290 THE CRUSTACEA 



grouped together as Catometopa these penes either lie for a short 

 distance from their base within grooves excavated in the last 

 thoracic sternum, or else they perforate the sternum directly, the 

 vasa deferentia in this case not entering the coxopodites of the 

 legs at all, as they do in most other Decapods. 



In most Macrura and in the Dromiidae the spermatozoa when 

 discharged are enclosed by a sheath of secretion which sets to a 

 firm membrane, forming a continuous cord-like mass. In Scyllarus 

 and in the Anomura this is broken up into separate spermatophores 

 attached by one end in a row on a strip of membrane. In the 

 Brachyura (except the Dromiidae) the spermatophores are quite 

 separate. 



The spermatozoa are remarkably varied and complex in 

 structure. As a rule they are provided with stiff radiating pro- 

 cesses which serve to attach them to the surface of the egg, and, 

 in some cases, an " explosive " apparatus is present which effects 

 penetration of the egg-membrane. 



The ovaries generally resemble the testes in shape and position. 

 In the Penaeidae they may extend through the whole length of 

 thorax and abdomen, but in most cases they are of less extent. In 

 Leucifer, and in most if not all Thalassinidea and Paguridea, they 

 lie wholly in the abdomen. They are always united across the 

 middle line, sometimes at more than one point. In the Crayfish 

 the two ovaries (like the testes) are joined together posteriorly so 

 that the organ has a trilobed form. Except in Leucifer and in the 

 Brachyura, the oviducts are simple in form and open on the 

 coxopodites of the sixth thoracic appendages (third legs). In 

 Leucifer, which is peculiar in so many points of structure, the 

 oviducts have receptacula seminis connected with them and unite 

 to open by a median aperture on the sternal surface of the thorax. 

 In the Brachyura, where intromittent organs are developed in the 

 male, the terminal part of the oviduct is of considerable length and 

 serves as a vagina, while a lateral pouch, sometimes double, with 

 glandular walls, forms a receptaculum seminis. In the Dromiacea 

 the receptaculum seems to be a temporary structure formed at the 

 time of copulation. In the Brachyura, with exception of the 

 primitive Dromiacea and of certain Oxystomata (Raninidae, some 

 Dorippidae), the oviducal apertures are removed from the coxo- 

 podites of the legs and open on the sternum of the corresponding 

 somite. 



In addition to the internal receptacula seminis mentioned above, 

 an external organ having apparently the same function is found 

 in certain Decapods. It is best known in the Penaeidae, where it 

 has been named the tlielycum and affords characters of systematic 

 importance. It lies on the sternal surface of the thorax and 

 appears to be formed by two or more outgrowths from the last 



