70 bulletin: museum of comparative zoology. 



Where the number of eggs is so very large, their size is usually 

 only from one sixth to one twelfth of a millimeter in diameter. 



All account of the processes of fertilization and development of 

 the egg is given on pages 73-75. 



A few species of neraerteans are viviparous, the eggs l)eing ferti- 

 lized within the body. Here they develop into young worms which 

 at the time of birth are fully provided with all the organs of the 

 adult except the sexual glands. Most of the known viviparous forms 

 belong to Prosorhochmus, although the terrestrial Georiemertes 

 agricola {Coe, :04''), one or more s])ecies of Tetrastemma and 

 Stichostemma, and a single species of Lineus (X. viviparus Isler) 

 likewise give birth to living young. 



In Carcixoxemertes and SxiciiosTEinrA, as well as in Geone- 

 mertes australiensis, the eggs are often fertilized within the body and 

 are usually deposited shortly after fertilization. Tn some cases, how- 

 ever, a portion of them may remain in the body until the later 

 stages of cleavage or even until after the formation of free swim- 

 ming embryos. 



In all these forms it seems probable that cross fertilization nor- 

 mally occurs, the spermatozoa passing from the body of one individ- 

 ual into the ovaries of another. This process has been observed for 

 several species, the male lying side by side with the female and the 

 spermatozoa j^assing from one body to the other through the inter- 

 vening mucus. The spermatozoa thus reaching the ovaries through 

 the oviducts may remain alive for some days and fertilize the ova as 

 they mature. Dendy ('93) has noticed such spermatozoa in the 

 ovaries of Geoxj;mertes, and I have seen them in STiciiosTEaurA. 

 Child (:01), on the other hand, was led by his observations to the 

 belief that self-fertihzation normally occurs in Stichostemma, for 

 the eggs and spermatozoa ripen in the same gonad at the same time. 

 In *S'. graecense also both kinds of sexual elements mature at the 

 same time (Bohmig, '98), while according to Montgomery ('95) 

 S. eilharcU is protandric. 



In Geonemertes «^^r/coZa the sexual phases are com])licated by the 

 appearance of («) forms with ovaries only, {h) individuals with 

 developing embryos, and with immature ovaries or s])ermarie8, or 

 both, and (c) purely male forms of small size whose bodies are dis- 

 tended with spermatozoa. The conditions observed in this species 

 (Coe, : 04^^) may be ex})lained by the hypothesis that at least a por- 



