548 



depression between the posterior two pairs of periopoda is sometimes filled 

 with a grey mass of firm and leathery texture" Again on p. 70 of his 

 article on the Penaeidea in the Ann. Mag. Nat. Hist. Vol. VIII, 1881, 

 he says of the female „a ventral plate varying in form and appearance, 

 according to species , lies between the last two pairs of legs : connected 

 with this, varying also in form according to species a large mass of 

 brown membranous material is attached, in some species large and fan- 

 like in shape, in others it is an irregular oval disk; but what relation it 

 has to the true history of the animal, observation has yet to determine.'' 

 In the lack of observation we may suppose that both the leathery, or 

 gelatinous , or membranous mass is in all cases a spermatophore , or at 

 least a secreted mass that has been poured out in connection with the 

 sperm, to protecfit, and to aid in its safe disposal within the cavity of 

 the Thelycum. The specific shape of these attached masses must be 

 connected with the specific form of the abdominal legs, which probably 

 shape the mass as well as transfer it to the Thelycum. 



That so little is said of these membranous masses in the Challenger 

 report is probably due to their loose, temporary nature, to their having 

 fallen of in the manipulation of specimens. 



We then conclude that in the Penaeidea the Thelycum is generally 

 used to hold the spermatophor mass, or to store up the sperm, and 

 that the Petasma is used to transfer the sperm , or the spermatophores, 

 to the Thelycum. 



In the exceptional cases such as Lucifer where there is no Thelycum 

 the Petasma probably transfers the spermatophores directly to the 

 mouths of the oviducts. This difference seems correlated with the mode 

 of carrying the eggs, since in both Lucifer and the Schizopoda where 

 there is no Thelycum and the spermatophores are inserted into the ovi- 

 ducts the eggs are carried upon the legs of the thorax, which is not 

 known to be the case in other decapods. 



An illustrated description of the anatomy of the Thelycum will be 

 published later. At present its resemblance to the sperm receptacles 

 of the lobster and of Cambarus leads to the inference that the ances- 

 tors of the crayfish must have had something like a Thelycum in the 

 female and something like the Petasma in the male. In Cambarus the 

 structures have been greatly changed in both sexes but still have the 

 same uses, in general. 



In Astacus the male abdominal leg is still modified but is used to 

 transfer spermatophores to the exterior of the shell of the female, the 

 female lacking any sperm receptacle. In crayfishes of the Southern 

 Hemisphere the semale organs also are lacking. Rather than assume 

 that the resemblances between the organs of the higher crayfish and the 



