CHAPTER XVIII 

 THE EARTHWORM- co/!^vN/ed 



The earthworm is hermaphrodite (monoecious), being furnished 

 with two pairs of testes and one pair of ovaries, together with 

 accessory generative organs, which will now be described. 

 The ovaries are a pair of small pear-shaped bodies lying in 

 somite 13. They are attached by their broader ends to the 

 hinder face of the septum separating the 12th from the 

 13th somite, and lie low down in the body-cavity not far from 

 the ventral nerve cord. Essentially an ovary arises as a 

 local multiplication of cells of the ccelomic epithelium. Its 

 thicker end consists of a solid mass of cells or oogonia, and 

 the narrower end contains ripe ova, the intermediate part 

 being occupied by ova in different stages of maturation. The 

 mature ovum is surrounded by a distinct follicle consisting of 

 a single layer of flattened cells. On the bursting of the 

 follicle, the ovum, which is a large round cell with granular 

 cytoplasm and a very distinct nucleus and nucleolus, passes 

 into the cavity of the somite and is thence conducted to the 

 exterior by a special oviduct. There is a pair of oviducts 

 opening into segment 13 by wide ciliated funnel-shaped mouths. 

 Each duct passes through the septum separating segments 

 13 and 14, dilates on the other side of it to form a small 

 chamber, the receptaculum ovorum or ovisac, and turns 

 sharply outwards and downwards to open to the exterior by 

 the oviducal pore on segment 14. The ova remain for some 

 time in the ovisac before they pass to the exterior, and in it 

 they undergo the final phases of maturation. 



The two pairs of testes are small digitate organs situated in 

 somites 10 and 11, in positions exactly corresponding with 

 those of the ovaries. Like the ovaries, they are local thick- 

 enings of the ccelomic epithelium, but the sperm mother-cells 

 produced from them do not develop into spermatozoa in the 

 testes themselves, but pass into two large sacs which surround 



II. c 33 



Digitized by Microsoft® 



