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 Ammoccetes ? 
“ 
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 
branchie, 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 required 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- 
ccetes 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 
