37 



Dolklialer (Xiphosura). Det synes altsaa som om 

 disse Dyr, uagtet deres Organisation idethele har 

 naaet et forholdsvis meget hoit Udviklingstrin, dog 

 ved Siden heraf har conserveret flere af de primitive 

 Characterer, der maa antages at have ixdmærket de 

 ældgamle Stamformer, hvorfra alle de moderne Cru- 

 staceer i sidste Instans har tåget sit Udspring. 

 Ogsaa Udviklingen synes at støtte denne baade af 

 Prof Claus og Dr. Dohrn fremholdte Anskuelse. 

 Saagodtsom hos alle Phyllopoder begynder nemlig 

 den frie Udvikling med det overordentlig simple 

 saakaldte Nauplius Stadium, og Larven gjenuemgaar 

 derpaa en Række af successive Omformninger, der 

 lidt efter lidt forbereder Phyllopodestadiet. Det bør 

 dog her bemærkes, at Dr. Packard i sit fortjenst- 

 fulde Værk over Nordamerika's Phyllopoder hævder 

 en herfra meget forskjellig Anskuelse. Efter denne 

 Naturforskers Mening er Phyllopoderne tværtimod 

 af meget ny Oprindelse og fremstaaet ved en videre 

 Udvikling af Cladocer-Typen. Dette kunde maaske 

 til Nod lade sig høre, hvor der er Spørgsmaal om 

 den ene af Phyllopodernes 3 Sectioner, de saakaldte 

 Conchostraca, der ganske sikkert viser en meget ud- 

 præget Affinitet til Cladocererne. Men langt van- 

 skeligere bliver det at faa udledet de 2 øvrige Phyl- 

 lopode-Typer fra Cladocererne. Der gives ikke en 

 eneste Cladocer, der viser den fjerneste Tilnærmelse 

 til de for G-rupperne Anostraca og Notostraca cha- 

 racteristiske Eiendommeligheder, og det gaar heller 

 ikke an, at aflede disse 2 Typer fra den 3die Con- 

 chostraca. De 3 Phyllopodetyper staar i Virkelig- 

 heden paa en Maade helt isolerede og har rimelig- 

 vis et meget forskjelligt Udspring. Langt natur- 

 ligere end den af Packard fremsatte Hypothese om 

 Phyllopodernes Afstamning fra Cladocererne, synes 

 det mig at være at vende Sagen helt om, og altsaa 

 antage, at Cladocererne er af yngre Oprindelse end 

 Phyllopoderne og har udviklet sig som en Sidegren 

 fra Gruppen Conchostraca. Raadsperger vi Palæ- 

 ontologien, vil ialfald intet Modbevis mod en saadan 

 Antagelse kunne hentes herfra; tværtimod. Medens 

 man nemlig endnu ikke kjender en eneste Cladocer 

 i fossil Tilstand, finder man talrige fossile Skaller 

 af utvivlsomme conchostrake Phyllopoder, nærmest 

 henhørende til Slægten Estheria, lige op til den De- 

 voniske Periode; noget der jo viser, at ialfald denne 

 Gruppe af Phyllopoder ikke kan være af saa ny 

 Oprindelse, som man efter Packards Hypothese synes 

 at maatte antage. Nu er der forskjellige morpho- 

 logiske Forhold, der gjør det hoist usandsynligt at 

 antage, at Gruppen Conchostraca skulde repræsen- 

 tere de ældste og oprindeligste Phyllopoder. Vi 

 kommer ad denne Vei snarere til en stik modsat 

 Slutning, nemlig at denne Gruppe er af en betyde- 

 lig yngre Oprindelse end de 2 øvrige. At man ikke 

 kjender nogen forverdenske Former af disse sidste 

 Grupper, kan naturligt forklares af de herhen hø- 



the division Notostraca, at any rate, is an unmistak- 

 able habitual resemblance with the sword-tails (Xi- 

 phosura), likewise passing far back in geological 

 times It appears therefore, as if these animals, 

 although their organisation has, upon, the whole, 

 attained, relatively, a very high stage of develop- 

 ment, still have retained several of the primitive 

 characteristics which must be assumed to have 

 distinguished the ancient ancestors, from which 

 all the modern crustaceans have finally had their 

 origin. The development also seems to support 

 that view, advocated both by Prof. Claus and Dr. 

 Dohrn. In almost all the Phyllopods the free 

 development begins, namely, with the extraordi- 

 narily simple so-called Naixplius stage; and the 

 larva thereupon undergoes a series of successive 

 transformations, wliich, little by little, prepare the 

 phyllopod-stage. It ought to be noted here, how- 

 ever, that Dr. Packard in his admirable work 

 upon the Phyllopods of North America, maintains 

 a very different view. According to the opinion 

 of that naturalist, the Phyllopods are, on the con- 

 trary, of very late origin, and produced by a further 

 development of the Cladocera-type. That might per- 

 haps, in the absence of anything better, be accepted, 

 when the question concerns one of the 3 sections of 

 Phyllopods, the so-called Conchostraca, which quite 

 certainly exhibits a very distinguished affinity to 

 the Cladocera. But far more difficult does it become, 

 to trace the 2 other phyllopod-types from the Cla- 

 docera. There is not a single Cladoceran that exhi- 

 bits the slightest approximation to the characteristics 

 peculiar to the groups Anostraca and Notostraca, 

 and neither is it permitted for us to trace these 

 2 types from the 3rd, the Conchostraca. The 3 types 

 of Phyllopods stand in reality, in a measure, quite 

 isolated, and probablj' have a very different origin. 

 Far more natural than the liypothesis of the deri- 

 vation of the Phyllopods from the Cladocera, pre- 

 sented by Packard, does it appear, to me, to be, to 

 quite reverse the case, and consequently assume that 

 the Cladocera are of later origin than the Phyllo- 

 pods and have become developed, as a lateral branch, 

 from the group Conchostraca. If we consult palæ- 

 ontology, we will, at any rate, find no testimony 

 rebutting such an assumption to be obtained there- 

 from; on the contrary, while we do not yet know 

 of a single Cladoceran in fossil condition, we find 

 numerous fossil remains of indubitable conchostracan 

 Phyllopods most closely approaching to the genus 

 Estheria, even up to the Devonian period; a fact 

 that certainly shows that that group of Phyllopods, 

 at any rate, cannot be of such late origin as we, 

 according to Packard's hypothesis, seem obliged to 

 suppose. Now there are various morphological rela- 

 tions that make it extremely improbable to suppose 

 that the group Conchostraca represents the oldest 



