72 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



observed the direct development of Zygonemertes virescens ('99*^). 

 Ill no other species as yet known from the Pacific coast has the 

 process of development been studied. 



The direct type of development occurs in Cephalothrix and in 

 all of the hoplonemerteans which have been studied embryologi- 

 cally. In these forms the segmentation of the egg results directly 

 in the formation of the body of the nemertean without any inter- 

 mediate free-swimming stage. The direct type of development has 

 been observed in Cephalothrix and Zygonemertes, as stated 

 above, in Carcinonemertes carcinophild by Barrois ('77), in Pkoso- 

 ROCHMUS by Burger ('94), in Monopora {z= Prosorochmus ?) by 

 Salensky ('84), in Amphiporus by Barrois ('77), and myself ('99"), 

 in Drepanophorus by Barrois ('77), and Lebedinsky ('98), in Te- 

 TRASTEMMA by Barrois ('77), Hoffman ('77), and Lebedinsky ('98), 

 in Stichostemma by Child (:01), in Malacobdella by Hoffman 

 ('77''), and in Geonemertes by Dendy ('93"), and by myself (104"). 



The most complete general account of the direct type of develop- 

 ment is that given by Lebedinsky ('98) for Tetrastemma vermicidus 

 and Drepanophorus spectabilis. The develo])ment of the various 

 organs of the body is described by Salensky ('84) for Monopora 

 vwipara {= Prosorhochmus ?), by Burger ('95) for Prosorhooh- 

 Mus, and by myself (:04'') for Geonemertes. 



The indirect type occurs in all the heteronemerteans in which 

 the course of development has been followed, although unfortu- 

 nately all observations have thus far been confined to the Lineidae. 

 In this type the segmentation of the egg results in the formation of 

 a ciliated embryo, inside which the body of the young worm 

 develops. In Linens viridis tlie metamorphosis is not very com- 

 plete, and the larval form is known as Desor's larva. In Micrura 

 and Cerebratulus, on the other hand, an embryo is prodviced 

 which has no reseml)lance to the future Avorm, and which swims 

 actively in the water for a season, and finally by a complicated 

 metamorphosis produces the body of the young worm. The larval, 

 free-swimming form is known as the pilidium (Text-tigs. 29, 30) and 

 bears a certain sui)erhcial resemblance to the trochophore of anne- 

 hds. The pilidium occurs in Lineus (?) lacteus (Metschnikoff, 

 '69), in Micrura (Coe, '99") , and in Cerebratulus (C. B. Wilson, 

 '98, and : 00, and Coe, '99»). 



A brief resume of the process of fertilization and cleavage of the 



