204 ARTHUR WILLEY. 



that take place are the completion of the oral hood and the 

 increase in the size of the gill-slits and endostyle, but no fresh 

 slits are formed. 



The close of the critical period may, therefore, be taken as the 

 close of the entire larval phase of development. Up to this 

 point all the gill-slits of the left side are primary — that is to 

 say, they appeared at first as a single series on the actual right 

 side of the larva, and were pushed across to the left side by the 

 developing secondary slits. We shall find reason to suppose 

 that the actual right side of the larva is not precisely identical 

 with the morphological right side. 



Summary of the Stages. 



In the following summary it will be found desirable to give 

 a few measurements, which, however, have only an average 

 value, the unit employed being a hundredth of a millimetre. 

 It is also intended, for the most part, to ignore the variations 

 which have been detailed above, and to state only what 

 appears to be the usual condition of things, as inferred from 

 observations on numerous larvae. 



Shage I. Right Side. — Fourteen primary slits, none 

 closing ; six secondary thickenings above the primary slits ; 

 endostyle in front of club-shaped gland, and therefore in front 

 of all the gill-slits ; atrium widely open anteriorly. 



Left Side.— Large lateral mouth; apparent length (in this 

 case the actual length), 45 units; 1 to 2 elements of buccal 

 skeleton. 



Stage II. Right Side. — Fourteen primary slits, four- 

 teenth closing ; secondary thickenings perforated ; endostj'le 

 unaltered ; atrium somewhat less widely open anteriorly. 



Left Side. — Lateral mouth beginning to bend round in 

 front; apparent length, 36; 3 to 5 elements of buccal skeleton. 



Stage IIL Right Side. — Thirteen primary slits; se- 

 condary slits simple ; endostyle unaltered ; atrium slightly open 

 anteriorly. 



Left Side. — Mouth still further bent round in front; 

 apparent length ranges from 28 to 12, average about 25 ; 5 to G 



