36 THE LEECHES OF MINNESOTA 



but al of Somite XXV is partially divided and al of both XXVI 

 and XXVII is larger than a2. Neither annulus of XXVII is com- 

 plete, al reaching- only to the sides of the body and a2 not so far. 

 The disc is composed of somites XXVIII to XXXIV. Fig. 4 rep- 

 resents the arrangement of the furrows in a young animal. Somite 

 XXIV is the last segment of the body proper and its posterior 

 boundary forms in contracted specimens a fold which envelopes the 

 contiguous portion of the narrowed peduncle. The latter continues 

 to narrow to the sucker, to the middle portion of which it is strongly 

 attached for rather more than the posterior half. The posterior 

 sucker is large, circular and directed strongly ventrad. The nephrid- 

 iopores are in the sensory annuli of somites VIII to XI, and XIII to 

 XXIII and are placed similarly to those of P. parasitica. 



The mouth is very small and situated far forward near the 

 anterior rim of the sucker in somite II. As in related species the 

 proboscis is slender, and the crop provided with seven pairs of large 

 caeca reaching nearly to the margins of the body. The caeca, 

 however, are less deeply and finely divided than in P. parasitica, 

 each of the first six pairs exhibiting only two or three rather short 

 lobes. The intestine reaches to the posterior part of somite XXIV 

 or even beyond and then bends abruptly forward toward the 

 dorsum as an extremely narrow rectum reaching to the minute anus 

 situated at XXIII/XXIV. The forward curvature of the rectum and 

 the anterior position of the anus are unique features in the 

 family. The salivary glands are widely scattered through the an- 

 terior two thirds of the body. On either side of the oesophagus, in 

 somites X and XI, lie a pair of compact oesophageal glands which 

 join the oesophagus by a short duct in somite XL 



The reproductive organs are essentially similar to P. parasitica. 

 The male and female external orifices are situated at XI/XII and 

 XIIa2/a3 respectively. Six pairs of testes are crowded between the 

 bases of the gastric caeca. The large sperm sack and ejaculatory 

 duct of the vas deferens form a compact snarl in somite XII in the 

 immediate neighborhood of the atrium. 



THE ANATOMY OF THE CENTRAL NERVOUS SYSTEM 



OF LEECHES. 



A brief Historical Sketch. 



The first writer to describe in any way the central nervous sys- 

 tem of the leech was Poupart who, in 1697, spoke of a knotted 



