168 



PRINCIPLES OF ANIMAL BIOLOGY 



To work in this manner, the ducts discharging the germ cells must be 

 in front of the clitellum, by which the cocoon is secreted. Their arrange- 

 ment is shown in Fig. 137. The male organs are two pairs of testes, 

 three pairs of seminal vesicles, and one pair of vasa dcferentia. Male 

 germ cells are originated by the first of these organs, are developed in the 

 second, and are discharged thrqugh the third. 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 seminal receptacles. 

 The eggs, after leaving the ovaries, are held temporarily in the ovisacs 

 and then discharged through the oviducts. The seminal receptacles 

 receive spermatozoa from another worm and hold them until a cocoon 

 passes by their openings. 



Fig. 137. — 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; vsb, base of vesicula 

 seminalis; 9, opening of oviduct; cf, opening of vas deferens. {Modified from Hesse.) 



While in the earthworm and in some other hermaphroditic species 

 an elaborate mechanism ensures cross-fertilization, in other hermaph- 

 roditic species no such devices exist and, indeed, self-fertilization (fertili- 

 zation of eggs by spermatozoa of the same individual) is well known either 

 as a regular or occasional occurrence. Some plants as wheat and beans 

 regularly self-fertilize. Other plants as the violet produce some flowers 

 which are regularly cross-fertilized and others which can only be self- 

 fertilized. Among parasitic flatworms (tapeworms and flukes) and 

 among snails both cross- and self-fertilization have been observed. 



As stated in an earlier section, Paramecium is to be regarded as 

 hermaphroditic. One individual conjugates with another for exchange 

 of micronuclei. Besides this, at intervals there is, without conjugation, a 

 reorganization of the nuclei of a single individual which results in rein- 

 vigoration, but which seems not to correspond to self-fertilization since 

 there is no fusion of nuclei. 



