THE LEECHES OF MINNESOTA 19 



and socald sinuses, some of which, if not all, represent portions of 

 a true body cavity. Portions of this system have contractile walls. 

 The blood is kept in circulation by the more or less regular pulsa- 

 tions of these vessels and the irregular contractions of the very 

 muscular body. 



Respiratory System. 



None of the leeches found in Minnesota have any special organs 

 of respiration. That function is performd almost wholly by the 

 skin. Two marine genera, Branchellion and Ozobranchns, have gills on 

 certain somites. 



The Excretory System. 



The excretory system consists of a series of more or less highly 

 differentiated tubes cald nefridia. Each somite, excepting a few at 

 the anterior end and a few at the posterior end, contains a pair of 

 these nefridia, one on each side. Typically the nefridium consists of 

 a funnel-shapt "mouth" that communicates with a small space rep- 

 resenting the body cavity, a glandular portion well supplied with 

 blood-vessels and a non-glandular, bladder-like reservoir which 

 opens to the exterior thru a small pore (the nefridiopore) on the 

 ventral side of the body. When a living leech is wipd dry on the 

 •ventral side and is then carefully comprest laterally, small drops of 

 the secretion of the comprest nefridia may be made to appear at 

 the nefridiopore and make evident the position of the pore. 



The Nervous System. 



The central nervous system is essentially a series of paird 

 ganglia (collections of nerve cells and nerve fibers) connected by 

 a double nerve. In this chain we recognize a pair of small, some- 

 what pearshapt ganglia near the mouth on the dorsal side of the 

 farynx or esofagus. They are connected with each other at their 

 larger ends and are cald the brain or supra-esofageal ganglia. The 

 smaller end of each ganglion is continued into a nerve that passes 

 around the farynx to meet the one from the other side beneath the 

 farynx on the midventral line in a paird ganglion often cald the sub- 

 esofageal ganglion. 



From the sub-esofageal ganglion the double nerve extends along 

 the midventral line of the body wall, immediately under the di- 

 gestive tract, to the posterior end, connecting a series of ganglia, 



