346 DISCOVERY REPORTS 



pouch (Fig. 26 a-c). Anteriorly, directly below the ventral body wall of the animal, the 

 two coxal plates remain unfused in the middle line, forming an open groove, which runs 

 from the wide mouth of the thelycum a to its posterior opening p between the genital 

 pores, and which in Fig. 23 is closed by the sperm mass spm. 



Zimmer (191 3) described the thelycum as being derived entirely from outgrowths 

 from the sternum, but the view of Raab (191 5), that both the sternum and the coxo- 

 podites contribute to its formation, is confirmed in the present iresults, by examination 

 of transverse and longitudinal serial sections. 



FERTILIZATION 



Fertilization must be external. The spermatozoa, which are very characteristic oval 

 cells with large nuclei, have never been seen within the body of the female, although 

 Zimmer (1913) described them as penetrating into the thorax. It is clear from his 

 account, as Raab (191 5) pointed out, that he mistook the system of diffuse shell glands 

 sgl for sperm masses. In all the fertilized females examined during the course of the 

 present investigation, the sperm mass was always confined to the thelycum. The 

 specimens selected for sectioning also carried spermatophores, the contents of which 

 could be clearly seen in the same position (cf. Plate V, figs, i and 3) and were never 

 found to have penetrated within the body. 



When transference of spermatophores is about to take place, the first two pairs of 

 pleopods of the male, which are specially modified for the purpose, are extended for- 

 wards under the thorax. The spermatophores are expelled from the ejaculatory duct and 

 are caught by the pleopods, which transfer them to the thelycum. This operation must 

 be a rapid one, for although male specimens have been obtained with spermatophores 

 extruding from the genital pores, they have never been seen holding them in the 

 pleopods. 



Zimmer gives a detailed account of this transference and Raab agrees with it. If the 

 first pair of pleopods is bent forward under the thorax, the modified endopodites do 

 not lie under the genital pores, and could not therefore take up the spermatophores 

 when they are extruded. Further, all the special hooks etc. are on the under side of the 

 endopodite, away from the ventral surface of the animal. The second pair of pleopods, 

 however, if extended forward, coincides with the genital openings, the furrows on the 

 endopodites being turned towards the ventral surface of the body. They are thus in 

 a position to receive the spermatophores. The first pair of pleopods would be in the 

 way, and therefore they are probably bent out laterally during the process. 



According to Zimmer, fertilization takes place with the animals lying abdomen to 

 abdomen. In this position, however, the furrows on the second pair of pleopods are 

 opposed to the ventral surface of the female, whereas the organ on the first pair is 

 turned towards her. The spermatophores are evidently passed from the second to the 

 first pair of pleopods, which then fix them into the thelycum of the female, who must 

 be firmly held during this complicated proceeding. Zimmer states that the elasticity 



