The Evidence from Sexual Characters. 181 



One of the most remarkable characteristics of the 

 Barnacles is that, with a few exceptions, they are her- 

 maphrodite. The Arthropoda include a very consider- 

 able 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 hermaphro- 

 dite 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 accom- 

 panied 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 cornplemental 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 bar- 

 nacle, 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 hermaphrodite- 

 like 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- 



