which are pretty closely related to the copepods, which they resemble somewhat, during the early stages of their development. The young swim freely in the water for a time, but finally attach themselves to foreign bodies, head downwards, by their antennæ, and are sedentary for the rest of their life. In the stalked or pedunculated barnacles, the antennæ of the free larva become replaced in the adult by a long peduncle, at the top of

which there is an irregularly triangular box, the capitulum, made up of a number of calcareous plates. Inside this box the animal is placed, head downwards, and although it is greatly modified to fit it for this protected sedentary life, it still presents unmistakable evidences of its crustacean affinity, such as the mouth-parts, the segmented body and limbs.

One of the most remarkable characteristics of the Barnacles is that, with a few exceptions, they are hermaphrodite. The Arthropoda include a very considerable proportion of all the animals which are known to us, and as all of them, except the Barnacles and a few closely related parasitic forms, have the sexes separated, the fact that these few sedentary forms are hermaphrodite is certainly very remarkable, and we must believe that they are the descendants of crustacea with separate sexes. The stalked barnacles resemble typical crustacea much more closely than do the sessile ones, and we must regard the former as more closely related than the latter to the ancestral form with separated sexes. It is, therefore, interesting to find that a few species of stalked barnacles are male and female, and also that in a few others the ordinary hermaphrodite form is accompanied by a parasitic male, which has been called by its discoverer, Darwin, a complementary male.

The study of the few species with separate sexes and of those with complemental males has brought to light some of the most remarkable phenomena of natural science, and the subject is well worthy of extended notice.

Figure 16 is an ordinary hermaphrodite stalked barnacle, Pollicipes. It belongs to a genus in which no true males or true females are ever found.

Figure 17 is a species belonging to a closely related genus, Scalpellum, and it will be seen at once that it closely resembles Pollicipes, even in the arrangement of the plates of the capitulum. It is an hermaphroditelike Pollicipes, but with a difference, for it carries inside its shell a small parasitic complemental male, which is shown in Fig. 18. This male is very much smaller than the hermaphrodite, and Fig. 17 is considerably magni

fied, while Fig. 18 is of nearly the natural size; but with this exception the complementary male is essentially like the hermaphrodite, and it has the structure of an ordinary stalked barnacle. There is a distinct peduncle, which carries a triangular capitulum, and although the plates are somewhat reduced in number they agree in form and position with the chief plates of such a species as Pollicipes. The animal inside the capitulum is much like an ordinary barnacle, the essential difference being the total absence of female reproductive organs. It is a male and nothing more.

S.

FIG. 17. An hermaphrodite barnacle, Scalpellum villosum.

FIG. 18. Complemental male of the same species,

Figure 19 shows the female of another species, Ibla Cummingi, which does not differ essentially from the forms shown in Figs. 16 and 17, but the female of Ibla Cummingi is a true female instead of an hermaphrodite, and there are no traces of male reproductive organs, but inside her shell, and planted by a long rootlike process, there is a minute parasitic male, shown in Fig. 20, magnified thirty-two times, while the figure of the female is magnified only five times. In Fig. 20, b is part of the wall of the body of the female, and a is the long root by which the parasitic male is planted.

The male has a capitulum, but no calcareous plates, and its antennæ, an., are not completely merged in the peduncle. It also differs from the female in the possession of an ocellus, or eye-spot It has mouth-parts. and limbs, and, except for the fact that all its parts are somewhat rudimentary, it does not differ very greatly from other barnacles, except as regards its reproductive organs.

In other species of Scalpellum, however, as in Scal

Fig. 19. Female specimen of Ibla FIG. 20. Parasitic male of the same

pellum Regium, the male is still more rudimentary, and has no mouth or digestive organs.

In two other genera, Alcippe (Fig. 21) and Cryptophyalus, the females, which are true females, with no trace of male reproductive organs, differ very essentially from ordinary barnacles, and they have fastened. to the outside of their bodies a number of very small males. In the males of these two species, which are shown greatly magnified at Fig. 22, there are a few faint traces of muscular fibres, but the organs of digestion

are entirely gone, and the inside of the body is entirely filled with a great testis, while the posterior end is prolonged into an enormous penis; and the animal hardly deserves to be called an animal at all, as it is scarcely more than an independent male reproductive organ attached to the body of the female.

This is certainly one of the most remarkable cases of difference between the sexes, and no one who compares Figs. 18 and 22 with Figs. 16, 17 and 19, can doubt that

Fig. 21. Female of Alcippe lampas.

Fig. 22. Male of the same species.

among these barnacles the males differ from each other much more than the females.

Among the higher crustacea we find great numbers of cases where the young male is like the adult female, or the young of both sexes, but at maturity acquires distinctive sexual characters. Any one who is familiar with the crustacea will acknowledge the existence of this phenomenon, and it will only be necessary to give a few illustrations. The adult male Lucifer is distinguished from the adult female by the posses