1883.] Heterogenetic Development in Diaptomus. 383 



larged variety or age form of D. sanguineus, but the writer must 

 still express his decided belief that these must all be referred to 

 the European D. castor. 



It must be admitted that the intensity of coloration does not 

 depend upon season but upon the conditions of the water which 

 may or may not be influenced by the time of year. In the same 

 month I have found the same species of all colors, from colorless 

 to deep crimson-red or variegated red, yellow and blue or purple. 

 Weissmann seems to have neglected these facts in referring the 

 coloration of many species of Cladocera to sexual selection. I 

 have found in every case where the Diaptomus was intensely red, 

 the species of Cyclops, usually green or bluish, would be more 

 or less red also. In Swan lake, near Decatur, nearly all the 

 Cladocera were brilliantly marked, sida and sirnoceplialus (?) be- 

 ing most so ; in the neighboring lakes these species were pale 

 as usual. 



With respect to the identity of our species, ist. The armature 

 of the last segment of thorax is usually obscured by doubling 

 over. Fig. 12, Plate vi of Brady gives the large thorn but omits 

 the lower process. 2d. Fig. 7, Plate vi of Brady figures the pro- 

 cess on the antenna of male. (Claus gives the best figures.) 3d. 

 Fig. 5, Plate i .of Cyclopidae of Minn., Herrick shows that the 

 inner branch of the male fifth foot is armed by three spines (as 

 figured by Brady) in Minnesota specimens. It must be remarked 

 that this applies to young forms only, and that the later forms lose 

 them and become shorter. Brady has probably transposed his 

 numbers as regards the female feet of fifth pair. 



The serrature of the spines differs between different age forms. 



Diaptomus pallidus Herrick (Plate vii. Figs. 1-6) is quite rare 

 as compared with the foregoing. The peculiarities mentioned in 

 the original notice suffice to distinguish it from any other known 

 to me. D. sicilis Forbes, seems to sustain the same relation to 

 pallidus that giganieus does to castor. The two-jointed character 

 of the inner branch of female feet in sicilis confirms this view. 

 There are several differences however. 



Char. spec. — Extremely slender ; head separate from thorax by 

 a suture ; antennae longer than the caudal setae ; setae very long ; 

 fifth foot of female very short, inner branch with only one ter- 

 minal seta at end ; the male fifth feet long ; the jaw is like that of 

 castor. 



VOL. XVII. — NO. IV. 87 



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