498 ALFRED GIBBS BOURNE. 



Hatschek^ has pointed out, the posterior sucker is composed 

 of a certain number of fused segments. The segmentation 

 here is, however, only part of the segmentation which has 

 gradually asserted itself in the course of the ancestral develop- 

 ment of the Leeches. 



The general arrangement of muscles corresponds with that 

 obtaining in the Triclada. The oblique muscular layers in the 

 body-wall and the dorso-ventral muscles are unknown in any 

 Chsetopod. 



The structure of the pharynx in the K-hynchobdellidse, as 

 Lang has pointed out, agrees very minutely with that in 

 Gunda segmentata. 



The Leeches differ markedly from the Chsetopoda in the 

 absence of parapodia and of setse, but it must be remembered 

 that Polygordius is devoid of setse, and Branchiobdella 

 also has no setse. 



The Leeches agree with some Chsetopods in the presence of 

 clitellum and in the practice of forming cocoons. The bran- 

 chial apparatus resembling that of some Chsetopods, though 

 present only in a well-developed condition in Branchellion, 

 is represented, as I have shown, in all the other genera of 

 Rhyncobdellidse. 



The segmented condition of the Leeches stands upon rather 

 a diflferent footing. Animals presenting a metameric segmen- 

 tation are not necessarily allied ; it is quite certain that such 

 segmentation has arisen at different times in different groups 

 of the animal kingdom, and it is quite possible that the seg- 

 mentation of a Leech has come about by a process of dysmero- 

 genesis, as opposed to that of a Chsetopod, which has been 

 attained by a synthesis of a eumeristic colony — a process of 

 eumerogenesis.^ 



With regard to what represents coelom in the Leeches, it 

 seems certain that this has once been more fully developed ; 

 the existence of remnants of coelomic epithelium, of a highly 



' ' Arb. aus dem Zool. Inst.,' Wien and Triest, torn. 1, 1878, p. 340. 

 ^ Cp. E, Ray Lankester, ' Encyclopaedia Brit.,' 9th edition, art. "Hydro- 

 zoa — Hypotliesis of the Individuation of Organs." 



