190 University of California Publications in Zoology. [ VOL - 4 



borax carmine. Congo red, erythrosin, eosin, and orange-G were 

 used as counter stains. Of these Meyer's haemalum counter- 

 stained with congo red for the early stages, and Mallory's con- 

 nective tissue stain for advanced stages that were fixed in Zenker's 

 fluid proved to be the most satisfactory. 



Living material was examined with a Zeiss-Greenough stere- 

 oscopic microscope. The depth of field and appearance of three 

 dimensions afforded by this instrument added greatly to the 

 accuracy and facility of observations, particularly in the detailed 

 study of swimming. 



PERIODS OF DEVELOPMENT. 



The following statement made in a previous paper (Kitter- 

 Davis '04, p. 173) in regard to periods of larval life of Tornaria, 

 applies to D. pusillus: "In the larval life of the enteropneusta 

 three periods should oe recognized; namely, a period of larval 

 development; a climactic period; and a metamorphic period; -i.e., 

 a period of development again, but this time development toward 

 the adult animal." 



As I shall recognize these periods in my account of the early 

 life history of D. pusillus, it will be necessary to modify some- 

 what the definition previously given, particularly since no account 

 was taken of the earliest developmental stages of tornaria. 



The first period, or period of larval development, includes the 

 time passed wholly within the egg-membranes. It may be noted 

 that my observations on the time of hatching do not reveal any 

 such irregularity as described by Bateson for B. kowalcvskii. 

 After describing that stage of the larva where the collar area is 

 definitely marked off by anterior and posterior grooves, he says : 

 "The animal remains in this condition for some hours and is 

 generally hatched without the occurrence of any further alter- 

 ation. The time of hatching is, however, quite irregular. Larvae 

 may frequently be found swimming freely whose organization is 

 not much in advance of Stage C (the stage before either groove 

 has appeared), and on the other hand, I have seen them in the 

 condition of Stage G (after appearance of first gill opening) in 

 the eggshell." (Bateson '84, p. 211.) 



