mata Anglica, the animals are not hermaphrodites, as earlier writers had supposed, but that the males, which are rarely met with, are very much smaller than the females. The latter sex is furnished with a digestive tract which is quite complicated in structure, and is armed at the mouth with a highly specialized masticating apparatus. The digestive organs of the male, on the other hand, are almost absent. The jaws, the œsophagus and the mouth are wanting, and the stomach and intestine are reduced to a functionless rudiment. The males receive no nourishment after they leave the egg, and they live only a short time. The presence of a digestive tract is characteristic of all groups of animals above the protozoa, so we are compelled to believe that the ancestral form from which the Rotifera are descended had, like the ordinary metazoa, a mouth, a stomach, and an intestine; and no one who is at all familiar with comparative anatomy can doubt that the male, in which it is absent, rather than the female, in which it is present, is the sex which has been modified. The digestive tract is usually one of the first parts to be developed in the embryo, and its disappearance or absence in the adult male rotifer is therefore very different from the absence of the wings in certain female insects. Wings appear very late in life, and the failure of the female to acquire them is simply an arrest short of perfect development, while the absence of digestive organs shows active degeneration. In 1855 Leydig verified Dalrymple's observation (Zeit. f. Wiss. Zool. vi. p. 96) in the same species, and also in a second species of the same genus; and as he was able to distinguish the outline of the male inside the egg, while this was still contained within the body of the female, he removed all reason for doubting that the two sexes belong to one

species. In these two species the females were much alike, while the males were not only very different from the females, but also from each other.

Since the year 1855 the subject has been studied by many naturalists, and the males have been found in such a number of species that it is probable that the

sexes are separate in all the Rotifera. In some forms the males are even more simplified than in Notommata, while in others they are less so, and in a few they are like the females in size and structure, and have the digestive organs perfectly developed.

ANNELIDS. Among the marine polychatous annelids there is often considerable difference between the sexes, and the points in which the male differs from the female are also points in which the males of various species differ from each other.

ARTHROPODA.-Among the Arthropods, the Insects, Crustacea, etc., the female is often very greatly modified, and in some cases the females of allied species differ from each other much more than the males, and in other cases it is hardly possible to say whether the males or the females of allied species differ most, but, taking the group as a whole, the Anthropods seem to follow the law which prevails in other groups of animals, and male modifications are more numerous than female

modifications.

In the Branchiopod Crustacea the males are smaller than the females, and are much less abundant. The male differs from the female in the possession of a number of secondary sexual characters. The second antennæ of the male are more richly supplied with sensory hairs than those of the female, and various appendages of the male may be so modified as to form clasping organs for holding the female. In Branchippus the second antennæ of the male are greatly modified for this purpose. Figure 3 shows the head of a female specimen of Branchippus Grubei, figure 4 the head of the male of the same species, and figures 5 and 6 the heads of the males in two closely allied species. These figures show how much the males of the various species

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.

1

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