THE DECAPODA 



291 



sternal surface of posterior tlioracic somites nf 

 female Lobster (Homarus (lammaru^), showing tlie 

 receptaciilum seminis. VI-VIII, bases of the last 

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

 ducts ; /.p, lateral i)rocess on penultimate sternum ; 

 ni.p, median process on last thoracic sternum ; up, 

 slit-like opening of receptacuUim. 



thoracic somite enclosing a, cavity -within wliich may sometimes 1)6 

 found the large foliaceous spermatophores deposited by the male. 

 In the Lobster (Ilomarus) (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 

 Antaeus) 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 

 the structures occupying a 

 similar position in the Syn- 

 carida would yield important 

 results. 



In the great majority of 

 Decapods the eggs after ex- 

 trusion are carried hy 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), where the pleural plates are 

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

 the PagLiridae they are distributed over the ventral integument of 

 the abdomen. In the Braehyura, 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 diff"erent 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 

 closel}^ similar series of changes has been found by Coutiere and by 



