in the Decapod Crustacea. 341 



VI. 



Generally we think that the modifications due to parasitic 

 castration must be assimilated to those which are the result of 

 progenesis. We say that there is progenesis when in an 

 animal sexual reproduction occurs in a more or less preco- 

 cious manner, that is to say, when the sexual products (ova or 

 epermatozoids) are formed and matured before the creature 

 has attained its full development. As examples may be cited 

 the axolotls and larvajof Tritons, which, the former normally, 

 the latter occasionally, oviposit while they still possess their 

 branchiffi. 



Very often progenesis affects only one sex. Sometimes it 

 is the female sex tliat ripens in the larval state, as in the 

 Aphides *, 8tylop>s^ &c. Sometimes it is the male sex, as in 

 Bo7ieUia, the complementary males of the Cirripedes, the 

 pigmy males of the Rotifera, the male of the salmon and the 

 eel, &c. 



In other cases again the, animal presents the two sexes 

 successively with progenesis in one of them. Thus there is 

 protandric progenesis in the Cymothoadian Crustacea, which 

 are males when young, and become females as they grow old 

 and complete their development. The case of old female 

 Gallinaceee with masculine plumage and instincts, on the 

 contrary, seems to be an imperfect example of protogynic 

 progenesis, since these females have laid eggs when they had 

 still the livery of the young, and have subsequently continued 

 their development and presented the characters of the males, 

 without, however, the production of spermatozoids having 

 been seen. 



In extreme cases of female progenesis reproduction even 

 takes place without the assistance of the male element, thus 

 reverting to the primordial agamic form. These cases have 

 been long known under the name oi pcedogenesis. They have 

 been observed in the larvse of Miastor and Gliironomus , and 

 in certain Aphides, The supposed alternation of generations 

 in the Trematoda must also be regarded as a very strongly 

 marked case of female progenesis (paidogenesis), and so also 

 perhaps in other cases still regarded as examples of alterna- 

 tion of generations. 



Whenever there is progenesis in a particular type we there- 

 fore recognize either momentarily or definitely an arrest of 



* According- to a very recent note by M. R. Moniez there is in certain 

 Apliides [Coccidje] pnxjenesis in the male. In this case {Lecan'mm hespe^ 

 ridum) the male remains rudimentary and in a manner parasitic on the 

 female, like the male of Bonellia (' Comptes Rendiis,' February 14, 1887). 



Ann. & Mag. N. Hist. Ser. 5. Vol. xix. 24 



