PHYLUM ANNUIATA 



473 



sense-organs, with which they are obviously homologous. The 



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



(Fig. 377), 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 goUet- 

 shapcd organs, which are very prob- 

 ably organs of taste. The minute 

 structure both of these and of the 

 segmental sense-organs is very simi- 

 lar to that of the eyes. The function 

 of the segmental sense-organs is un- 

 known. 



Reproductive Organs. - The 

 Leech is monoecious. There are nine 

 pairs of testes (Figs. 373 and 374, ts.), 

 in the form of small spherical sacs, 

 situated in segments 12-20. Each 

 gives off from its outer" surface a 

 narrow efferent duct, which opens into 

 a common vas defer ens (v. d.). In the 

 tenth segment the vas deferens in- 

 creases in width and forms a complex coil, the vcsicula seminalis 

 (v. scm.\ from which is continued anteriorly a somewhat dilated 

 muscular tube, the ductus cjaculatorius (d. ej.). From each ejacula- 

 tory duct a narrow tube passes to the base of the penis (jj.), a curved 

 eversible muscular organ which opens on the ventral surface 

 of the second annulus of the tenth 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 w T alls 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 

 second annulus of the eleventh segment, i.e. one segment behind 

 the male aperture. 



FIG. 377. Section of eye of Leech 

 c. cuticle ; d 3, gland-cells ; cp. epi- 

 dermis ; g, nerve-cells ; n. nerve; 

 /<, refractive cells ; pi. pigment. 

 (From Lang's Coinparatttc Anat- 

 omy-) 



