344 



ZOOLOGY. 



are usually very plainly visible on the surface (fig. 174), 

 The oviducts are shorter, and only pass back into somite 

 14 : at the base of its funnel, each oviduct bears a little 

 pouch, the receptaculum ovorum. 



So much for the essential organs : next there are certain 

 important accessory structures. The cells from which 

 spermatozoa develop are set free into the ccelom and there 

 undergo further development before they pass out along the 

 vasa deferentia. To allow for their development, the septa 

 of the somites concerned become greatly modified, and there 

 project out from them, into adjacent somites, large white 

 pouches, called the seminal vesicles. It is in the cavities of 

 these vesicles (really a portion of the co3lom) that the early 

 development of the spermatozoa goes on. Quite distinct from 

 these are two pairs of little sacs called spermothecse, de- 

 veloped between somites 9 to 10, and 10 to 1 1, from ingrowths 

 of the skin, and having no communication with any internal 



organ. When the sperma- 

 tozoa have undergone a 

 certain amount of develop- 

 ment, they are transferred 

 from the seminal vesicles 

 (through the vasa deferentia) 

 of one worm to the sperm o- 

 thecse of another worm, the 

 exchange being mutual. 

 This is effected by the two 

 worms placing their ventral 

 surfaces in contact with one 

 another, their heads point- 

 ing in opposite ways. Hence 

 in a mature earthworm 

 both seminal vesicles and 

 spermothecse contain sper- 

 matozoa, those in the latter 

 (derived from another worm) 

 being in a more advanced 



stage of development than those in the former. The 

 several stages in their development are shown in fig. 

 179. In the testes spermospores or sperm-ro other-cells 



Fi& 179. DEVELOPMENT OF SPERMATOZOA 

 OF LUMBRICUS. 



