648 FRESH-WATER BIOLOGY 



penis and provided with a prostate gland. The ovaries, also 

 coelomic sacs, are a single pair, usually elongated and folded and 

 opening directly into a small median bursa. In the Hirudinidae 

 they have special ducts provided with an unpaired albumen 

 gland and a muscular vagina. Accessory copulatory glands may 

 occur. 



A most striking characteristic of leeches is the great reduction of 

 the body cavity which, besides the ovarian and testicular coelom, is 

 represented only by a system of sinuses, the extent and arrange- 

 ment of which vary in the several families. In addition there is a 

 true blood vascular system consisting of dorsal and ventral longi- 

 tudinal trunks and a peri-intestinal sinus united by transverse 

 loops and in the caudal sucker by a circle of radiating loops. Ex- 

 cept in some Ichthyobdellidae, which have lateral gills or pulsating 

 vesicles, leeches respire solely by virtue of the capillary network 

 underlying or even penetrating the hypodermis. 



The nephridia in general resemble those of the Oligochaeta, but 

 the funnels especially are more complex and variable, being some- 

 times branched and sometimes having the opening occluded. Not 

 more than seventeen pairs usually occur, they being absent from 

 both ends of the body and often from one or more clitellar segments. 



Pigment occurs in the form of excreted matter contained in 

 wandering cells and reserve cells and is usually deposited along 

 the line of muscle bundles in either metameric or non-metameric 

 spots or bands. The eyes are highly developed sensillae, several 

 of which are sometimes united in a common pigment mass. They 

 occur rarely on the caudal sucker as well as on the head. 



Leeches are among the most interesting and beautiful of the 

 invertebrate inhabitants of our fresh waters. They abound in 

 ditches, pools, ponds and lakes, few species occurring in swift, cold 

 streams. In the small lakes of our northern borders they fairly 

 swarm. 



They are predatory hunters or scavengers, temporary or nearly 

 permanent parasites, or they may change from one mode of life 

 to another. The few fresh-water Ichthyobdellidae attach them- 

 selves chiefly to the fins and gills of fishes. Several Glossiphonidae 

 have similar habits and one remarkable species is a nearly permanent 



