Classification of the Crustacea. 447 



scqncntlj tlic abundant scc^mcntation of the body of Estheria 

 appears to be a primitive condition, and likewise the 

 diminislied number of body-segments in the other Estlieridce, 

 Limnadia and Limnetis, when viewed from this standpoint, 

 is seen to be of a secondary nature. If therefore the 

 Estlieridaj distinguished by a smaller number of body- 

 segments {Limnadia and Limnetis)^ and the Cladoeera are 

 to be derived from more richly segmented forms, the process 

 must be imagined to have taken place in such a way that 

 developmental stages of Estheria with a smaller number of 

 scgmentsconstituted the starting-point for theother Estheridae, 

 which are composed of fewer segments, and likewise for the 

 Cladoeera. 



A further argument in favour of the theory that the rela- 

 tively unwieldy Estheria was the ancestor of the Cladoeera is 

 furnished by the peculiar shape of the end of the furca, which 

 is adapted for motion on the bottom. If we consider the 

 mode of life of the Cladoeera we must designate it as pelagic. 

 The Cladoeera move about in the water with a hopping 

 motion. It is true that tiiere are also forms which live in the 

 mud, like certain Linceids [Monospilus) ^ but these are not 

 representatives of primitive Cladoeera. As such must be 

 regarded the Sidida?, which live in clear water. Now, since 

 the furca of the Cladoceran body points, by reason of its 

 shape, to a mode of life upon the bottom, such as we actually 

 see in the case of Estheria^ the occurrence of such a furca ia 

 the case of the Cladoeera is intelligible only if we derive 

 them from forms living upon the bottom. Such a mode of 

 life is, however, usually combined with a larger and heavier 

 body ; from this there results a further reason for deriving the 

 Cladoeera from an ancestral form distinguished by such 

 characteristics, and for regarding them as Crustacea of the 

 Estheria-type which have become adapted to the pelagic mode 

 of life, in consequence of which their development has been 

 arrested at a certain point. 



A third piece of evidence in support of the view that the 

 Cladoeera are to be derived from a young form of Estheria 

 is furnished by the condition of the compound eyes. In the 

 Cladoeera also the two compound eyes are united into an 

 eye-bulb and overgrown by a reduplicature of the skin. As 

 has already been shown in connexion with the discussion of 

 the peculiarities of the Estheria-type, the forcing asunder and 

 fusion of the two lateral eyes in Estheria is connected with 

 the strong lateral compression of the head, and this again with 

 the roofing-over of the head by the shell, and as being due to 

 the same cause is also to be explained the overgrowth of the 



