COE : NEMEETEANS OF WEST AND NORTHWEST COASTS. 71 



tion of the individiials pass through several phases of sexuaUty, 

 being (a) first hermaphroditic and protandric, then (b) maturing a 

 brood of large embryos which absorb a large portion of the tissues 

 of the adult, so that it then becomes either (c) a smaller sized indi- 

 vidual Avhich develops spermatozoa, and is looked ujion as a male, or 

 (d) matures or absorbs its small spermaries and retains its ovaries so 

 that it again appears as a pure female at the beginning of the follow- 

 ing season. 



Development. 



Since the publication of Burger's Monogi-aph on the nemerteans 

 of the Gulf of Naples ('95) a number of papers have apj»eared 

 dealing exclusively Avith the development of certain species. These 

 papers include accounts of the development and maturation of the 

 sexual cells, the processes of egg-laying, fertilization, cleavage, gas- 

 trulation, and the formation and structure of the pilidium in certain 

 Lineidae. The important features of most of these papers have 

 been included in Burger's comprehensive treatise on the develop- 

 ment of the nemerteans in Bronn's Thierreich (vol. 4, supplement,, 

 p. 308-384, 1903), and to this important treatise the reader is 

 referred for a critical review of the literature previous to the year 

 1900. 



Embryological studies have been confined mainly to the Lineidae, 

 Amphiporidae, Cephalothrix, and certain viviparous Hoplonemer- 

 tea, so that at the present time practically nothing is known of 

 the developmental processes in any of the Paleonemertea except 

 Cephalothrix, nor in the Taenisomidae, nor in any save a few spe- 

 cies of the Hoplonemertea. 



The general course of development is better known in Linens 

 oiridis than in any other nemertean, oAving to the researches of Desor 

 ('48), Barrois ('77), Mcintosh ('73), Hubrecht ('85), and Arnold 

 ('98). In another form Avhich alsf) occurs on the Pacific coast, Cere- 

 bratulus niarginatus^ the maturation and fertilization of the eg^ has 

 been carefully studied by Kostanecki (: 02), and by myself ('99). I 

 have also studied the gastrulation of the egg of this s])ecies, and 

 the formation of the early i)ilidium ('99*^). 



Barrois ('77), JMcIntosh ('73''), and myself ('99a) have observed 

 the direct development of Cephalothrix linearis, and Barrois ('77), 

 and Hoffman ('77) that of Tetrastemma dorsale. I have also 



