514 



ZOOLOGY 



The principal sense-organs are the eyes, of which there are five 

 pairs (Figs. 404 and 411) situated round the margin of the anterior 

 sucker, on the dorsal side, one pair in each of the first five segments. 

 They occupy positions taken in the succeeding segments by lateral 



sense-organs, with which they are 

 obviously homologous. The structure 

 of the eyes is peculiar: they are 

 cylindrical in form (Fig. 411), 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.), sur- 

 rounding 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 pro- 

 bably 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 segmental sense-organs is un- 

 known. 



Reproductive Organs. — The 

 Leech is monoecious. There are nine 

 or ten pairs of testes (Figs. 407 and 

 408, 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 in width and forms 

 a complex coil, the vesicula seminalis (v. sem.), from which is con- 

 tinued anteriorly a somewhat 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 



Fig. 411. — Section of eye of Leech 

 c, cuticle ; d 8, gland-cells ; ep. epi 

 dermis ; g, nerve-cells ; n. nerve 

 p, refractive cells ; pi. pigment 

 (P'rom Lang's Comparative Anat 

 omy.) 



