494 ALFRED GtBBS BOURNE. 



the Trematoda. Carl Vogt^ suggested, and Lang has insisted 

 upon, their connection with the Triclada, and especially with 

 Gunda segmentata. Professor Lankester refers them to 

 the Platyhelminthes in his * Notes on Embryology and Classi- 

 fication,' 1876. 



On the other hand, Cuvier, Leydig, and de Quatrefages have 

 dwelt upon the resemblances in their structure to Annelids, more 

 especially as regards nervous and circulatory systems, and 

 Huxley placed them with the Chsetopods and Gephyrsea in his 

 group Annulata. Hatschek has insisted upon their close 

 relationship with the Chsetopoda. Balfour also removed them 

 from among the Platyhelminthes, and treated them as an 

 independent class allied to the Chsetopoda. 



In drawing phylogenetic conclusions with regard to any 

 group of animals, we are led to consider certain series of facts 

 — viz. (1) the amount of variability in any particular system of 

 organs within the group itself; (2) the adult conditions of the 

 systems of organs in the group in relation to that condition in 

 allied groups; and (3) the ontogenetic history of the indivi- 

 dual genera. I shall only consider here, as 1 have put forward 

 no new developmental facts, the variability of structure in the 

 group and the anatomical relations with other groups. 



1. — Variability of Structure within the Group. 



In few groups of the animal kingdom (in relation to their 

 size) does the consideration of this q^iestion afford us more 

 valuable results than in the Hirudinea. The importance of 

 such considerations has been recently pointed out by Hubrecht^ 

 ■with respect to the nervous system of Nemerteans. 



In the Hirudinea we have in this respect to note the striking 

 correspondence — a correspondence evidently due to true homo- 

 geny and not to mere homoplasy — in certain minute details, 

 remarkable in themselves, between members of the group 

 which, in other respects, present the widest divergence. 



Let us consider, in the first place, the constancy of structure 

 which occurs. The nervous system presents the greatest simi- 



» Carl Vogt, ' Zoologische Briefe,' Frankfort, 1851. 

 ' 'Quart. Journ. Micr. Sci.,' vol. xx, 1880, p. 276. 



