226 ARTHUR WILLEY. 



either in front of or behind the unmodified slit with which it 

 was primitively paired. As a matter of fact, of course, it 

 passes in front of that slit, and so it has come — possibly in some 

 measure owing to the curious torsion which has affected this 

 region — to arise in front of the first gill-slit, with which it 

 really on this view corresponds as a pair. It thus belongs 

 morphologically to the region of the second myotome. 



5. The atrophy of the club-shaped gland occurs simulta- 

 neously — or nearly so — Avith the atrophy of the first primary 

 slit. This fact is worthy of the utmost emphasis. 



6. In the formation of the secondary slits the first of them 

 pairs with the second primary slit, and no true secondary slit 

 ever appears to correspond with the first primary slit. 



7. After the simultaneous formation of the club-shaped 

 gland and the first primary slit (the mouth and anus being also 

 present) there is a prolonged interval, during which no further 

 formation of gill-slits occurs. 



This might indicate a distinction between the first primary 

 slit and the succeeding ones. 



These are the reasons, drawn from facts, all of which, except 

 Nos. 4 and 7, are illustrated by figures accompanying the 

 present paper. 



Deduction. — The club-shaped gland and first primary gill- 

 slit represent an ancestral pair of gill-slits, which atrophies 

 in the course of larval development. 



How or for what special object a gill-slit could be modified 

 into such a tubular collateral organ is quite another matter, 

 which I do not now propose to consider farther (vide p. 210). 

 Suffice it if the reasons given above are enough to lead us to 

 suppose that such a thing has indeed happened in the present 

 case. 



The closure of the hinder primary slits presents an interest 

 different from that of the first, and is the decisive step in what 

 I have before called the " symmetrization" of the larva. The 

 transverse growth which carries the more anterior slits from 

 the right to the left side does not extend back so far as the 

 hinder slits, so that they are left in the middle ventral line. 



