448 ALFRED GIBBS BOUKNE. 



continued to the extreme, giving rise, in fact, to the thin-walled 

 capillaries. As we find that the regular distribution of pigment 

 obtaining in Pontobdella (i.e. the more branched the cell 

 the more superficial its position) disappears to a varying extent 

 in other allied genera, so here, while a regular distribution of 

 pigments holds in Aulostoma, this distribution is partially- 

 modified in Hirudo, branching pigment-cells occuring in great 

 numbers in the walls of the dorsal and ventral sinus (fig. 62), 

 and to a certain extent in other parts of the body ; and finally, 

 in Nephelis we have an extreme condition (this may well be 

 placed side by side with the extreme condition of Nephelis in 

 relation to the development of botryoidal tissue) . The branched 

 pigmented cells in that genus become excessively branched, 

 degenerate in character, and as in the similar cases of Pis- 

 cicola and Branchellion, dark or even black in colour, 

 and distributed throughout the body. 



This theory which accords so well with the blood supply of the 

 parts and with the homology of these cells with those in other 

 leeches, seems to be rendered not only possible but highly 

 probable in the light of the facts which I have brought 

 forward. 



4. Eutoplastic metamorphosis. Vacuolation to form capil- 

 laries. 



The indirect method of capillary formation described above 

 can hardly be universal. 



There are certain thin-walled, non-contractile capillaries in 

 direct connection with true vessels (see p. 468), e.g. 

 the capillaries upon the gastro-ileal portion of the ali- 

 mentary canal which have no relation at all to botryoidal or 

 any pigmented tissue (fig. 47). Until embryological evidence 

 can be obtained respecting this matter, I can only suggest that 

 they are probably formed by the vacuolation of primitive connec- 

 tive-tissue corpuscles. In this connection must be considered the 

 fact tliat, those leeches where pigmented tissue never becomes 

 vascular, i. e. in forms where no canalisation of pigmented cells 

 has taken place, the -blood is always colourless, while in all forms 

 with red blood such canaliculisation of pigmented tissues has 



