92 ARTHUR E. SHIPLEY. 



marked difference in size between the largest ovarian ovum 

 and the smallest floating one — a difference I am quite unable 

 to account for. 



The floating ova are oval in shape, the largest about 1 mm. 

 long, with a thick zona radiata, in which the radial markings 

 can only be detected with very high powers (fig. 30). This 

 membrane stains deeply except its outermost layer, which does 

 not absorb any staining fluid. The protoplasm is very granular, 

 aad stains well. The nucleus is very large, and sometimes 

 reaches almost from one side of the cell to the other ; it does 

 not stain at all. No micropyle was to be seen. 



The testis occupies in the male a position similar to that of 

 the ovary in the female. The mother-cells of the spermatozoa 

 separate from the testis before or whilst dividing. Whilst 

 floating in the perivisceral fluid the nuclei of these cells com- 

 menced to divide, and the whole floats about as a multi- 

 nucleated mass of protoplasm. The stages which most com- 

 monly occurred were those with eight or sixteen nuclei 

 (fig. 8). The males were much rarer than the females, and 

 none of them contained ripe spermatozoa. 



Summary. 



The following is a brief summary of the more important 

 points described in detail in the body of the paper. 



(1) The head of Phymosoma is surrounded by a stiffened 

 vascular horseshoe-shaped lip, the dorsal ends of which are 

 continuous with the ends of a hippocrepian lophophor. The 

 lophophor bears a crown of about eighteen tentacles — the 

 number is always even. In the hollow of the lophophor lies 

 the brain, which is continuous with the ectoderm of the 

 prseoral lobe. The inner surface of the tentacles and the 

 ectoderm above the brain is crowded with dark brown pigment- 

 gianules, and the ectoderm of the prseoral lobe is curiously 

 wrinkled. Between the hippocrepian lophophor and the 

 vascular lip is the crescentiform opening of the mouth. 



(2) At some little distance behind the lip is a thin but very 



