502 



ZOOLOGY 



The structure of the eyes is peculiar : they are cylindrical in form 

 (Fig. 418), the long axis of the cylinder being at right angles to the 

 surface of the body. The outer layer is formed of black pigmented 

 tissue (pi.), surrounding a layer of large, clear, refractive cells (p.), 



which occupy the greater part of the 

 organ. A nerve (n.) enters at one side, 

 and is continued up the axis of the 

 cylinder by a row of sensory cells. 



The margin of the anterior sucker 

 also bears a large number of goblet- 

 shaped organs, which are very probably 

 organs of taste. The minute structure 

 both of these and of the segmental 

 sense-organs is very similar to that of 

 the eyes. The function of the seg- 

 mental sense-organs is unknown. 



Reproductive Organs. The Leech 

 is monoecious. There are nine or ten 

 pairs of testes (Figs. 414 and 415, ts.), 

 in the form of small spherical sacs, 

 situated in segments 12 to 20 or 21. 

 Each gives off from its outer surface a 

 narrow efferent duct, which opens into 

 a common vas deferens (v. d.). In the 

 tenth segment the vas deferens increases 



FIG. 418. Section of eyejf Leech. . . , ,V 



c, cuticle ; dr. gland-cells ; ep. epi- in Width and forms a Complex COll, 



the vesicula semmalis (v. sem.), from 

 Lang ' S Comparative Anat ' which is continued anteriorly a some- 

 what dilated muscular tube, the ductus 

 ejaculatorius (d. ej.). From each ejaculatory duct a narrow tube 

 passes to the base of the penis (p.), a curved eversible muscular 

 organ which opens on the ventral surface of the fourth annulus of 

 the eleventh segment, in the middle line. The base of the penis is 

 surrounded by a number of unicellular glands, which constitute 

 the prostate, and secrete a substance by which the sperms are 

 aggregated into masses called spermatophores. 



The ovaries are coiled filamentous bodies, each enclosed in a 

 small globular ovarian sac (ov. s.), situated in the eleventh segment. 

 From each ovarian sac a short oviduct passes inwards and back- 

 wards, and unites with its fellow into a median duct, the walls of 

 which are supplied with albumen-secreting gland-cells. The 

 common oviduct opens into a curved muscular tube, the vagina (va.), 

 which opens in the middle line on the ventral surface of the fourth 

 annulus of the twelfth segment, i.e. one segment behind the male 

 aperture. 



It will be noticed that the ovaries of the Leech form a single 

 pair, while the testes are multiple and segmental : also that, while 



