AMPinOXUS AND AMMOCCETES. 169 



The last-mentioned median ciliated groove would appear 

 to be unrepresented in Amphioxus, but the downward 

 curvature of the ciliated bands of the latter behind the gill- 

 slits can be observed (Fig. 93). 



In Ammocoetes the ciliated peripharyngeal grooves, 

 where they curve upwards in front along the anterior wall 

 of the pharynx, apparently occupy the same position which 

 was previously occupied by the first pair of gill-pouches. 

 Since the latter have already entirely disappeared, there is 

 nothing in the way of their occupying this position (Fig. 

 92 C). In Amphioxus, where the corresponding gill-slit 

 remains open for a long time, the peripharyngeal band exists 

 without connexion of any sort with the portion of the wall 

 occupied by the slit, and when the latter closes up, it leaves 

 no trace behind."^ 



Thyroid Gland. 



When the metamorphosis of Ammocot;tes into Petromy- 

 zon takes i)lace (which happens after a larval existence of 

 some two years' duration), the hypobranchial groove loses 

 all connexion with the pharynx and becomes broken up 

 by the ingrowth of connective tissue into a number of 

 se]3arate capsules which collectively constitute the thyroid 

 gland of I'etromyzon. 



The thyroid gland is one of those enigmatical ductless 

 glands which form such a curious and constant feature of 

 the Vertebrate organisation. 



There is considerable doubt as to the specific physio- 

 logical function which it has to perform, but at the same 

 time it is a necessary factor in the Vertebrate economy, 

 and is of great importance from a pathological point of view. 



In the higher forms it is attached to the lower side of 

 the larynx, and appears to have received its name on 



