634 FRESH-WATER BIOLOGY 



except a few anterior ones and one at the posterior end. Not infre- 

 quently some of the nephridia may fail to develop, when a more or 

 less irregular and asymmetrical distribution results. 



The reproductive organs of the fresh-water Oligochaeta are simi- 

 lar to those of the terrestrial earthworms. One or two pairs of 

 male gonads (spermaries or testes) are attached to the anterior 

 septa of certain somites and extend freely posteriad into the cavities 

 of the somites. One or two pairs of ovaries are correspondingly 

 situated in somites posterior to those which contain the spermaries. 

 The sperm ducts of most species have their internal openings or 

 spermiducal funnels in the somites which contain the spermaries, 

 and the external openings, or spermiducal pores, on some somite 

 posteriad; but in a few species both openings may be in the same 

 somite. In many species the sperm ducts are modified in various 

 ways, givjng rise to prostates, atria and storage chambers (Fig. 

 990). The internal openings of the oviducts, the oviducal funnels, 

 are in the ovarian somite, and the oviducal pores are either at the 

 posterior boundary of the same somite or, more commonly, on 

 the following one. Accessory reproductive organs are commonly 

 present. Evaginations of the septa of the somites which contain 

 spermaries form sperm sacs in which the sperm cells may complete 

 their development and be temporarily stored before they pass out 

 through the sperm ducts during copulation. Evaginations of the 

 posterior septa of the ovarian somite form ovisacs. Invaginations 

 of the body wall of certain somites produce spermathecae, usually 

 paired, which serve for storage of the sperm cells received during 

 copulation, from another individual. 



Sexual reproduction occurs in all families of fresh-water Oligo- 

 chaeta at more or less definite seasons of the year. In the two 

 families Naididae and Aeolosomatidae, asexual reproduction by 

 budding is the mode by which the majority of new individuals 

 are produced. Figure 980 exhibits the main features of the process 

 and renders an extended description unnecessary. The body wall 

 thickens anterior to the middle of the budding somite and forms a 

 budding zone, the anterior half of which gives rise to an indefinite 

 number of new somites which form the posterior part of the ante- 

 rior daughter-worm. The posterior half of the budding zone 



