Grupper i et System lienæviies Familier, Slægter. Under- 

 slægter eller hvad man nu vil kakle dem; Hovedsagen er, at 

 det Princip. efter hvilket Systemet er bygget, gjennemfores 

 konsekvent, saa at man over hele Linien benytter ligevær- 

 dige Kjendemærker til at karakterisere sideordnede Grupper. 



Naar man l)etragter Rækken af de forskjellige Hyd- 

 roidefamilier og Slægter, ser man iios dem en Udvikling fra 

 Ensartethed til mere eller mindre sterkt udviklet Differen- 

 tiation, paa samme Maade som man trætler den overalt ellers 

 saavel under den ontogenetiske som phylogenetiske Udvikling. 



Hos Hydroiderne ytrer denne Differentiering sig mest 

 paafaldende ved Generationsstoffenies (Gonoforernes) Plads 

 i Kolonien. Betragter man f. Eks., med Clava som Ud- 

 gangspunkt. de athecate Hydroider, der har spindelformige 

 Hydranther med traadformige Tentakler, da finder man her 

 represonteret alle de forskjellige Trin i en saadan Udvikling. 

 Alle Hydranther i Koloni af Clava bærer Gonoforcr; der 

 er altsaa inden Kolonien fald Ensartethed, og det samme 

 finder man hos Perigonimus, hvor Gonoforerne findes spredt 

 over Stamme og Grene i hele Kolonien. BougainviUidae 

 viser os et noget hoiere Udviklingstrin, idet her Gonofo- 

 rerne findes samlede paa enkelte Hydranthers Stilke, medens 

 Kolonien forresten er steril. (Første Anlæg til Blastostyl- 

 dannelse). Hos Dicorgne og hos Hgdradinia er Blastostyl- 

 dannelsen fuldstændig, idet de Hydranther. der liæn-er Gono- 

 forerne, er mere eller mindre atrophierede, og saaledes 

 ikke Irenger staar i Ernæringens Tjeneste. 



Herfra kommer man saa over til de thecapJion' Hgd- 

 roider, hos hvilke den atrophierede Hydranth og de paa 

 den siddende Gonoforer, yderligere er udpræget som en 

 Enhed derved, at de er bedækkede af et fælles Chitinhylster, 



Ved at gaa ud fra Corgne og Ijetragte de Familier, 

 der maa afledes fra denne, ser man samme Udvikling, men 

 her med store Sprang. I denne Eække ser man Ditferen- 

 tieringen endnu videre fremskreden, end hos Hydractinia, 

 idet Blastostylerne (de atrophierede Hydranther) fuldstæn- 

 dig omdannes og fra at representere Individer i en Koloni, 

 gaar de over til at være Organer paa et Individ. Denne 

 Forandring indledes hos MgriotJieJa. hvor Blastostylerne, 

 der fuldstændig ligner dem. man finder hos Hydractinia, 

 har sin Plads paa den nederste Del af Koloniens eneste 

 ikke atrophierede Hydranth. Og hos Tnhidaria findes den i 

 sin hoieste Udvikling, idet Blastostylerne, der neppe kan gjen- 

 kjendes som saadanne, sidder som Kreds af klaselignende 

 Organer indenfor Hydrantiens proximale Tentakelkreds. 



Jeg, tror, at denne Differentiering indenfor Kolonien 

 er en god Ledesnor ved Gruppering af Hydroiderne, All- 

 UKIH har flere Steder benyttet den, idet han har skilt fra 

 hinanden Arter med og uden Blastostyldannelse; men da 

 dette kun staar som et af de mange ofte hoist uvæsentlige 

 Kjendemerker i Arternes Bygning, der danner Grundlaget 

 for hans nye Slægter, faar man ikke Indtryk af at han 



groups in a system are called families, genera, suljfamilies, 

 or by any other name; the main thing is that the prin- 

 ciple upon which the system is built is carried out consi- 

 stently, so that, in every case, distiuguishing marks of equal 

 value are employed in characterising co-ordinate groups. 



On looking at the series of the varios hydroid fami- 

 lies and genera, there is seen to be a development from 

 homogeneity to a more or less highly-developed difterentia- 

 tion, similar to that whicli it universally met with during 

 hoth ontogenetic and phylognetic development. 



In the hydroids, this diilerentiation is most noticeably 

 expressed in the position of the generative products (gono- 

 phoi'es) in the colony. If, for instance, with Clava as our 

 point of departure, we look at those athecate hydroids 

 which have spindles haped hydranths with filiform tentacles. 

 we find represented there all the difterent stages in a 

 development of this description. AU the hydranths in a 

 colony of Clava bear gonophores; there is thus complete 

 homogeneity wihtin the colony, and the same is the case 

 with Perigonimus, where the gonophores are found scattered 

 over stems and branches throughout the colony. Bougaln- 

 vilJia presents a somewhat higher stage of development, 

 the gonophores being congregated on the stems of a few 

 hydranths, while the rest of the colony is sterile. (Earliest 

 commencenient of blastostyle formation). In Dicorgne and 

 Hgdradinia, the blastostyle formation is complete, as those 

 hydrants which bear the gonophores are more or less 

 atrophied, and are thus no longer employed in the service 

 of nutiition. 



From these we come to the thecaphore hydroids, 

 where the unity of the atrophied hydrantli and its gono- 

 phores is vet more distinctly emphasised by their being 

 covered with a common chitinous sheath. 



Starting from Corgne, and looking at the families 

 that must be derived from it, we see the same development, 

 but with greater breaks. In this series, the difterentiatiou 

 is found to he vet farther advanced than in Hgdradinia, 

 the blastostyles (atropied hydrants) being conipletely trans- 

 formed, and from representing persons in a colony, have 

 beconie organs in a person, This change is introduced in 

 Myriothda. where the blastostyles, whicii exactly resemble 

 those in Hgdradinia, are found on the lover part of the 

 only unatrophied hydranth in the colony. In Tnhidaria it 

 is found at its liighest degrce of development, the blasto- 

 styles, which are scarcely recognisable as sucb, being pla- 

 ced like a circle of raceme-like organs within the bydranth's 

 proximal crown of tentacles. 



I think this differentiating within the colony is a good 

 guide in the grouping of hydroids. Alhnan lias employed 

 it in several places. separating from one anotlier species 

 with and without blastostyle formation: l>ut as this is 

 only one of niany, often extremely unimportant, distiugui- 

 shing features in the structure of the species that form 

 the foundation for his new genera, it does not appear that 



