346 Ralph S. Lillie 



sliaped respectively, and are elongated and already capable of a 

 very considera ble degree of protrusion and retraction. The neuro- 

 podial setae consist of a single short hook on each side of the ven- 

 tral surfaee of each setigerous somite. As grovvth proceeds, additional 

 hooked setae appear ventrally to those already laid down; at the 

 swarming stage, however, a single hook onl}' is as a rule present 

 on either side in each somite. In the third setigerous somite, the 

 setae in the majority of larvae are as yet incompletely formed; as 

 elongation proceeds, however, they enlarge and acquire the same 

 characteristics as those of the preceding somite. Locomotion in the 

 swarming stage is effected by means of the two ciliary rings, proto- 

 troch and paratroch; these are connected by a median ventral band 

 of shorter and stouter cilia. The continued activity of the cilia pro- 

 pels the larvae at a uniform rate in a forward direction toward 

 the source of light. On the prostomium are two simple "'eye-spots", 

 each of which consists of a compact dump of pigment or excretory 

 granules on the surfaee of the brain. Large clear cells, apparently 

 of a glandulär nature, occupy a large portion of the prostomium, 

 and similar but smaller cells are found at the posterior extremity 

 of the body. These cells probably furnish^ in part at least, the gluti- 

 uous material by the aid of which the larvae later form the rough 

 tubes in which they live subsequently to the termination of the 

 swarming period. In fixed and stained preparations, the cells bave 

 a clear and reticulate appearance (Figs. 7 and 8, Piate 22); they 

 dimiuish and disappear shortly after the beginning of the crawling 

 and burrowing period. 



The swarming stage apparently serves simply for the dissemi- 

 nation of the individuals of the new generation, and differentiatiou of 

 internai organs remaius very incomplete until after its termination. 

 The intestine at this period stili consists largely of yolk, and its 

 lumen is as yet very incompletely formed. There is, however, a 

 partial subdivision into Oesophagus, stomach and rectum — a sub- 

 division which becomes well defined shortly after the beginning of 

 the crawling stage, and persists througli part of the larvai period 

 and into adult life. Mouth and anus bave not yet appeared; the 

 anterior part of the intestine, however, which later forms an ever- 

 sible proboscis, exhibits already a relatively high degree of diffe- 

 rentiation (Fig. 7) — a peculiarity in evident correspondenee with 

 the early appearance of its activity and the important part which 

 it plays in larvai life. The brain, oesophageal ring and ventral 



