398 EIOHARD EVANS. 



therefore comparable to type B, intermediate between types A 

 and C. Such a larva when settled down would contain 

 yolk bodies in the same way as type B does soon after 

 fixation. Hence it is to be recognised that, whether we deal 

 with larvae developed along the line of histological differentia- 

 tion passing from type A to type C, or from type A to type 

 D, we may come across specimens which possess cells with 

 vesicular nuclei which contain numerous yolk granules instead 

 of a few, as in types C and D. 



(a) The Features Common to Both Larvse at the Time 

 of Fixation and Metamorphosis — the Disap- 

 pearance of the Flagellated Layer from the 

 Surface, the Formation of the Flattened Epi- 

 thelium and of the Marginal Membrane. 



The flagellated cells may pass into the interior either 

 individually or in groups of several cells. They generally 

 tend to pass in groups from the lower surface, that is the 

 surface of fixation, and individually from the upper sur- 

 face. The usual result of this difference is, that all the 

 flagellated cells which once occupied the lower surface are 

 well within the body of the young individual, while those of 

 the upper surface still form a more or less complete layer, for 

 a time retaining their flagella, though the cells are by no 

 means so closely packed as in the free-swimming larva 

 (fig. 29). However, there is a considerable amount of variety 

 in the mode of flattening out on the part of the different larvae. 

 Some appear to flatten so quickly that the best way to describe 

 it is to say that the larva appears to tumble into pieces almost 

 instantaneously. These differences are due, probably, to the 

 number of points on the larva througli which the cells with 

 granular nuclei make their way out. If these cells break 

 through the flagellated layer in several places at the same 

 time, the result is a quick and rapid metamorphosis. If, on the 

 other hand, they burst out merely at one point on the surface, 

 that point will become the area of fixation, from which the 



