonophor;v with the different kinds of Hydroids 

 and naked-eyed Medustv. Beginning Avith Pliysalia {Fiij. 20), 

 it is not ditficult to perceive that the various kinds of 

 appendages which hang from the floating air-bag of that 

 animal may l)e compared to the heterogeneous individuals 

 of an inverted Hydractinia. Fancy the channelled layer 



PlIYSALIA AliETHUSA, Til. ./ .' 



: Blunt ciKi of the air s.ac, supporting wliicli foHUS tlic attaclicd Ijasc of Hydractiuia to be swollen 



the whole community, at whicli the . n i ■ i 



youngestbudsniaybe'found. — (< Open UltO il hu'gC oljloUg l)ag, aud tllC COmpanSOU may be car- 

 end of the air sac. — e Crest of the air . * i i*ii' 



sac.-jn Bundles of single individu- Ticd cvcu luto tlic dctallsj for tlic csscntial diiierence 



als. — 71 Tentacle contracted. — / ( Ten- , . . 



tacles of the largest kind extended. betWCeU tlieSC tWO gCUCra doCS UOt SO UlUCll COUSlst WX a 



