differ from each other in this respect. The shape and structure of the first antennæ and of the abdomen may

also show considerable modification in the males of various species of Branchiopods.

Among the Cladocera, of which the common waterflea of our fresh-water ponds and lakes is an example, the female is provided with a brood pouch, within which the eggs are carried and the young developed. In the male these structures are absent, and the second antennæ are especially modified as organs for discovering and holding the female. They are richly supplied with sensory hairs, and they are often armed at their tips with grappling hooks, which differ in the males of closely allied species.

FIG. 7. Antenna of male Cyclops serrulatus.

FIG. 8. Antenna of male Cyclops canthocarpoides.

The Ostracoda present sexual differences like those in the Cladocera, and in many of them it is certain that the male part deviates, more than the female part, from the typical form.

In the non-parasitic Copepods, of which the freshwater Cyclops (Fig. 9) is an example, there is not very much difference between the sexes, although certain appendages, which are unmodified in the female. and retain their typical form, sometimes differ greatly in the males of allied species, and may be specially mod

ified for discovering or holding the female. The modification of the first antenna of the male for this purpose is quite general, and a comparison of this part in the males of various species of Cyclops (Figs. 7 and 8) with the same part in a female (Fig. 9), shows how much the males of allied species differ in this respect. The second antennæ, the maxillary feet, and the last pair of

FIG. 9. Female specimen of Cyclops canthocarpoides.

FIG. 10. Female specimen of Notodelphys Allmani.

swimming feet, are sometimes modified in the same way in the male. In the male Saphirrina the wonderful display of brilliant colors is due to the presence of peculiar color-producing organs, which are absent in the female.

Among the parasitic Copepods we find a departure from the ordinary typical structure, which is so remark

able that no one, on first examining one of the more modified parasitic forms, such as the one shown in Fig. 15, would detect any resemblance to the free or nonparasitic members of the group, or would even suspect that the animals are crustaceans.

The females, which are known as "fish-lice," are parasites upon fishes and other aquatic animals, while the males are parasites upon the bodies of the females, and are usually of minute size as compared with the females.

The adaptation to a parasitic life has not only produced the most profound changes in the general structure, but it has also brought about an almost unparalleled difference between the sexes. It is true that this is not due to the modification of males alone, for the females as well as the males exhibit the most extreme departures from the organization which is characteristic of typical or non-parasitic crustacea, and it is difficult. to decide from structure alone whether the male or the female is most modified. The fact that the male has been adapted to a life as a parasite upon the body of the female, while the female has simply become adapted to a parasitic life on other animals, seem to show that the male organism is somewhat more plastic than the female. Simple parasitism may be brought about by indefinite variability, but parasitism upon a parasite demands definite variation to meet the definite changes which have taken place in the host.

The highly specialized parasitic Copepods are joined to the non-parasitic forms by a long series of intermediate species, in which the parasitic habit is only slightly developed, and I give a few figures to illustrate some of the steps in this most interesting series. The female Notodelphys (Fig. 10), which lives in the body cavities

of marine invertebrates, and has very limited powers of locomotion, hardly differs from the non-parasitic Cyclops (Fig. 9), except that two of the body segments are modified to form a chamber in which the eggs undergo their development.

The male (Fig. 11) is somewhat

FIG. 11. Male specimen of the FIG. 12. Female specimen of same species.

Lernentoma corunta.

smaller than the female, but bears a close resembance to her, and to ordinary copepods.

The female Lernentoma (Fig. 12) is very different from the male (Fig. 13), and both depart very greatly from the typical copepod structure, although a slight resemblance can be traced between the female and cyclops. The female is very much larger than the ordi