188 University of California Publications in Zoology. [Vol.4 



INTRODUCTION. 



In November, 1902, I .secured a few eggs and free larvae of 

 D. 2)ifsilhts Ritter, a short account of which is given in a recent 

 paper (Ritter-Davis '04). In this account it is stated that owing 

 to destruction of collecting grounds at San Pedro, California, it 

 would be impossible to secure a complete series of stages necessary 

 for detailed study, but that it was hoped to obtain sufficient ma- 

 terial at San Diego, California. On January 18, 1905, I had the 

 good fortune to find a small area of mud flats on the west side of 

 Whaler's Bight, San Diego Bay, containing D. pusillus at the 

 height of its breeding season. During the month following (Jan- 

 uary 18 to February 18) I succeeded in getting all stages from 

 unsegmented egg to metamorphosis, and was able to study these 

 stages in the living condition. 



The only account we have of direct development of Enterop- 

 neusta is that by Bateson '84- '85. This has become one of the 

 zoological classics. However, Bateson 's series of early stages was 

 not complete, and he did not give a detailed account of the ani- 

 mal's activities at different periods of its development. 



Further investigation of the subject is an urgent desideratum. 

 In this paper I give an account of the habits and activities of the 

 early life of D. pusillus, and also the stages of development up to 

 and including that of the formation of the body cavities. 



At another time I hope to follow out the later stages of de- 

 velopment, particularly of the connective tissue system and 

 supporting framework. I take pleasure in acknowledging the 

 helpful suggestions and directions which I have received in the 

 preparation of this paper from Professor William E. Ritter. 



MATERIAL AND METHODS. 



The portion of the mud flats on which the animals live is 

 uncovered at mean low tide so that collecting is possible for 

 several hours during five or six days of each low tide period. 

 This fact is important, for the work of collecting is slow and 

 tedious, as a great many burrows must be examined before one is 

 found containing eggs. 



