476 FRESH-WATER BIOLOGY 



sory pieces. These accessory pieces are usually from one-fourth to 

 two-thirds as long as the spicula themselves, and not uncommonly 

 possess an inward or backward extending apophysis whose function 

 is to anchor them firmly in position, or serve for the attachment 

 of special muscles. Long-necked unicellular glands are often 

 seen to empty into the cloaca near the distal ends of the spicula. 

 These probably serve a special purpose at mating time. The 

 form of the spicula and of their accessory pieces is useful in 

 distinguishing the various species, and as these organs are usually 

 viewed in profile the various terms used to describe them are 

 understood to apply to this aspect. The various forms and terms 

 are shown in the accompanying illustrations, Fig. 771. 



Among the male accessory organs the bursa is, in a number of 

 genera, the m^ost important, though there is no trace of it in the 

 greater number of the fresh-water genera. The bursa is a thin, trans- 

 parent flap-like expansion of the lateral cuticula of the tail end of the 

 male, and serves as a copulatory clasping organ. It may consist of 

 two distinct halves, one on each side of the tail, and each ending short 

 of the extremity, or the two parts may extend to the extremity 

 and coalesce to form a continuous flap encompassing the tail. 

 The bursa springs from the submedian or lateral regions, though it 

 is usually on the ventral side of the lateral lines and, furthermore, 

 is bent toward the ventral side. Typically the flaps spring from 

 the body somewhat in front of the anus, grow wider as they pass 

 backward, and reach their maximum development about opposite 

 the anus; thence onward they usually diminish, — though in some 

 cases not very much. In its maximum development the bursa may 

 possess flaps as wide as the body itself; from this maximum it 

 varies to rudiments that may easily be overlooked (pp. 484, 493). 



The bursa functions as a male clasping organ through the pres- 

 ence of muscular fibers adapted to close it ventrally, and through 

 the presence of so-called ribs which appear to be in the main, if not 

 altogether, tubular outlets for a cement-like secretion used to 

 fasten the male more or less permanently to the female at mating 

 time. No chemical examinations have been made of the cement 

 substances of the bursa and the caudal glands, but both are insol- 

 uble in water and seem otherwise similar. Some genera in which 



