17 



ikke hx?ggG for stor vægt paa deu særck-les karakteristiske 

 specielle bygning (form), som Epicardiot bar bos QarcJina 

 og Amarmicium. Hovedmomcntft er for mig den betragt- 

 ning, at tarmtraktus og peribrancliialsæk dog er saa over- 

 ordentlig forskjellige dannelser, og at da de udbugtningei-, 

 de danner, maa opfattes paa forskjellig niaade." Hertil kom- 

 mer nu det, at epicardiet bos Amaroucimn og Clavélina 

 aldeles ikke omklæder tarmens bagre parti, men danner et 

 ganske enkelt cylinderformigt ror. der fortsætter sig ud i 

 postabdomen. respektive stolenerne som disse organers 

 .. skille vægge". Fra et anatomisk synspunkt formaar jeg 

 saaledes ikke at se nogensombelst overensstemmelse mellem 

 disse organer. Derimod sogte jeg at forklare peribrancbial- 

 sækkens store udvidelse bos Bvfri/Uus som en tilpasning til 

 det forbold, der er saa karakteristiske for denne gruppe 

 (ligesom for Pyyosoma), nemlig at de to aabninger, inge- 

 stions- og egistionsaabningen, rykker saa langt fra binanden. 

 Herved faar det parti af peribrancbialsækken, som man 

 almindelig kakler kloakken, en overordentlig størrelse. Dy- 

 ret stræJikes desuden, saaledes at anus i modsætniug til alle 

 andre sammensatte Ascidier næsten bgger i kropsaksens 

 bagre pol. Paa mine figurer (31) pl. 37, tig. 1 og 4 er 

 dette meget instruktivt fremstillet. (Se Pl. IX, fig. 5).. 

 Sammenligner man denne figur med pl. X, tig. U. som jeg 

 bar beskrevet ovenfor, saa vil man kunne forestille sig, at 

 iiaar egestionsaabningen (for tydeligheds skyld ikke indteg- 

 net paa Pl. IX, fig. 5) rykker benimod den bagre pol af 

 dyrets længdeakse, saa vil ogsaa peribrancbialsækken (eller 

 kloakken) faa en større udbredelse, end den bar bos Ania- 

 roiiriuiii, livor den er begrændset til tarmens pars resjjira- 

 toria. Man vil ved denne sammenligning samtidig se, bvor 

 forskjellig en saadau udvidelse af peribrancbialsækken vikle 

 være fra epicardiet bos Amaroitcinm. 



Er der nu saa store anatomiske forskjelligbcder bos 

 de to dannelser, saa ibrekommer det mig paa forbaand 

 vanskeligt at „bomologisere" dem af fysiologiske grunde, 

 saa meget mere som det i den sammenlignende anatomi 

 turde være ukjendt, at nærstaaende familier bar homologe 

 organer af saa væsensforskjellig- art. Betragter vi imidlertd 

 den fysiologiske side af sporgsmaalet, saa maa jeg først 

 bemærke, at epicardiets opgave bos Clavélina og Amarou- 

 dum hverken er at beklæde tarmtraktus eller at være 

 kloak. Jeg nævnte ovenfor, at Pizon sammenligner peri- 

 brancbialsækken bos Ascidierne med ■perifoneum hos andre 

 Dyr, en tanke, som ban vel har faaet ved studiet af de 

 eiendommelige og specifikt modificerede forbold hos Botr/jlhi» 

