Classification of the Crustacea. 455 



The second antenna lias preserved the form of the biramous 

 swimming-foot, while this is likewise retained in the palp of 

 the mandible. The maxillae and the double pair of maxilli- 

 peds, produced by the separation of the outer and inner rami 

 of the second maxilla, exhibit the shape of the Phyllopod 

 limb. As opposed to Apus the most primitive Eucopepods, 

 the CalanidiB, with reference to what must be assumed to 

 have been the ancestral forms of all existing Crustacea, 

 possess more primitive characters in the retention of the 

 biramous second antenna and of the mandibular foot, as well 

 as of the foliaceous foot-shape of maxillae and raaxillipeds. 

 In Apus, indeed, the second antenna is wanting in the adult 

 state or is only present in a vestigial condition, and similarly 

 the mandibular palpi are absent and both raaxillai reduce 1 in 

 size. 



The thoracic feet of the Eucopepods exhibit the swimming- 

 foot shape which is characteristic of the group — a two-jointed 

 stem and a pair of three-jointed elongated rami. For their 

 special shape a connexion is to be found in the case of Apus 

 among the Euphyllopods. The special swimming-foot-like 

 development of the limbs of Apus may here be emphasized 

 once more. The Copepod foot, howevei", has lost the epipo- 

 dial appendages by degeneration ; the segmentation of its 

 two rami was probably originally more extensive, as we may 

 conclude from the shape of the foot in Argulus. The elon- 

 gated form of the Eucopepod furca, too, occurs again among 

 the Apodida3, and in this connexion the larval conditions of 

 the latter must in the first place be considered. 



The compound lateral eyes have usually disappeared in 

 adult Eucopepods, and are retained in a modified form only 

 in the Pontellidae ■^. On the other hand, I was able to provef 

 that in the Nauplius-stages of Cetochilus (and this in all 

 probability also applies at least to the other free-living 

 Eucopepods) extensive rudiments are present for the paired 

 lateral eye, but these nevertheless undergo degeneration after 

 they have severed their original connexion with the integu- 

 ment. Consequently, after the facts which have been adduced 

 there can be no doubt that the ancestors of the Eucopepods 

 possessed compound lateral eyes, and that these merely 

 underwent degeneration at a later date. A comparison 

 between the sinking-in of the eye of Apus and the severance 

 of the eye-rudiments from the integument in Cetochilus as a 



* Cf. C. Claus, " Das Medianauge der Crustaceen," Arbeit der zoolog. 

 Instituts zu Wien, Bd. 9, 1891, p. 26. 



t C. Gvobben, " Die Eutwicklungsgeschichte von Cetochilus septen- 

 trionalis," ibid. Bd. S, 1881, pp. 20 aud 36. 



