82 



Part III. — Eighteenth Annual Report 



size of maturity. The following crabs under 6 inches were found with 

 ripening ovaries, viz. two crabs at 5 inches ; two at 5^ inches, one at 

 each of the following sizes — 5J, 5|, 5^, 5|, 5-J ; two at 5f. In the 

 smallest size we have a close agreement with the data afforded by the 

 berried crabs and those with empty egg-capsules — viz., 4J, 



So far the term mature has been limited to those crabs which give 

 proof that they have actually reproduced their species (berried) or are 

 preparing to do so (ripening ovary). It is now necessary to include in 

 the term mature those which are capable of reproducing their species. 

 A female crab may be physiologically mature when of a certain size, 

 and still its ovary may not ripen. The male crab does not, as in most 

 forms, impregnate the eggs directly. It introduces its sperms into the 

 spermatheca of the female immediately the latter casts its shell, and the 

 sperms are retained in the specialised diverticulum of the vagina for 

 probably in most cases about a year before they are used to impregnate 

 the ova. It then follows that if a crab is not fertilised when it casts 

 it cannot reproduce its species till the next cast. A crab measuring 4g 

 inches across is capable of fertilisation, but many crabs measuring from 

 4^ to 5| inches are not fertilised. The latter will not be functionally- 

 mature till they are of larger size. So far the evidence will not 

 permit me to say that every crab at 4J inches is thus mature, nor is 

 it quite certain that no crab smaller than that is mature. Of 54 crabs 

 measuring 4^-5§ inclusive, 29 were fertilised and 25 unfertilised. 

 No fertilised crab was found among the 27 less than 4| inches across ; 

 and every crab above 5| inches, viz. 76 in number, was fertilised. 



Fertilised and Unfertilised Female Crabs. 



Analysis of 157 Female Crabs, measuring from 3-8-1- inches. 



Inches. 



1 



3-4 



41 







4? 



4| 





5 



51 



5i 



5§ 





Unfertilised 



27 



4 



5 



■ 



3 



2 



3 



3 



2 



1 



2 





Fertilised . 



• 



2 



1 



l 



1 





2 



7 



3 



7 



3 



76 



A female crab which is capable of being fertilised differs structurally 

 from an actually immature specimen, in that the vagina of the former 

 is of large diameter, and the spermatheca is a sac of considerable size ; 

 in the latter the vagina is narrow and the spermatheca is so small that 

 it is with difficulty made out. The latter condition has been* found in 

 crabs 3|, 3| (2), 3|, (3) 4 T 1 g -, 4| (2) inches. These crabs could not be 

 fertilised because the vagina would not admit the penis of the male. 



Fertilisation. 



The male crab, it is very probable, only has connection with the 

 female at the moment the latter casts its shell, and while the new shell 

 is still pliable. The male copulatory organ is paired, as also are the 

 vagina and spermatheca paired. Each penis consists of two elements — 

 the anterior, which is tubular, situated on the first abdominal segment ; 

 the posterior, which is solid, rod-like, situated on the second abdominal 

 segment (Plate I., fig. 8). The posterior element is received into the tube 

 of the anterior. The penis is inserted into the spermatheca and the 

 spermatophores are by it conveyed there. In the female the genital organs 

 consist of two vulvae, two vaginas, and two spermathecaa. When 

 the female casts, the inner lining of the vagina and spermatheca are 

 shed with the old shell. At this time the glands of the spermatheca 



