CONTRIBUTIONS TO THE ANATOMY OF THE HIRUDINEA. 445 



the same extent as in Pontobdella. The rounded cells may 

 attain a huge size in the central regions of the body. The 

 colouring varies, being generally rather greener than in 

 Pontobdella (see fig. 10, c, d). 



In Piscicola similar arrangements obtain; fig. 31 repre- 

 sents cells in the deeper subepidermic layers ; fig. 32, 

 cells from the more superficial portion. Much branched, 

 cells may also occur in the central region of the body (see 

 Leydig's beautiful figures of cells from the walls of the testes 

 and ovarian sacs.)^ 



In Branchellion none of these cells preserve their rounded 

 shape, but all become exceedingly finely branched, and neigh- 

 bouring cells become joined together by their processes 

 (fig. 34). The fine branches may be seen scattered through 

 the body, but become specially developed in the walls of blood 

 sinuses. 



In the subepidermic region and in the branchiae the develop- 

 ment of pigment is carried to the extreme, resulting in the 

 black colouring of, at any rate, the species I have examined. 

 All the pigmented cells present a black pigment in Bran- 

 chellion. It seems to be a general law that the more branched 

 such cells are, the deeper the colour of their pigment. 



b. The cells take part in the formation of a vascular system, 

 botryoidal tissue, vaso-fibrous tissue. Such an arrangement 

 obtains in all the Gnathobdellidae. 



Whether all the cells which develop pigment in the Gnathob- 

 dellidge become vascularised, even though they may subse- 

 quently lose all connection with the vascular system, is a 

 difficult point to determine. I am inclined to think they do 

 from the manner and region of the body in which they primi- 

 tively develop. 



The mode of development is perhaps best studied in Aulos- 

 toma, where the processes seem to be actively taking place 

 in the adult animal, doubtless owing to the enormous extent of 

 its digestive powers; a meal which will last a Hirudo two 



' ' Zeit. f. w. Zool.,' 1849, pi. ix, fig. 45, and pi. x, fig. 52 ; see also 

 fig. 11, d. 



