: PHYLUM ANNULATA 467 



In front of the first and behind the last pair of nephridiopores 

 one important external mark of segmentation fails, but a further 

 indication is furnished by the presence on the first ring of each 

 undoubted metamere of a number of delicate transparent elevations, 

 the segmental papillce (s. p.), which have probably a sensory func- 

 tion. These structures are found along the whole length of the 

 body, and as they mark the first ring of all those segments the 

 extent of which can be checked by the nephridiopores, it is legiti- 

 mate to assume their segmental value in the anterior and posterior 

 regions, where the controlling excretory apertures are absent. By 

 the clue thus furnished it is found that there are five segments in 

 front of that bearing the first pair of nephridiopores, and four 

 behind that bearing the last pair, making a total of twenty-six 

 metameres : of these the first six and the last four have less than 

 the normal number of rings. 



The anterior sucker bears on its dorsal surface five pairs of small 

 black spots, the eyes (e. 1, e. 5), the arrangement of which shows 

 them to be special modifications of sensory papillae, since they 

 occupy in the first five segments the precise position occupied in 

 the sixth and following segments by segmental papillae. 



The perfectly definite and comparatively small number of 

 metameres in the leech offers a striking point of contrast with 

 what we have met with in the Cha3topoda, and is to be looked 

 upon as a mark of higher differentiation. 



Body-wall. The body is covered externally by a thin cuticle 

 (Fig. 371, cu.), which is constantly being cast off in patches and 

 renewed. Beneath it is an epidermis (d. ep.) consisting of hammer- 

 shaped cells, separated at their inner ends by spaces in which 

 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 cells are large, branched, and abundantly pigmented, 

 and have their branches directly connected with the smaller blood- 

 vessels. Surrounding the enteric canal is the peculiar and charac- 

 teristic botryoidal tissue (b. t.) consisting of branched blood-vessels, 

 the walls of which are formed of large cells loaded with black 

 pigment. 



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. 474) : the segments 

 in question therefore constitute the clitellum. 



The muscular system is well developed, and consists of 

 outer layer of circular (c. in.} and an inner of longitudinal (l.m.) 



H H 2 



