128 



PRINCIPLES OF ANIMAL BIOLOGY 



are secondary soxiiiil functions. The essential feature of any genital 

 system, therefore, is one or more gonads, the glands or organs which 

 produce eggs or spc^-ms. Those; gonads which produce eggs (or ova) 

 are the ovaries, and those which produce sperms the spermaries, or testes. 

 All other parts of the reproductive system are accessory to the gonads. 

 The complexity of these accessory genital organs bears a close relation to 

 the mode of life of the animal, its breeding habits, and the place where fer- 

 tihzation occurs, that is, whether external or internal. In the simpler 

 coelenterates (Fig. 60) the sperms are discharged from the spermaries 

 directly into the water, and to fertilize the ova they must penetrate 

 the ovary. In others the sperms are discharged into the gastrovascular 

 cavity whence they escape through the mouth into the water. In this 

 group there are no tubes, or gojioducts, for the transfer of germ cells from 

 the gonads to the exterior but the germ cells must break through the 



m. 



^ysb i/s^__vsb 



m 



ys 



21 



22" 



Zff 



Fig. 95. — Reproductive organs of the earthworm, schematic representation of the 

 side view. IX-XV, numbers of somites; cm, circular muscles; ep, epithelium; /, funnel 

 of vas deferens; Im, longitudinal muscles; ov, ovary; ovd, oviduct; ovs, ovisac; rs, recep- 

 taculum seminis; ts, testis; vd, vas deferens; vs, vesicula seminalis; vsh, base of vesicula 

 seminalis; 9, opening of oviduct; (S^, opening of vas deferens. {Modified from Hesse.) 



body wall. In certain polychaste worms also the germ cells are shed into 

 the water by the bursting of the body wall in certain somites of the body. 

 Systems with Accessories. — In the oligochajte worms, represented 

 by the earthworm, a much more complicated group of organs serve the 

 reproductive functions. The male organs are two pairs of testes, three 

 pairs of seminal vesicles and one pair of vasa deferentia. The testes 

 (Fig. 95) are imbedded in the bases of larger organs, the seminal vesicles, 

 in which the sperms undergo the later stages of their development and 

 await the time of copulation. Delicate tubes, the vasa deferentia, which 

 originate in funnels in the base of the seminal vesicles and which open to 

 the surface in the common earthworm on somite 15 conduct the sperms 

 to the exterior. The same worm also possesses a set of female reproductive 

 organs consisting of one pair each of ovaries, ovisacs and oviducts, and two 

 pairs of receptacula seminis. The eggs are discharged from the ovaries 



