76 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



margin ten minute eyes can be recognized as black spots. A 

 large round sucker (Fig. 23), also facing ventrally, occupies the 

 posterior end of the body, and the small dorsal anus opens just 

 in front of it. There are two other unpaired openings, both in 

 the median ventral line, one, the male aperture, in the second 

 annulus of the 6th segment, the other being the female aperture 

 in the second annulus of the 7th segment. There are also 17 

 pairs of minute excretory pores by which the nephridia open in 

 the hindmost annuli of segments 2-18. The setae of the earth- 

 worm are quite unrepresented in the leech. 



2. The Skin consists of cuticle, epidermis, and dermis. The 

 cuticle is thin, elastic, and frequently cast and renewed. It is 

 secreted by an underlying epidermis, which is mainly made up 

 of a single layer of cells shaped like mallets with the handles 

 turned inward. There are also numerous epidermal glands, each 

 of which is unicellular and opens on the surface by a very narrow 

 tube or duct. These glands are of two kinds (1) mucous glands, 

 scattered all over the body and secreting the slime which makes 

 the body of a leech so slippery, and (2) clitellar glands, occurring 

 in segments 5-7, and secreting the materials from which the 

 cocoons are made. There is no swollen clitellum as in the earth- 

 worm, and a further difference is that the reproductive organs 

 open in, not in front of, the clitellar region. The dermis is a 

 moderately thick gelatinous layer containing a number of branched 

 cells, and traversed by pigmented fibres and a network of capillary 

 vessels, both of which penetrate between the internal " handles " 

 of the epidermal cells. There are also muscle-fibres in the 

 dermis. 



Eespiration is effected by the skin (i.e., is cutaneous), and 

 absorption of oxygen from the exterior with corresponding elimi- 

 nation of carbon dioxide is rendered possible by the superficial 

 position of the capillary network, as described above. 



3. The Digestive Organs (Fig. 23) consist of a tube running 

 straight from mouth to anus, and divisible into fore-gut (buccal 

 cavity and pharynx), mid-gut (gullet, crop, and stomach), and 

 hind-gut (rectum). 



The fore-gut (stomodseum) commences with a conical buccal 

 cavity, situated at the bottom of the anterior sucker with three 

 flat muscular jaws projecting into it. One of these is dorsal, the 

 others ventro-lateral, and they are arranged in a three-rayed 



