THE DECAPODA 



291 



33. 



thoracic somite enclosing a c t avity within which may sometimes he 

 found the large foliaceous spermatophores deposited by the male. 

 In the Lobster (Homarus) (Fig. 169) a median pouch enclosed by 

 three processes on the sterna 



of the last two thoracic 



somites has the same function, 

 and in the Crayfishes of the 

 genus Cambarus (but not in 

 Astacus) a more complicated 

 organ in the same position 

 is known as the " annulus 

 ventralis." These structures 

 have not hitherto been studied 

 from a comparative point of 

 view, but it seems likely that 

 an investigation of their mor- 

 phology and their relation to 



3 P- 



L P 



f- 



Fio. 169. 



the structures occupying a 



Similar position in the Syn- , Sternal surface of posterior thoracic somites of 



" J female Lobster (Honutrus r/c iiimn /.-), showing the 



Carida Would yield important receptacuium seminis. VI-VIII, bases of the last 



lf three pairs of legs; <?, external openings of ovi- 



reSUltS. ducts ; l.p, lateral process on penultimate sternum ; 



Tn tlio fTi-oof rnainril-TT r>f '^-P. median process on last thoracic sternum; sp, 



111 tne great majority <I 8h like opening of receptacuium. 



Decapods the eggs after ex- 

 trusion are carried by the female attached to the abdominal append- 

 ages. Only in the Penaeidea they appear to be shed free into the 

 water immediately on extrusion, or carried for a short time only, as 

 in Leucifer, where they have been found attached to the posterior 

 thoracic limbs. The attachment of the eggs to the abdominal 

 appendages of the parent is effected by means of a cementing 

 material. As a rule this material seems to be produced by dermal 

 glands, which are found abundantly developed on the inner faces of 

 the pleural plates of the abdomen and on the uropods. In some 

 cases (Stenopus and Thalassinidae), Avhere the pleural plates are 

 slightly developed, the glands occur mainly on the pleopods, and in 

 the Paguridae they are distributed over the ventral integument of 

 the abdomen. In. the Brachyura, however, such glands are absent 

 or only little developed, and the function of producing the cementing 

 material is stated to be discharged by the receptacuium seminis. 



Secondary sexual characters among the Decapods are numerous 

 and varied. In many cases the males are distinguished from the 

 females by the greater size and different shape of the chelipeds and 

 by the narrower abdomen. Dimorphism of the males has been 

 noted in many cases, and Faxon discovered that in Crayfishes of the 

 genus Cambarus the two forms are alternating breeding and non- 

 breeding phases in the life -history of the same individual. A 

 closely similar series of changes has been found by Coutiere and by 



