442 ALFRED GIBBS BOURNE. 



Clepsine (fig. 25, fig. 9, «, and fig. 10, a), and are very 

 numerous. They may remain small in size or increase greatly ; 

 certain cells of this nature which are relatively enormous occur 

 near the nerve cord. The nucleus always presents a size com- 

 parable to that of the cell. The most usual modification which 

 takes place is that a semifluid substance accumulates in drop- 

 lets in the cell, giving it a reticulate, vacuolated appear- 

 ance : such cells closely resemble Waldeyer's plasma cells 

 which are found in mammalian areolar tissue (see fig. 9, b, 

 fig. 25, c). 



Another modification is the formation of highly refringent, 

 almost greenish-looking granules, which are arranged in smaller 

 or larger groups (fig. 26). Similar cells occur in Piscicola, 

 but in this genus and in Clepsine the most common 

 modification results in an accumulation of fat (fig. 27, and 

 fig. 10, b). Such cells closely resemble the cells of the 

 developing adipose tissue of Mammalia. The fat accumulates 

 in small globules, which run together and form a large 

 globule ; the nucleus is pushed to one side, but the protoplasm 

 never disappears so completely as in mammalian fat-cells. In 

 a variety of these cells, which obtain a very large size, the 

 fat-globules never run together, but always remain as scat- 

 tered globules. The globules present the usual microscopic 

 appearance of fat-globules, and behave in the same manner 

 towards reagents. They stain perfectly black when treated 

 with osmic acid. 



These fat-cells do not occur in the Gnathobdellidae. The 

 only genera among the Gnathobdellidae which present these 

 rounded connective-tissue cells areTrocheta and Aulostoma. 

 They occur in Trocheta in enormous numbers (fig. 14, a), 

 and, remaining packed closely together, form dense masses. 

 There is, however, a distinct tendency for them to be arranged 

 in rows, probably prior to their conversion into botryoidal 

 tissue (see below). 



In Aulostoma they are apparently undergoing active 

 metamorphosis, forming simultaneously side by side fibres and 

 pigmented cells (botryoidal tissue) (fig. 28). 



