76 TAGE SKOOSBERG 



It may, however, be pointed out here in passing that G. W. MUller has assumed that 

 the brush-shaped organ might possibly be found in the males of the Cypridinids; he 

 writes as follows about this: .,Man mufi an die Moglichkeit denken, daC der Penis der C y p r i- 

 d i n i d e n daraus hervorgegangen ist, oder daB es in den Penis aufgegangen ist und einen Theil 

 desselben bildet. An dem Penis verschiedener C y p r i d i n i d e n, nicht aller, liiBt sich eine 

 borstentragende Platte nachweisen, welche bei Cylindroleberis'' (= Asterope) ,,entschieden an das 

 biirstenformige Organ erinnert, dem sie auch in ihrer Lage im Wesentlichen entspricht." 

 Without entering in any detail into this difficult problem I will only point out that these assump- 

 tions are to be considered as, to say the least, very uncertain — a fact that has, of coui'se, been 

 admitted by G. W. MCller himself. If we regard the penis in the Cypridinids as homo- 

 logous to the brush-shaped organ, we pre-suppose, of course, that the former organ is not homo- 

 logous with the penis in the C y p r i d s, N e s i d e i d s, C y t h e r i d s and C y t h e r e 1- 

 1 i d s. It is true that some investigators have denied the homology of these organs — because 

 the penis in the C y j) r i d i n i d s does not include the vas deferens — but this view seems 

 to be untenable for several reasons. It also seems to be very improbable that the bristle-bearing 

 plate on, for instance, the penis of Asterope is homologous to the brush-shaped organ. It seems 

 more probable that it represents one of the two branches of the biramous limb from which the 

 penis probably has developed; cf., for instance, G. W. MOller, 1894, pi. 5, fig. 41. 



We thus find that the brush-shaped organ is situated in a number of forms close to the 

 fifth limb, in others close to or behind the seventh limb. Are these homologous organs? 

 G. W. MOller is of the opinion that they are because of the similarity in shape of the organs in 

 all the groups and also because in almost all the groups in which it has been observed, C y p r i d s, 

 N e s i d e i d s, C y t h e r i d s and C y t h e r e 1 1 i d s, it is confined to the same sex. I 

 have nothing to add to this. At any rate the possibility is not excluded. The position behind 

 the seventh limb is taken by G. W. MOller to be the original one; the brush-shaped organ 

 would represent the eighth limb. With regard to the cause of the hypothetical displacement 

 this author writes as follows, p. 76: ,,Die Bedingungen, welche das Organ zwischen die anderen 

 Beinpaare drangt, sind wohl in der Verkiirzung des gesammten Korpers zu suchen. Nachdem 

 es einmal zwischen die Beinpaare gerathen war, wurde es bei den Formen, welche seitlich stark 

 comprimirt sind (z. B. Paradoxostoma) bei denen kein Platz mehr zwischen den der Mittellinie 

 sehr genaherten Beinen blieb, bis vor das vorderste Beinpaar gedrangt." In support of the 

 assumption that the position behind the seventh limb is the original one it may be pointed out 

 that the forms in which this position has been observed are considered to be more or less primi- 

 tive in several respects. It is perhaps also supported by the late appearance of this organ during 

 ontogeny; it appears only ,,wenn alleGliedmaaBen anniihernd ihre definitive Form besitzen"; with 

 regard to the last argument it may, however, be pointed out that reduced organs often appear com- 

 paratively late, as, for instance, the reduced second maxilla in the Cladocera. Other authors assume 

 that the position in front of the fifth limb is original; according to these authors the brush-shaped 

 organ would represent the second maxilla of other Crustacea. No reasons of any importance have 

 been put forward in support of this view; on the other hand we must say, I think, that this pos- 

 sibility must be regarded as being present. This problem thus seems so far to be unsolved. 



