HIRUDINEA. 



395 



surrounding the mouth. There are no parapodia 

 few exceptions, are absent. A sharply distinct 

 head is never developed, since the first rings are 

 not essentially different from those following 

 and are never furnished with tentacles or cirri. 



Alimentary canal. The mouth is situated 

 near the anterior end of the body, sometimes 

 at the bottom of a small anterior sucker 

 (RhynchobdelUdce), sometimes at the base of a 

 projecting spoon-shaped hood, which resembles 

 a sucker (G nathobdellidcr} (fig. 319). The 

 mouth leads into a muscular pharynx provided 

 with glands. The anterior part of the pharynx, 

 which may be distinguished as the buccal 



cavity, is armed 

 (Gnathobdetti- 

 dce) with three 

 serrated chiti- 

 nous plates (fig. 

 319, a, b), or 

 more rarely 

 with a dorsal 

 and ventral 

 plate (Branchi- 

 obdellidce), or 

 it is provided 

 with a protru- 

 sible proboscis, which lies free in its anterior part 

 (Shynchobdellidce). The pharynx leads into a 

 stomach, which forms a straight tube in the 

 axis of the body and sometimes shows con- 

 strictions, which correspond with the segments ; 

 sometimes it is produced into a larger or smaller 

 number of lateral caeca. From the stomach a 

 short rectum, which is sometimes also provided 

 with cfeca, leads to the anus. The anus is placed 

 at the posterior pole of the body, dorsal to the 

 sucker. 



Excretory organs. Segmental organs are pre- 

 sent, one pair to each segment in the middle 

 region of the body. Their number, however, 



and setje, with a 



FIG. 319. c Cephalic region of the 

 Medicinal Leech. The three jaws are 

 visible, b, One of the jaws isolated 

 with the finely serrated free edge. 



FIG. 320. Lonsritudina 

 section through the 

 Medicinal Leech (after 

 R. Leuokart). D, in- 

 testinal canal; G, 

 cerebral ganglion ; 

 Gk, ganglionic chain ; 

 Ex, excretory canals 

 or segmental organs 

 (water vascular sy.~- 

 tem). 



