500 ALFRED GIBBS BOUENE. 



and by the strongly-marked metameric segmentation of its 

 nerve-cord and some other organ-systems. But these cha- 

 racters, although they are shared by the Leeches with the 

 Cha^topoda, do not appear to me to be of value as indications 

 of affinity. They are merely the expressions of a necessary 

 method of elaboration of structure which probably occurs in 

 several parallel groups. A coelom and a segmentation of the 

 body are not specific indications of community of ancestry, and 

 cannot be recognised in a phylogenetic classification by asso- 

 ciating the Leeches in the same group with the Chaetopoda. 



On the other hand such minute details as the structure of 

 the muscles of the body-wall, the structure of the pharynx, 

 the median position of the male and female genital pores 

 united in one individual, and the general features of the struc- 

 ture of the reproductive organs — common to the Leeches and 

 the Triclada — cannot, I venture to think, be regarded with 

 any probability as owing their agreement in the two groups to 

 mechanical necessity apart from heredity. They imply the 

 existence of an ancestor common to the Leeches and the 

 Triclada (with which of course go also the Trematodes and the 

 Cestodes) possessing these characters. Whether the Leeches 

 have advanced very greatly beyond the degree of elaboration of 

 organisation exhibited by that common ancestor, or whether 

 the ancestor was near in all points except segmentation to the 

 existing Leeches, whilst the Triclada and Trematodes have 

 degenerated, is a matter for further inquiry. 



