444 ALFRED GIBBS BOURNE. 



all in the Gnathobdellidse is a question to which I shall 

 recur. 



In Pontobdella, outside the walls of the alimentary canal, 

 here and there among the muscles, and in large quantities in 

 the subepidermic tissue, occur cells which preserve to a large 

 extent their rounded form, but which have become closely 

 packed with fine brownish or greenish-brown pigment-granules. 

 These are modifications of the primitive rounded, or perhaps 

 slightly branched, corpuscles. In the adult leech no cells in- 

 termediate between the two are to be found, but in the young 

 Pontobdella (recently hatched) pigment-granules may be 

 seen forming in such cells. The development of the pigment 

 goes on side by side with the branching growth. 



These cells form a complete network in the deeper layers of 

 the subepidermic tissue, being arranged in rows, as are the cells 

 described above in Trocheta, and as are also cells which are 

 about to give rise to botryoidal tissue (figs. 35 and 36, 

 and fig. 9, d). 



Superficial to these in the subepidermic tissue occur cells 

 which are exactly similar to them in their structure, but 

 much smaller and arranged in groups; they appear, in fact, to 

 have arisen by a division of pigment-cells of the lower layer. 

 Their arrangement is very irregular, and they exhibit a ten- 

 dency to give rise to processes (fig. 37 and fig. 9). Superficial to 

 these, and doubtless derived from them, there is a layer of cells 

 which have become much broken up and branched, some of them 

 becoming perfectly ragged in appearance. Their colouring be- 

 comes a trifle darker. It is processes of such cells which force 

 their way in between the epidermic cells, and give rise to the 

 colouring on the surface of the body (fig. 38 and fig. 9,/) . These 

 three varieties are the only pigmented cells present in Pon- 

 tobdella. They are evidently modifications of the primitive 

 connective-tissue corpuscles which have become more branched 

 and ragged ; in other words, degenerate in character as they 

 approach the surface. Similar cells, presenting similar varie- 

 ties, and similarly disposed, are present in Clepsine in vary- 

 ing quantity, according to the species, but never developed to 



