516 



ANNELIDA. 



in the common leech lie in front of the brain, and send their nerve- 

 plexuses to the jaws and pharynx, are considered 

 by Leydig to be enlargements of cerebral nerves, 

 and very likely control the movements which 

 occur in swallowing. 



The alimentary canal. The mouth leads into 

 a muscular pharynx provided with salivary glands. 

 In the Rliyncobdellidae the anterior part of the 

 pharynx is protractile and highly muscular; it 

 forms the so-called proboscis. In the Gnatlio- 

 bdellidae the anterior part of the pharynx projects 

 as three prominent ridges, over which lie the 

 three serrated chitinous plates or jaws (Fig. 417). 

 Occasionally the jaws are reduced in number 

 (Greobdella), or 

 they may be vesti- 

 gial (Trochetia), 

 or entirely ab- 

 sent (Leptostoma 

 edentuluni). The 

 pharynx (Fig. 

 418) is followed 

 by a tubular 

 oesophagus (in- 

 conspicuous in 

 Hirudo), which 

 leads into a thin- 

 walled stomach 



(sometimes called the crop or proventriculus) pro- 

 vided with paired, segmentally arranged lateral 

 caeca. These vary in number in different forms, 

 and the last pair are much the largest, and extend 

 backwards on each side of the intestine. The 

 blood swallowed by the animal is stored up in 

 these caeca, consequently they are best developed 

 in forms which feed most rarely ; in Aulastoma, 

 which does not suck blood at all, but feeds on 

 small worms, etc., the last pair alone are present. 

 From the stomach a short intestine, the anterior 

 part of which is often provided with two dorsal 

 caeca (Hirudo), leads back to the anus. The 



a 1) 



FIG. 417. ft, cephalic region of the 

 Medicinal Leech ; the three jaws are 

 shown, b, one of the jaws isolated 

 with the finely serrated free edge. 



FIG. 418. Diagram of a 

 longitudinal section 

 through the Medi- 

 cinal Leech (after 

 Leuckart). D stom- 

 ach with 11 lateral 

 caeca ; G cerebral 

 ganglion ; Gk ventral 

 nerve-cord; Ex 

 nephridia. The in- 

 testinal caeca are 

 not shown. 



