528 BERTRAM G. SMITH 



Aeration of the blood i.s afforded, not only by the external gills, 

 but by the capillaries lying close to the surface over all the body. 

 On account of its great exposed surface, the tail may be of espe- 

 cial importance as a respiratory organ. 



As in the preceding stage, cilia persist over the entire surface 

 of the body, and the ciliary currents are strongest in the vicinity 

 of the gills. 



B. Larval development, and the metamorphosis 



The changes in the form of the body, and the gradual increase 

 in size, during the first year of larval development, are shown 

 by the photographs (figs. 262 to 267). 



Year-old larvae reared in the laboratory reach a length of 

 about 5 to 7 cm. ; three two-year-old specimens reared in the lab- 

 oratory measured respectively, after preservation in alcohol, 7 

 cm., 8 cm. and 9.5 cm. Near the close of the second summer 

 these latter specimens lost their external gills. The few speci- 

 mens with external gills taken in August from their natural 

 environment (they were found under small fiat stones in shallow 

 water) measured as follows: 6.4 cm., 6.8 cm., 7.0 cm., 7.3 cm., 

 7.7 cm., 12.0 cm., 12.3 cm. It will be noticed that these speci- 

 mens sort into two lots, one lot containing those with body lengths 

 ranging between 6 and 8 cm., the other lot containing specimens 

 approximately 12 cm. in length. Though this data is rather 

 meager, the rather considerable gap between the two lots suggests 

 that we are dealing with larvae of the first and second summers, 

 respectively. In comparing the larger larvae taken from their 

 natural environment with the two-year-old specimens reared in 

 the laboratory, allowance must be made for the fact that the 

 latter were measured after being shrunken by preservation in 

 alcohol; moreover the body form of the laboratory specimens 

 seems to be shorter and stouter than the normal. Specimens 

 with a body length of 14 cm. and more, taken from their natural 

 environment, have invariably lost their external gills. The com- 

 bined evidence from specimens reared in the laboratory and 



