PHYLUM ANNUL ATA 



509 



blood-capillaries run. The blood is thus brought into close relation 

 with the surrounding water, and the skin becomes a highly effi- 

 cient respiratory organ. The space between the epidermis and 

 the enteric canal is filled by a peculiar form of connective-tissue, 

 consisting of a gelatinous matrix with interspersed cells and fibres, 

 many of the former large and branched. More immediately 

 surrounding the enteric canal is the peculiar and charac- 

 teristic botryoidal tissue (b. t.) consisting of branched canals, the 

 walls of which are formed of large cells loaded with black 

 pigment. This system of canals is in communication on the one 

 hand with the blood- vascular system and on the other with the 

 greatly reduced coelome. 



Numerous unicellular glands are produced from the epidermis : 

 the gland-cells themselves lie in the connective-tissue, and are con- 

 tinued into long ducts which open on the surface. Special glands 

 in the ninth, tenth, and eleventh segments secrete the substance 

 from which the cocoon is formed (vide infra, p 515) : the segments 

 in question therefore constitute the ditellum. 



The muscular system is well developed, and consists of an 

 outer layer of circular (c. m.) and an inner of longitudinal (/. ?..) 

 fibres. There are also dorso- ventral fibres (d. v. m.) passing vertically 

 between the .pouches of the crop (vide 

 infra), and radial fibres extending from 

 the wall of the enteric canal to the in- 

 tegument: these take the place of the 

 septa of Chretopods. 



The alimentary organs are greatly 

 modified in accordance with the blood- 

 sucking habits of the animal. Surround- 

 ing the mouth are three jaws, one median 

 and dorsal (Fig. 408, d. j.\ the other two 

 ventro-lateral (v. I. j.). Each (Fig. 406) 

 has the form of a compressed muscular 

 cushion, with a sharp, evenly curved, free 

 edge covered with chitin, which is pro- 

 duced into numerous serrations or teeth : 

 by means of its muscles each jaw can be 

 moved backwards and forwards through 

 a certain arc, and the three, acting to- 

 gether, produce the characteristic triradiate bite in the skin of the 

 animal upon which the Leech preys. 



The mouth leads into a muscular pharynx (Figs. 407 and 408, 

 ph.) situated in the fourth to the eighth segments. Radiating 

 muscles pass from its walls to the integument, and by their con- 

 traction dilate its cavity and suck in blood from the wounds made 

 by the jaws. Around the pharynx are numerous unicellular 



FK;. 406. One of the jaws of 

 Hirudo medicinalis. 

 (After Leuckart.) 



