48 FREE AND FIXED ANIMALS 



singly in the first annulus of each segment (i.e., 

 in the annulus following that which carries the 

 segmental nephridiopores) ; each shows a dark 

 centre surrounded by a pale border on the top 

 of a retractile papilla. These segmental ocelli- 

 form organs commence in the segment in front 

 of the first nephridiopore, and there are twelve 

 of them in each segment, i.e., six on each side 

 disposed as follows : two lateral rows, one above 

 and one below the lateral band, a dorso-lateral 

 and a ventro - lateral row, a submedian dorsal 

 and a submedian ventral row. 



The preceding notes serve to illustrate, in 

 respect of external characters alone, the highly 

 specialised organisation of the land leech, which 

 is already indicated by the concentration of its 

 stereotropic mechanism, by its lack of regenera- 

 tion, and by its passion for blood which amounts 

 to a hsematotropism, as no one can doubt who 

 has read Sir Emerson Tennent's account of it, 

 or who has experienced it in the flesh. Hardly 

 anything proclaims a finished organisation, the 

 culmination of a phyletic career, so plainly as 

 an exclusive diet. In point of fact the leeches 

 represent the culmination at the present age of 

 the Annelid branch in a definite direction. 

 Branches of the tree of life often terminate in 

 a brush of points, not necessarily in a single 

 point ; and leeches constitute one of these 

 terminations, 