 (ligesom Della Valle), og vi maa da erindre, at hau baade 

 bos knopper og larver mener, at pcribranchialsækken er en 

 ndbugtning af entodermen, tielv om imidlertid saa var til- 

 fældet, saa forekommer det mig umuligt paa nogen maade 

 at sammenligne peribranchialsæk og et peritoneum, hvad 

 ogsaa ran Benedcn og Jalin og Seeliger tidligere meget 

 distinkt har fremhævet. Et peritoneum er dog vel overalt 

 en niesoderinal-dannelse, og selv om bos 'Botv^WviS.lnwpyerne 

 peribranchialsækdannelsen har en /oyj/dighed med cælom- 

 sæ-kdannelseu hos f. eks. Aiiijioxns eller Sar/itfa, saa er dog 

 derfor dette organ hgesaa lidt et peritoneum. som skal- 



will not lav too great stress upon the highly cliaracteristic 

 special structure (form) of the epicardium in Clavélina and 

 Awaroucium. The chief point for me is the consideration 

 tbat the ahmentary canal and the peribranchial cavity are 

 such exceedingly difierent formations, and that therefore the 

 evaginations they form must be interpreted in difierent 

 wa3's. To this must be added the fa et tbat in Amaroiwiuin 

 and Clavélina, the epicardium does not encase the poste- 

 rior part of the intestine at all. but forms (piite a simple 

 cylindriral tube, which is continued out into the post-abdo- 

 nien. the stolons, as the walls of thcse organs. From an aua- 

 tomical point of view, I cannot see any harmony whatever 

 l)etween these organs. On the other hand I have tried to ex- 

 plain the great expansion of the peribranchial cavity in Bo- 

 tryJlus as an adjustmeut to the conditions so characteristic of 

 this gi-oup (as also of Pyrosoma), namely, tbat the two aper- 

 tures, the oral and the atrial are so far removed from one 

 another. By this means, tbat part of the peribranchial cavity 

 generally called the cloaca, becomes exceedingly large. The 

 animal too is extended so that the anus, contrary to that of all 

 other Compound Ascidians, lies almost in the posterior pole 

 of the axis of the body. In my figures (31) Pl. 37, figs. 1 

 and 4, this is very instructively represented. (See Pl. IX, 

 fig. 5). If we compare this figure with Pl. X, fig. 9, whicii 

 I have described above, we sball be able to imagine that 

 when the atrial aperture (for the sake of distinctness not 

 shown on Pl. IX, fig. h) is pushed towards the posterior 

 pole of the long axis of the animal, the peribranchial cavity 

 (or cloaca) will also acquire a greater extent tban it has 

 in Amuroucinm, where it is limited to the pars respira- 

 toria of the intestine. By this comparison it will also be 

 seen bow different such an expansion of the peribranchial 

 cavity would be from the epicardium in Amaroucium. 



If now tliere are such great anatoraical differences in 

 the two formations, it already seems to me difficult to „bo- 

 mologise" tbem, for pbysiological reasons, the more so as, 

 in comparative anatomy, the circumstance of nearly-allied 

 families baving bomologous organs of such essentially diffe- 

 rent kinds, is [)robably unknowu. If, however, we consider 

 the pbysiological side of the question, I must first remark, 

 that the function of the epicardium in Clavélina and Ama- 

 ronciuiu is neither to line the intestinal region, nor to be 

 cloaca. I mentioned above tbat Pizon compares the peri- 

 branchial cavity in the Ascidians with the yeritonemn in 

 other animals, an idea which be, like Della Valle, has pro- 

 bably got by the study of the peculiar and specitically 

 modified conditions in Botri/llus, and we must then recol- 

 lect. tbat both in buds and larvæ, he considers the peri- 

 branchial cavity to be an evagination of the endoderm. 

 Even if. however, such were the case, it seems to me im- 

 possible in any way to compai-e the peribranchial cavity 

 and the peritoneum, an opinion wliicb van Beneden and 

 Julin, and Seeliger bave previously empbasised very clearly. 

 A peritoneum is presumably always a mesodermal-forma- 

 tion, and even in the buds of Botri/llus, the peribranchial 

 cavity has a similarity of form to the cæloma, e. g. in 

 Amphioxus or Sagitta, yet this organ is not for that 



Den norske Nordhavsexpedition. Juliau Hjort: Kim 



blndstudier paa gnuidlag; af Astidiernes ndvikling. 



