CONTRIBUTIONS TO THE ANATOMY OF THE HIRUDINEA. 447 



though not so much so as in Nephelis, but outside, a regular 

 muscular wall is developed and only one or two definite com- 

 munications exist with the surrounding botryoidal tissue 

 (fig. 51). As I shall show when discussing the vascular 

 system in Hirudo, the blood after entering the more 

 deeply lying portions of the " cutaneous " network (botryoidal 

 tissue) passes into the intermediate portion which consists of 

 vessels with brown pigmented walls ; these are constituents of 

 the " vaso-fibrous " tissue (Lankester) . 



Branches of the botryoidal tissue can be traced passing 

 outwards between the muscles, the cells becoming smaller 

 and the walls very irregular ; these are connections between 

 the botryoidal tissue and the vaso-fibrous tissue (figs. 13, 14). 



The vaso-fibrous tissue in its turn communicates with the 

 most superficial " cutaneous " network, which is composed of 

 thin-walled capillaries. 



The vaso-fibrous tissue is actually developed doubtless from 

 botryoidal tissue, the gradual transition is shown in such con- 

 nections as the one figured (fig. 13, k). The homologous 

 tissues described in Pontobdella, which have not become 

 vascularised, are connected in the same way, the deeper lying 

 cells giving rise to the more branched superficial cells. 



I would suggest, interpreting my own and Professor Lan- 

 kester's observations (loc. cit.), that capillaries of the botry- 

 oidal tissue become converted into capillaries of the "' vaso- 

 fibrous " tissue, the cells dividing and remaining small, the walls 

 at the same time thinning and partially breaking down, becom- 

 ing very irregular, their nuclei dropping into the blood-stream. 

 It seems that this process might be carried further in one of 

 two ways, either — a, the lumen might become obliterated (which 

 would account for fig. 4, PI. XXVII, of Professor Laukester's 

 memoir), the cells or portions of cells remaining would in 

 this case become very finely branched, and, as in Pontobdella, 

 distributed in the subepidermic region, penetrate between 

 epidermic cells, and varying in quantity from spot to spot, form 

 the coloured pattern upon the body in Hirudo, or — b, they 

 might remain as capillaries, the degeneration of the Avail being 



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