THE CCELOMATA INVERTEBRATA 183 



same individual. In spite of this, however, it is unable to fertilise 

 its own ova, and cross- fertilisation by another worn is brought about 

 in a manner we shall discuss shortly. The reproductive organs lie 

 towards the anterior end, and are included in segments 9-15. 



The most conspicuous parts of the reproductive organs are large 

 white sac-like structures, lying ventrally and laterally to the oeso- 

 phagus, apparently in segments 9-12 or 13. These are the sperm 

 reservoirs, or vesiculse seminales, in which are to be found sperma- 

 tozoa in all stages of development, and although they vary in size 

 at different periods of the year they are always readily made out. 

 They consist of two mesial sacs, the anterior and posterior median 

 sperm sacs, situated in segments 10 and n respectively. From the 

 front corners of the former arise a pair of antero-lateral sperm sacs, 

 which push forward in front of them the septum between segments 

 9 and 10, and so appear to lie in somite 9. The hinder corners in a 

 similar way give off the median lateral sperm sacs, which push back 

 the septum between 10 and n, and so seem to lie in segment n. 

 The postero-lateral sperm sacs come off from the caudal corners of 

 the posterior median reservoir, and although really lying in somite 

 ii project backwards into segment 12 and often 13, carrying the 

 septum between n and 12 with them. They are very large struc- 

 tures. In almost any part of the vesiculae seminales there may be 

 living Monocystis agilis, the protozoon parasite already studied, which 

 may be found by examining a smear of their contents under the 

 microscope. The actual male reproductive organs are difficult to 

 see, as they lie within the sperm sacs, and the dorsal wall of these 

 structures must first be removed. There are two pairs of testes, 

 one pair in each of segments 10 and n. They are small flattened 

 digitate bodies, attached to the hinder surfaces of the septa between 

 somites 9 and 10, 10 and n, and projecting freely into the cavity 

 of the sperm sacs. Each is formed by a localised thickening of the 

 ccelomic epithelium covering the septum, and is composed of a mass 

 of cells, the spermatogonia or sperm mother cells. The spermatozoa 

 are not actually produced in the testes, which only shed into the sperm 

 sac small groups of from eight to sixteen spermatogonia, and there 

 these are transformed into sperms. On the anterior faces of the other 

 septa in the same somites and opposite to the testes are four fairly 

 large funnel-shaped apertures, which from the complexly folded nature 

 of their margins are known as the " ciliated rosettes." It is to 

 these structures that the large parasite Monocystis magna is to be 

 found attached ; it does not float freely, as does its smaller relative 

 M. agilis. These funnels lead through the septa into very fine 

 convoluted tubes, the vasa efferentia, which unite towards the posterior 

 end of the twelfth segment to form the single vas deferens on each 



