ZOOLOGY' AND BOTANY, MICROSCOPY, ETC. 353 



Eyes of Apus.*— W. Wenke finds that in the eyes of Apus productus 

 the cornea is without facets ; there are true crystalline cones which have 

 Sernper's nuclei in their sheath ; each retinula has usually seven optic 

 cells : the nuclei of these cells lie in the proximal part about the same 

 level. At the pointed pole of the crystalline cone there is a special zone 

 (Hesse's Schaltzone), in which the neurofibrils running parallel to one 

 another are seen emerging from the coalesced rhabdomeres (" Stif tchen "). 

 The neurofibrils bend round, insinuate themselves between nucleus and 

 wall of the retinula-cell, and penetrate the basal aperture of the retinula- 

 cell or the basal membrane. According to the illumination there is 

 movement of the pigment granules : they accumulate on the membranes 

 turned to the light. The structure of the eye is illustrated by ex- 

 ceptionally good figures. 



A dorsal frontal organ passes from each optic ganglion to the hypo- 

 dermic The median eye is tetrapartite. Its optic cells are prismatic 

 and without the rods. The chief nerve-strand of each prism-cell divides 

 into as many tufts as the prism has sides. Each tuft forms at the cell- 

 membrane a " Schaltzone," which passes into a rhabdomere. 



Varieties of Daphnia.t — R. Woltereck has studied two varieties of 

 Daphnia longispina, which seem to be elementary species. But extreme 

 and opposed nutritive conditions induce the production of variants that 

 lessen the divergence between the two varieties, though some difference 

 persists. These artificially induced variants tell against the idea of the 

 origin of the varieties by mutation. The group of lacustrine species of 

 Daphnia (excluding D. magna, but probably including D. pulex), is to be 

 regarded as a single species with numerous " fixed local varieties," or as a 

 large number of species. "Woltereck regards these varieties or species as 

 due to the ceaseless variability — in dependence on environment. Conver- 

 gence is very important in this connection, but selection is of secondary 

 moment as regards the differentiation of the external form of Daphnids. 

 There is very little evidence of mutation. 



Species of Cyclops.! — Esther F. Byrnes has studied fifteen species 

 of Cyclops in the fresh water of Long Island, with special reference to 

 their variability. A new species C. virido-signatus is described. Com- 

 parison with forms from the western lakes brings out the wide distribution 

 of some species. It is noted that great abundance of a species may be 

 followed by decline, till only a few representatives remain. There is 

 considerable variability in armature and proportions. Spines and seta3 

 are interchangeable. An apparent variation in the spines of Cyclops is 

 often due to a retardation of growth in the posterior appendages. Size 

 is greatly influenced by habitat. The study of variation is greatly 

 complicated by the occurrence of pedogenesis, by the occurrence of 

 heterogeneous forms in certain groups, and by retardations of nieta- 

 morphic changes which look like variations but are afterwards obliterated. 

 The greatest ranges of variability have been found among those forms 



* Zeitschr. wiss. ZooL, xci. (1908) pp. 236-65 (1 pi. and 13 figs.), 

 t Ver. Deutsch. Zool. Ges., 18 Jahres., 1908, pp. 234-40. 

 \ Cold Spring Harbor Monographs, vii. (1909) 43 pp., 15 pis. 



Ju ne 16th, 1909 2 b 



