ON PHYMOSOMA VARIANS. 91 



running round the introvert (fig. 21). One of these organs 

 is shown in fig. 12 ; the ectoderm-cells have multiplied and 

 increased in size, forming a small heap ; some of these cells 

 have then formed stiff processes, which project beyond the 

 level of the skin. These processes are gathered up into a small 

 brush by a chitinous ring which surrounds the base. 



The hooks (fig. 21) are very closely packed in a series of 

 ridges formed by the circular muscle-fibres of the introvert. 

 The point is directed backward, while the row of sense-organs 

 lies immediately behind them, embedded in the muscular 

 cushion. 



The Generative Organs. 



Phymosoma variaus is dioecious ; in no case are ova and 

 spermatozoa found in the body of the same individual. 



The ovaries are formed by a fold of the peritoneal epi- 

 thelium, elsewhere flat, which occurs at the base of the 

 insertion of the long ventral pair of retractor muscles. This 

 genital ridge extends beyond the inner edge of the muscle 

 attachment across the ventral middle line lying between the 

 nerve-cord and the skin ; it does not extend beyond the outer 

 or dorsal end of the muscle. The ridge is not quite con- 

 tinuous, but it is interrupted from time to time ; its free 

 border is also irregular, and this gives it a puckered or frilled 

 appearance (fig. 22). 



In transverse section — parallel to the long axis of the 

 Phymosoma — the ovary is seen to be much thicker at its free 

 border than at its base ; the latter indeed is formed of but two 

 layers of cells, thus giving the appearance of a simple fold of 

 endothelium. These layers, however, thicken towards the free 

 edge. Nearly all the cells have become ova, and are held 

 together by a very scanty matrix. The organ is solid, and 

 the ova dehisce from it into the body -cavity. 



In the ovary the ova increase in size towards the thickened 

 free edge, where the oldest are. Those found free in the body- 

 cavity also differ somewhat in size, and undoubtedly grow 

 whilst suspended in the perivisceral fluid ; but there is a veiy 



10—2 



