144 THE BIOLOGY OF AN ANIMAL. 



as in the figure. The spermatozoa are differentiated in the sper- 

 maries from cells forming the internal mass; they are at first 

 rounded like the primitive ova, but eventually become as it were 

 drawn out into the form of a filament. They are at first mo- 

 tionless, but gradually acquire the power of movement and swim 

 swiftly and vigorously about. 



Accessory Reproductive Organs. After the ova and sperma- 

 tozoa are ripe they are finally transported to the outside of the 

 animal by the oviducts and sperm-ducts. The former (o.d, 

 Fig. 80) are two short trumpet-shaped tubes, opening externally 

 by the smaller end upon the 14th somite. The inner end opens 

 freely by a wide ciliated funnel into the cavity of the 13th 

 somite. The oviducts are placed just behind the ovaries, above 

 and a little to the side of the nephridial funnel of the 13th 

 somite. The external opening lies between the two groups of 

 setas on the 14th somite (Fig. 72, o.d}. By means of the ciliated 

 funnel the floating ova are drawn into the oviduct and cast out 

 upon the outer surface of the body. 



The sperm-ducts (vasa deferentia] (s.d, Fig. 80) convey the 

 sperm (including the spermatozoa and the liquid in which they 

 move) to the surface of the 15th somite, where they open by a 

 conspicuous slit surrounded by fleshy lips (Fig. 72, s.d). They 

 are very long, slender tubes, open like the oviducts at both ends, 

 and somewhat trumpet-shaped ; they run straight forward to the 

 12th somite, where each branches like a Y. The two branches 

 pass obliquely forwards and inwards to join the seminal vesicle 

 (Fig. 80). The hinder branch joins the posterior vesicular cham- 

 ber. The other proceeds to the anterior chamber, with which it 

 becomes connected (Fig. 80). Just before joining the vesicle 

 each tube is twisted into a peculiar knot, and at the vesicle is 

 dilated into a funnel-shaped orifice closely attached to the lower 

 side of the vesicles. The sperm-ducts are ciliated, and closely 

 resemble the oviducts in mode of action. 



The obvious likeness between the reproductive ducts and nephridia 

 suggests the idea that the former may have been derived from the latter. 

 It is an interesting fact that in many worms nephridia act as ducts for the 

 emission of the reproductive elements, and that Lumbricus is exceptional 

 in having special ducts for this purpose. It has been suggested that the 

 earthworm perhaps had originally two pairs of nephridia in each somite, 

 of which one pair has disappeared from each somite, excepting the 14th and 

 15th, where they still persist as the oviducts and sperm-ducts. 



