THE NEMERTEANS 457 



As a rule a single egg is formed in each gonad, although some- 

 times there are two. Even when the eggs are fully formed there 

 remains in each gonad a portion of the spermatozoa previously 

 formed. This fact has led certain investigators to conclude that 

 self-fertilization may sometimes occur. The eggs are fertilized after 

 deposition, however, and the gregarious habits of the worms pre- 

 sumably insure the presence of spermatozoa from other individuals 

 which may effect cross fertilization in whole or part. 



The eggs are deposited in a double string embedded in a jelly 

 which attaches them to the water plants or other objects on which 

 the worms are found. They are beautiful objects by which to 

 illustrate the processes of fertilization, maturation, cleavage and 

 the development of the embryo. 



The worms may be kept alive for a long time in aquaria contain- 

 ing water plants, and under suitable conditions will continue to 

 live and breed year after year. They thrive under the conditions 

 found in botanical gardens, where large basins of water are used 

 in the cultivation of exotic water plants. 



Although the nemerteans are represented by numerous genera 

 and species in the oceans in all parts of the world, only a few forms 

 occur in fresh water and a few others in moist places on the land. 



In North America only a single genus is known from fresh water, 

 and of this genus the described species are so closely similar as to 

 lead to some doubt as to whether more than a single species is 

 actually represented. 



In 1850 Leidy published a brief and imperfect description of a 

 nemertean found in the vicinity of Philadelphia, which species he 

 described as Emea rubra. Silliman later found the same or a 

 very similar species in New York State, to which he gave the name 

 Tetrastemma aquarium dulcium, and included Leidy's species therein. 

 Montgomery, in 1896, described under the name Stichostemma 

 asensoriatum, a similar species from Pennsylvania, while appar- 

 ently similar forms have been recorded from Connecticut, Illinois, 

 Nebraska, and Washington. The worms are thus known to occur 

 from the Atlantic to the Pacific coasts and have probably been 

 observed at numerous unrecorded localities in the intervening 

 territory. 



