THE EVIDENCE OF THE THYROID GLAND 213 



Such is the conclusion to which the study of the thyroid gland 

 in Ammoccetes seems to me to lead, and one cannot help wondering 

 why such an unused and rudimentary organ should have remained 

 after its original function had gone. Is it possible to find out its 

 function in Ammocretes ? 



The Function of the Thyroid Gland in Ammoccetes. 



The thyroid gland has been supposed to secrete mucus into the 

 respiratory chamber for the purpose of entangling the particles of food, 

 and so aiding in digestion. I see no sign of any such function ; neither 

 by the thionin method, nor by any other test, have Miss Alcock and 

 myself ever been able to see any trace of mucous secretion in the thy- 

 roid, and, indeed, the thyroid duct is always remarkably free from any 

 sign of any secretion whatever. Not only is there no evidence of any 

 mucous secretion in the thyroid of the fully developed Ammoccetes, 

 but also no necessity for such secretion from Dohrn's point of view, 

 for so copious a supply of mucus is poured out by the glands of the 

 branchiae, along- the whole pharyngeal tract, especially from the cells 

 of the foremost or hyoid gills, as to mix up with the food as 

 thoroughly as can possibly be needed. Further, too, the ciliated 

 pharyngeal bands described by Schneider are amply sufficient to 

 move this mixed mass along in the way recpiired by Dohrn. Finally, 

 the evidence given by Miss Alcock is absolutely against the view that 

 the thyroid takes any part in the process of digestion, while, on the 

 other hand, her evidence directly favours the view that these 

 glandular branchial mucus-secreting cells play a most important part 

 in the digestive process. 



In Fig. 93, A is a representation of the respiratory tissue of a 

 normal gill ; B is the corresponding portion of the first or hyoid gill, 

 in which, as is seen, the whole of the respiratory epithelium is 

 converted into gland-tissue of the nature of mucous cells. 



To sum up, the evidence is clear and conclusive that the Ammo- 

 cartes possesses in its pharyngeal chamber mucus-secreting glands, 

 which take an active part in the digestive process, which do not in 

 the least resemble either in structure or arrangement the remarkable 

 cells of the thyroid gland, and that the experimental evidence that 

 the latter cells either secrete mucus or take any part in digestion 

 is so far absolutely negative. It is, of course, possible, that they 



