THE DECAPODA 291 



thoracic somite enclosing a cavity within which may sometimes be 



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 sir. 



is known as the " annulus 



ventralis." These structures :zni - 



have not hitherto been studied 



from a comparative point of l-p." 



view, but it seems likely that 



an investigation of their mor- ' 



phology and their relation to 



the structures occupying a FIG. 169. 



similar nnsitirm in thp Svn- Sternal surface of posterior thoracic somites of 



Vn female Lobster (Homdrus gammariis), showing the 



Carida WOUld yield important receptacnlum seminis. VI-VIII, bases of the last 



1 three pairs of legs; 9, external openings of ovi- 



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



Tn f>iA trivia f mainrifv nf m.p, median process on last thoracic sternum ; s;>, 



in tile great majority <I Bli like opening of receptaculum. 



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 Leutifer, 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), where 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 receptaculum 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 



