172 DEVELOPMENT OF A AI PHI OX US. 



3. They atrophy and disappear simultaneously during the 



metamorphosis of the larva. 



4. No secondary gill-slit ever arises to pair with the first 



primary slit. 



As the stage represented in Fig. 64 marks such a vital 

 turning-point in the development of the individual, being 

 the stage at which the embryo becomes a larva and the 

 struggle for existence in obtaining independent nourish- 

 ment genuinely sets in, it is important to be able to define 

 it accurately. In view of the above considerations, we 

 arrive at the conclusion that the larva is at this stage 

 possessed morphologically of a pair of gill-slits. 



It should be pointed out that this opening stage of the 

 larval development appears to be of the nature of a rest- 

 ing phase, during which the larva accumulates energy for 

 future growth. 



Pr&oral "Nephridium" of Hatschek. 



In the larvae of Amphioxus there is a structure lying at 

 the base of the notochord on the left side, immediately 

 above the praeoral pit, which we have not yet consid- 

 ered. (Cf . Figs. 8 1 and 82, ;r.) According to Hatschek, who 

 first described it, it arises in the larva as a mesodermal, 

 ciliated funnel and canal in front of the mouth, in the 

 region of the first metamere. It lies in a narrow division or 

 prolongation of the body-cavity, beneath the left aorta. (Cf. 

 Fig. 76 B.} At its hinder end it opens into the pharynx. 

 Hatschek interprets this structure as a nephridium. Its 

 true physiological, and especially its morphological, sig- 

 nificance is, however, very perplexing and requires further 

 study. 



