“Hence in these classes or subkingdoms, such as the Protozoa, Cœlenterata, Echinodermata, Scolecida, secondary sexual characters, of the kind which we have to consider, do not occur; and this fact agrees with the belief that such characters in the higher classes have been acquired through sexual selection, which depends on the will, desire, and choice of either sex.” There are some cases, however, in which animals low in the scale show a difference in the ornamentation of the two sexes. A few cases have been recorded in the roundworms, where different shades of the same tint distinguish the sexes. In the annelids the sexes are sometimes so different, that, as Darwin remarks, they have been placed in different genera and even families, “yet the differences do not seem to be of the kind which can be safely attributed to sexual selection.” In regard to the nemertian worms, although they “vie in variety and beauty of coloring with any other group in the invertebrate series,” yet McIntosh states that he “cannot discover that these colors are of any service.” In the copepods, belonging to the group of lower Crustacea, Darwin excludes those cases in which the males alone “are furnished with perfect swimming legs, antennæ, and sense-organs; the females being destitute of these organs, with their bodies often consisting of a mere distorted mass,” because these extraordinary differences between the two sexes are no doubt related to their widely different habits of life. Nevertheless, it is important to observe that such extreme differences may exist in cases where sexual selection cannot come in, because of the absence of eyes in the female. In regard to another copepod, Saphirina, Darwin points out that the males are furnished with minute scales, which exhibit beautiful changing colors, and these are absent in the females; yet he states that it would be extremely rash to conclude that these curious organs serve to attract the females. Differences in the sexes are also found in one species of Squilla, and a species of Gelasimus. In the latter case Darwin thinks that the difference is probably due to sexual selection. In addition to these cases, recorded by Darwin, there may be added the two remarkable cases, shown in our Figure 2 A, B, of _Calocalanus pavo_, the female of which has a gorgeous tail worthy of a peacock, and of _Calocalanus plumulosus_, in which one of the setæ of the tail is drawn out into a long featherlike structure. In the former, the male is much more modestly adorned, as shown in Figure 2 C; in the latter species the male is unknown. [Illustration: Fig. 2.—A male of the copepod, _Calocalanus plumulosus_. B and C, a male and a female of _Calocalanus pavo_. (After Giesbrecht.)]