340 MR. E. E. UNWIN : NOTES UPON THE 



for its release at the time of ecdysis. Each oostegite is a large free oval 

 plate, overlapping its neighbours and its fellow of the other side to form a 

 very safe brood-pouch for the reception of the eggs. 



The first 4 pairs of legs carry these oostegites attached to the coxopodites. 

 The general structure can be seen by reference to PI. 26. figs. 10 & 11. It 

 is interesting to find that the detailed structure of the oostegites is very like 

 that of the brancbial plates. Both surface observations of living oostegites 

 and transverse sections of fixed material reveal a very similar arrangement 

 to that found in the 4th pair of pleopods. It is easy to see in living 

 specimens the flow of blood through the oostegites, and they show blood- 

 spaces and "pillars" of much the same structure as those described by Kimus 

 in connection with the branchiae of Asellus. When the oostegites are 

 released, the next operation is the passage of the eggs from the ovary to the 

 exterior by way of the very short oviduct and reoeptaculum seminis. The 

 fertilization probably takes place at this time, and the eggs are passed out 

 through the aperture at the base of the 5th pair of legs and conducted into 

 the brood-pouch. The 4th pair of oostegites can, by a slight bending of the 

 body, be made to overlap the apertures through which the eggs are passed 

 out. In this way, and by the help of the last two pairs of thoracic legs, the 

 eggs are passed into the brood-pouch. 



This operation does not take place directly the brood-pouch is available : 

 a period of from 5 to 12 hours elapses before the brood-pouch is filled with 

 the eggs. 



The number of eggs laid at a time varies somewhat, the smallest 

 number which I have found being 39 and the greatest 62 ; the average 

 of many specimens is 46. 



The succession of events can now be summarized : — 



1. Male and female in association until the hind-half of the female 



cuticle has been cast. 



2. Impregnation of the female by the male. 



3. Female released by the male from the marriage-clasp. 



4. Fore-half of the female cuticle cast ; the oostegites are released 



thereby. 



5. Fertilization and passage of eggs into the brood-pouch. 



Development period. — The eggs are well supplied with yolk, and the whole 

 of the development takes places inside the brood-pouch, which is, as we have 

 seen, external to the body and contained between the ventral body-wall and 

 the overlapping oostegites. The problem of a constant supply of fresh water 

 is solved in several waj r s. 



The movement of the legs in walking will cause a certain amount of 

 movement of the oostegites and thus cause a slight changing of the water ; 



