184 AN INTRODUCTION TO ZOOLOGY 



side of the body. This fine duct passes straight back on the ventral 

 floor of the coelom to somite 15, where it passes through the body wall 

 to the external male aperture. 



The female part of the reproductive system is much less con- 

 spicuous than the male. Only one pair of ovaries is present, situated 

 on the hinder surface of the septum between segments 12 and 13, 

 and projecting freely into the cavity of the latter. The ovary 

 is again a localised thickening of the ccelomic epithelium, forming 

 a pear-shaped body about 2 mm. in length. At the anterior wider 

 end is a mass of small cells, the primordial ova and the oogonia, and 

 the narrower end is formed by a single row of large nearly ripe 

 ova, each surrounded by an envelope of thin cells constituting a 

 follicle. Two short ciliated funnel-shaped openings, the oviducal 

 funnels, are situated on the septum at the opposite end of the somite 

 vis-a-vis to the ovaries. They perforate the septum as oviducts, 

 and immediately on the posterior side of it each undergoes an 

 enlargement to form a small sac, the receptaculum ovorum or ovisac. 

 After a very short straight course the oviducts open to the outside 

 in segment 14. 



In addition to these sets of male and female organs we also find 

 four accessory structures, the spermathecse, whose function we shall 

 discuss shortly. These are two pairs of ovoid whitish or light 

 yellow sacs, lying ventro-laterally in the posterior halves of somites 

 9 and 10. 



The testis, as has been noted, sheds masses of protoplasm with 

 from eight to sixteen nuclei, but only indistinctly divided into cells, 

 the spermatogonia. In the vesiculae seminales these apparently 

 undergo two successive divisions, producing 32 to 64 much smaller 

 cells, thespermatids, around a central mass of protoplasm known as the 

 cytophore. These cells gradually alter until they form very typical 

 spermatozoa with small heads and long filiform tails, which still 

 retain a spherical arrangement around the central mass. These 

 groups of cells exhibit a very characteristic mulberry-like appear- 

 ance during these changes, and are in consequence known as the 

 sperm morulae. Finally, the ripe sperms are set free into the fluid 

 content of the sperm sacs. It is into the cytophore of the spermato- 

 zonia that the young sporozoite of M. agilis bores its way and 

 gradually consumes it while the surrounding spermatozoa are under- 

 going their maturation divisions. The tail of the ovary, as we have 

 seen, contains a string of large nearly ripe ova each enclosed in a 

 follicle, which presently bursts releasing the ovum, which is taken up 

 by the oviducal funnel and stored in the receptaculum. Here it 

 undergoes further ripening until it becomes mature. 



During the act of copulation two worms lie together in head 



