20 ARTHUR E. SHIPLEY. 



marked diflFerence 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, and 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- 

 granules, and the ectoderm of the pra?oral lobe is curiously 

 wrinkled. Between the hippocrepian lophophor and the 

 vascular lip is the crescentiform openiug of the mouth. 



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



