226 ZOOLOGY. 



ventral cord may show distinct right and left lobes or may 

 be completely fused into a median mass. From the brain, 

 nerves pass to the head-parts. From each of the segmental 

 portions of the ventral chain nerves pass to the walls, viscera, 

 etc. The ventral cord frequently lies in a blood sinus which 

 secures its abundant nourishment (leeches). 



The sense organs occur very unequally in the group. The 

 Polychaeta and the leeches are best supplied. The skin is 

 generally sensitive to contact and chemical stimuli. This sensi- 

 tiveness is perhaps specially localized in the tentacles, cirri, 

 and more movable parts. Otocysts, fluid-filled cavities 

 bounded by sensory epithelium, occasionally occur (see 108). 

 Some solid particles (otoliths) float in the fluid. These have 

 been described as organs of hearing but the sensation resulting 

 is probably quite different from what we know as hearing. 

 They are apparently organs of equilibration, enabling the 

 animal to appreciate its position in relation to the pull of 

 gravity. Eyes may consist merely of a group of pigmented 

 cells with nervous connections, or may be very complicated, 

 consisting of a capsule with refractive media and retina. 

 Images of objects are not formed, in all probability, but the 

 direction and intensity of light can be appreciated. In the 

 leech there are sense organs in each segment somewhat similar 

 in structure to the eyes. Their function is unknown. 



266. Reproductive Organs. The Oligochseta and the 

 leeches are hermaphrodite. In the Polychseta the sexes are 

 separate. The sexual products are developed from the ccelomic 

 epithelium, sometimes on the dissepiments, sometimes on the 

 body wall, or in other special regions. The elements may be 

 produced in many segments (Polychaeta), or in a few anterior 

 ones (Oligochseta). The region is usually distinguishable 

 only about the breeding time. In the hermaphrodite forms 

 the ova' and spermatozoa often mature at different times and 

 are produced in different segments. This of course insures 

 cross-fertilization. In the Polychaeta the conditions are rela- 

 tively simple. The elements are freed in the body cavity and 



