BARNACLES AND OTHER CRUSTACEANS 115 



an example. It is probable that the presence of ovaries 

 and spermaries in the same individual — the monoecious 

 condition — is an advantage to immovable fixed animals. 

 During the voyage of the " Beagle," and making use on 

 his return of the collections then obtained, Darwin 

 carried out a very thorough study of the Cirripedes of 

 all kinds from all parts of the world. He worked out 

 their anatomy minutely, classified the 300 different kinds 

 then known, and described many new kinds. The 

 stalked barnacles often occur in groups, the individuals 

 being of different ages and sizes, the small young ones 

 sometimes fixing themselves by their sucker-bearing 

 heads to the stalks of their well-grown relatives. In all 

 the varied kinds studied by Darwin he found that the 

 full-grown individuals were monoecious — that is, of com- 

 bined sex — as was known to be the case in those studied 

 before his day. But Darwin made the remarkable 

 discovery that in two kinds of stalked barnacles (not 

 the common ship's barnacles), comprising several species, 

 " dwarf males " were present perched upon the edge of 

 the shell of the large monoecious (bi-sexual) individuals. 

 These dwarf males were from one-tenth to one-twentieth 

 the length of the large normal monoecious individuals, 

 but usually possessed the characteristic details of the 

 shell-valves and other features of the latter. 



This existence of a sort of supernumerary diminutive 

 kind of male as an accompaniment to a race of normal 

 monoecious individuals was quite a new thing when 

 Darwin discovered it. That all the males in some 

 dioecious animals are minute as compared with the 

 females was known, and has been established in the case 

 of some parasitic crustaceans, in some of the wheel- 

 animalcules, and in the most exaggerated degree in the 

 curious worms, Bonellia and Hamingia. But the exist- 



