ANNELIDA 233 



fluid-filled cavities bounded by sensory epithelium, occasionally 

 occur (see §iii). Some solid particles (statoliths) 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 and to appreciate the action of water waves. 

 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 ob- 

 jects are not formed, ip 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. Even in the earthworm 

 and other forms in which there are no eyes, the skin is sensitive 

 to light. Most worms prefer dark places. 



272. Reproductive Organs. — The Oligochasta and the leeches 

 are hermaphrodite. In the Polychaeta the sexes are separate. 

 The sexual products are developed from the coelomic 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 (OUgo- 

 chseta). The region is usually distinguishable only about the 

 breeding time. In the hermaphrodite forms the ova and sper- 

 matozoa often mature at different times and are produced in 

 different segments. This of course insures cross-fertilization. 

 In the Polychaeta the conditions are relatively simple. The 

 elements are freed in the body cavity and when mature find 

 their way into the water where fertilization takes place. The 

 organs are much more complicated in the hermaphrodite worms. 

 The spermatozoa are produced in the testes, are passed into the 

 seminal vesicles where they are matured, and at the time of 

 copulation escape to the exterior by the vasa deferentia, to be 

 deposited in the sperm sacs or receptacula seminis of another 

 worm. From this place, any time after copulation, the sperm 

 is brought into contact with the ova of this second worm, as 

 they pass from the ovary, where they are produced, to the egg-sac 



