have den Betydning, Levinsen tillæo-gcr dem, du alle disse 

 Slægter — ligeoverior andre nærstaaeiide — dog staar som 

 et samlet Hele. 



Under Undersogelsen af N()rdliavsex])editionens athe- 

 cate Hydroider, blev jeg fuldstændig overbevist om, — hvad 

 Levinsen ogsaa havde fremhævet — at Sporgsmaalet om 

 Gonoforernes Udvikling er fuldstændig betydninp;slost tur 

 Systeraatiken, idet dor Idandt meget nærstaaende Arter, 

 stadig tindes saadanne, hvis Gonoforer sammen danner en 

 mere eller mindre tæt sluttet Ra'kl<e af Udvildingstrin — 

 fra Gonofor nden Radialkanaler og op til fri Meduser med 

 fuldsta'ndigt Kanalsystem. — En anden Sag er det. at saa- 

 danne Arter l)or skilles ud. hvis Gonoforer er hyggede efter 

 et Princip, forskjelligt fra det, soni er Grundlaget for 

 Medusebygningen. 



Denne min Opfatning frendioldt jeg (2'J) i min fore- 

 lobige Meddelelse om de athekate Hydroider. Desværre 

 fik jeg ikke, forend Korrekturlresningen paa denne var besør- 

 get, fat i Sclniciders Afhandling (107), der var udkommet 

 lige for mit ^Nfanuskript gik i Trykken, og jeg kunde saa- 

 ledes ikke referere til denne. Forfatteren giver her i korte 

 Træk en Fremstilling af Hydroidernes Systematik, og han 

 gjennemforer konsekvent det ovenfor omtalte Princip sani- 

 tidig med at han ogsaa ])aa andre Maader stadig arbeider 

 for en Forenkling af Systemet. Han fremholder, at alle 

 Arter danner en sluttet Række, og kun derved, at enkelte 

 Arter er uddode. andre endnu ukjendte, viser der sig for 

 os storre og mindre Huller i denne Eække; og saahenge 

 disse Huller existerer, og enkelte Arter derfor synes adskilt 

 ved et Sprang i Rækken, maa disse henføres til forskjellige 

 Slægter. der imidlertid smelter sammen, eftersom Overgangs- 

 former melleni dem opdages. Med denne Tanke som 

 Udgangspunkt gjennemgaar hau saa de tidligere opstillede 

 Slægtei' og reducerer den storste Del af dem til Syntinymer. 



Det System af Slægter. som da tilslut opstilledes af 

 Schneider, forekommer mig at være bygget paa en naturlig 

 Sammenhæng mellem Arterne, og jeg- niaa i alt væsentligt 

 givo det min Tilslutning^; og hvis Fremtidens Systematikere 

 under sit Arbeide vikle have Blikket rettet paa Overens- 

 stemmelsen mellem Artorne. lige meget som paa Forskjellen 

 mellem dem, da kunde der muligens være Haab om efter- 

 haanden at blive af med den Hær af Synonymei-, der nu 

 i saa hoi Grad vanskeliggjør enhver systematisk Llnder- 

 søgelse af Hydrøiderne. 



Men hvis man paa den anden Side gaar for vidt i 

 dette Forenklingsarbeide, og f. Eks. forener til en Slægt 

 alle Arter, mellem hvilke man kan tinde Overgangsformer, 

 da bliver jo Følgen kun den, at de gamle Benævnelser, 

 Familie, Slægt o. s. v., lidt efter lidt gbder ud af Systemet, 

 eller iallefald træder ud af Brug, medens man af praktiske 

 Hensyn maa indfore andre for at betegne nærmere og fjernere 

 Sammenhæng mellem Arterne indenfor en og samme Slægt. 

 Og det er jo forholdsvis ligegyldigt om de forskjellige 



attributes to them, as all these genera — comi)ari'd with 

 other nearly-allied genera — are like øne complete whole. 



During my examination of the N. Atl. Exp. athecate 

 hydioids, I l)ecame fuUy convinced of what Lerinf-en had 

 also asserted. vi/,, that the ipiestion as to the development 

 of the gonophores is of no imiiortauee whatever to the 

 systematisation, as among nearlyallied species tliere are 

 always some whose gonophores fonn together a more or 

 less complete series of develo|)mental stages, from gono- 

 phores witliout radial canals, up to free-swimming medusre 

 with a comi)lete canal system. Another question it is 

 that species, whose gonophores are constructed on a prin- 

 ciple different to that which forms the ])asis of the medusa 

 construction, ought to l)e separated from the others. 



I put forward this view in my preliminary communi- 

 cation (29) relating to the athecate hydroids. Unfortunately, 

 8cltneider's treatise (107), which had appeared just before 

 my MS went to press, did not fall into my hands until 

 after my proofs had been read. and I was thus un.-ible to 

 refer to it. The author theve gives a brief statement of 

 the systematisation of hydroids, and he consistently carries 

 out the above-mentioned principle, wliile at the same time 

 he is in other direction too striving towards a simplification 

 of the system. He maintains that all species form a com- 

 pact series, and that it is only because some species have 

 died out, aud others are still unknown. that we se large 

 or small gaps in the series; and as løng as these gaps 

 exist, and certain species, in consequence, seem to iie sepa- 

 rated by a break in the series, they must be referred to 

 different genera, which, however, merge together, as the 

 transition-forms between them are discovered. Starting 

 with this idea, he then goes thrøugh the previøusly- 

 established genera, and reduces the greater nundier of 

 them to synonyms 



The system of genera which is tinally drawn up by 

 Schneider seems to me to be built upon a natural connec- 

 tion between the species, and in all essentials, I must give 

 it my adherence.* If the systematists of the future, during 

 their labours, would turn their attention to tiie similai'ity 

 between the species as much as to the difference between 

 them, there would perhaps be some hope of gctting gra- 

 tlually rid of the host of synonyms which now so greatly 

 add to the difticulty of a systematic invcstigation of the 

 Hydroida. 



Buf if, on the other hand, this work of sim[)lifying 

 is carried too far, and one genas, for instance. is presented 

 wdiith all the species between which transition forms niay 

 be found, the consequence will be that the old names — 

 fainily, genus, etc. — gradually drop out of the system, or 

 at any rate out of use, while for practical considerations 

 others must be introduced to designate the close ør distant 

 counection between the species in a genus. It is indeed 

 a matter of comparative indiffi-rence whether the various 



' Hvor min Opfatning i Enkeltheder afviger fra Sclineiders, vil 

 det blive omtalt nedenfor. 



Where my opinion differ.s from Schneidcr"s in details, it will bo 

 mentioued below. 



