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



the physiological conditions would not be expected to be constant, 

 since they would be interfered with by growth, which, though 

 reduced, is nevertheless going on. But might not the cycles 

 which would otherwise be rhythmical due to one set of physio- 

 logical conditions be modified by another set, e.g., that of growth, 

 so as to bring about just such irregularity of swimming cycles as 

 is found? The fact that these cycles become more and more 

 irregular toward the end of period II, when internal changes are 

 becoming more active just previous to period III, lends some 

 credence to this explanation, and makes it plausible but by no 

 means certain. 



As has already been stated, the first phase of period III 

 is characterized by rapid crawling in which the now motile 

 proboscis begins to take part. This is an adaptation which is 

 important for preservation of the animal at low tide, for it is 

 enabled to burrow in the soft ooze and thus readily escape de- 

 struction. 



The second phase is really metamorphic. The animal's move- 

 ments are mainly muscular. The mucous glands are sufficiently 

 numerous and active to pour out abundant secretion for cement- 

 ing the walls of its bui-row and lubricating them with slime. 



The yolk supplied by the egg is now nearly used up as in- 

 dicated by the transparency of the animal (Bateson '84), and 

 by the absence of yolk granules as shown by microscopical exam- 

 ination. But in the m.eantime a mouth, digestive tract, and gills 

 have appeared, thus equipping the organism for self-support. 



EARLY GROW^TH STAGES. 



External Features. — Period I. 



Egg. The method of discharging the ova, and the general 

 characters of the esag itself are adequately described by Ritter- 

 Davis '04. The enteropneust egg has two membranes, a fact 

 hitherto unnoticed. At first, these are so closely adherent to each 

 other as to be indistinguishable, but later they are separated by a 

 narrow space. The outer membrane may then be removed, leav- 

 ing the inner one intact. This is often necessary in order to 

 study the larva within the capsule when the outer membrane is 



