174 University of California Publications. [z°oi i 



Since the specimens collected this summer were none of 

 them early enough to enable us to add anything to what is 

 already known about the first period, namely, that of larval 

 development, what we have to say concerning the structure and 

 form begins with the 



SECOND, OR CLIMACTIC PERIOD. 



This period is characterized by the maximum size and mini- 

 mum specific gravity reached during larval life; and by the 

 height of development of distinctly larval traits, viz., the ciliary 

 bands with their cilia; the several "fields" bounded by these 

 bands; the apical nerve spot and its sense organs; the bulb-like 

 form of .the stomach; and the great size of the blastocoel cavity 

 and the mass of secreted material by which this cavity is filled. 

 Negatively the larva is characterized not only by the absence of 

 several structural features which are present in the adult, but 

 what is of more significance, the remaining in developmental 

 abeyance of the whole coelom system, which made its appear- 

 ance in a comparatively early embryonic time. 



In treating of the form and structure of the larva, the full 

 and excellent published accounts of closely related species renders 

 it necessary for us to enter into details here on such points 

 only as a fuller knowledge of the species and the aims of our 

 present study, call for. The larva is somewhat larger than the 

 measurements given by Ritter, '93, indicate. The following 

 figures, in mm., were obtained by measurement of living animals 

 of the largest size, at the climax of larval life, and in full 

 expansion: 



Plate XVII. Figs. 1 and '2, present front and lateral views, 

 respectively, of the same individual. It is seen from these, as 

 from the measurements, that the larva is broad in proportion to 

 its height. Its anterior end is unusually flat, and the anal disc- 

 is bulging. The diameter of the body increases rapidly from 



