182 University of California Publications. [zoology 



larva?, it is quite short. Full-grown larva' invariably begin the 

 metamorphosis within a day or two after being taken from the 

 sea. 



THE THIRD, OR METAMORPHIC PERIOD. 



This, as already indicated, includes the whole series of 

 changes, extending from the climax of larval development to 

 and including the blocking out of all the organs and organ 

 systems of the adult animal. These changes, named approxi- 

 mately in their time sequence, are as follows: (a) The bilateral 

 thickening of the wall of the anterior mesoblastic vesicle as the 

 beginning of the proboscis musculature. (//) The beginning of 

 resorption of the ectodermal tentacles, (r) The reduction in 

 size of the larva. This is initiated by the narrowing of the 

 anterior end of the larva, and is very soon followed by the first 

 intimation of the constriction off' of the proboscis. {(I) The origin 

 of the first pair of gill pockets, (e) The transformation of 

 the gastric epithelium (as already shown, for the sake of 

 continuity in description, in the section on the first period) ; 

 and finally (g) the thickening of the somatic layer of the 

 two pairs of mesoblastic vesicles preparatoiy to the histogenic 

 changes in these. 



The purpose of the present study does not require us to follow 

 the details of these changes. We are, to repeat, concerned only 

 with certain points that present themselves with special distinct- 

 ness from the ecological point of view. 



Diminution in size is, as already shown, a general fact of a 

 good deal of interest that characterizes this period. The period 

 is, however, one of true development as well. The development 

 consists of certain retrogressive changes on the one hand, and 

 of progressive changes on the other. The retrogressive changes 

 are, in order of time, first the resorption of the tentacles and 

 flattening out of the ciliary bands and oral field; and second, 

 the loss, at a rather late period, of the cilia of the girdle. It is 

 well known from the published work of other observers that the 

 ectoderm of the oral field is very thin — considerably thinner than 

 that of the other regions of the larval body — and that this is due 

 to the pronounced flattening and concomitant expansion of a 



