582 



gill-slit. This gigantic enlargement ^) of (lie moutii indicates that the 

 larva, during its growth, must be a very gluttonous animal, if the 

 words gigantic and gluttonous may be applied to an animal that is 

 not even 5 m.m. long. The gluttony is also of a very inoffensive 

 nature and consists in swallowing water, for only minimal remnants 

 of food are found in the gut. 



During the metamorphosis remarkable modifications occur at the 

 mouth of the larva. 



One modification regards its size. In the first half of the period 

 of metamorpiiosis the gigantic mouth becomes constantly smaller, 

 till, in the middle of this period, it is an extremelj' little round 

 hole. But for a little sickle-shaped slit this hole is moreover closed 

 by the formation, at the rostral rim, of the first tentacle in the 

 shape of a little tongue. 



Kow the falling asunder and the resorption of the gill-muscles is 

 in full swing. The fil)res of these muscles let loose from their 

 insertions and have partly been broken into pieces. Just like the 

 loosened cells of the club-shaped gland these pieces float in the 

 fluid that is found in the body-cavity. 



From this phenomenon, from the cessation of the growth and 

 from the minimal size of the mouth 1 deduce, that the animal 

 ceases to take food from outside, and continues to live at the 

 expense of part of its own texture: the gill-muscles and the cells of 

 the club-shaped gland. 



Obviously the gill-muscles have l)ecome superfluous, in conse- 

 (juence of the formation of the alrium, which now regulates for the 

 greater part the movements of respiration. No trace of gill-muscles 

 appears at the slits of the right-side of the body, which never open 

 directly to exterior, but only indirectly by means of the atrium. 



In the second half of the metamorphosis the moulh enlarges 

 again gradually, and becomes the opening of the "velum", round 

 which three more tentacles develop, completing the number of 

 four, which this opening is provided with. 



The diminution of the moutli has already partly been observed 

 by Legros, but vehemently contested by Willky, who supposes this 

 diminution to Ic only apparent, an optical effect, caused by the 

 rotation of the mouth on a sagittal axis. 



According to Willey. who does not base his views on the study 

 of sections, but only on that of the larva in toto, this rotation should 

 commence already at the beginning of the metamorphosis. I found 



1) This enlargement of the moulh contributes to the temporary removal of the 

 foreraosl gill-slils of the lefl-side lo the ri^hl-side. 



