518 Howard Edwin Enders. 



Tkansfokmation from, Fkee-Swimming to Creeping Larva. 



The most advanced larvae that I have collected resembled the adnlt 

 worms in th.e form of the segments and in the transparency of the 

 integuments. These large-bodied larvse rarely swim about in the 

 aquarium into which they are placed but remain chiefly at the bot- 

 tom where they move among the diatoms. The parts of the body 

 wdiich correspond with the anterior, middle and posterior regions 

 of the adult are marked off more clearly here than in the earlier 

 stages. The larvse which correspond with my figures 11, 12 and 13 

 were 2 millimeters long. 



Anterior region. — The anterior region has shared in the growth of 

 the larva and its general appearance has changed considerably as a 

 result of the more rapid growth of some of its parts and the slower 

 growth of others. Thus^ its forward end has scarcely increased 

 while the posterior portion has grown in diameter ; the plastron has 

 grown so that it is wider than the body ; the pre-oral lobe has scarcely 

 increased in size while the post-oral lobe has grown into a large 

 spoon-shaped lip that either hangs downward over the plastron or 

 is directed forwards and kept in the same horizontal plane as the 

 body. This region has undergone a slight dorso-ventral flattening. 



The nine setigerous segments have grown so that the foremost 

 ones extend beyond the body of the larva. Septa have appeared 

 between the segments and each segment bears five lance-shaped set* 

 which are like those of the adult. The fourth segment has not yet 

 developed the club-shaped black seta? which are found in the adult 

 condition. 



The pre-oral lobe has relatively the same size and form, but is 

 tilted more than in the earlier stage so that its margin now is 

 almost in the same horizontal plane as the animal. It is covered, 

 as in the earlier stages, with cilia that vibrate towards the esophagus. 



The greatest increase in size has occurred in the post-oral lobe. 

 It is a large lip that either hangs, apron-like, over the ventral side 

 of the body and covers five or six of the setigerous segments (Figs. 

 11 and 12) when the larva is swimming, or is directed forward as 

 in the adult worm (Fig. 13) when it is creeping. The auriculate 

 lobes have begun to form by enlargement of its margin near the 



