254 Mm R- Alcock, On the Digestive [Nov. 9, 



I conclude then from these experiments that : — 



1. The proteid digestive ferment in the Ammoccetes is of the 

 nature of pepsin rather than trypsin. 



2. This ferment is very diffuse in position, being found in all 

 parts of the alimentary tract. 



3. It is found mainly in the anterior part of the tract, 

 especially in the respiratory portion of the pharynx and in the 

 liver. 



4. The so-called thyroid gland has nothing whatever to do 

 with the digestion of proteids. 



Perhaps the most novel and important feature of this series of 

 experiments is the evidence of the importance of the pharyngeal 

 cavity for proteid digestion in this primitive form of vertebrate ; 

 and when we come to examine the histological structure of the 

 alimentary tract we find that glandular secreting structures are 

 more conspicuous in this part of the alimentary canal than in the 

 intestine proper. In the pharynx the epithelium lining the body- 

 wall and the adjacent branchial surfaces is undoubtedly different 

 from the single layer of flattened epithelium cells which cover the 

 lamellar folds of the branchiae themselves. It is usually described 

 as consisting of several layers ; but very conspicuous amongst the 

 small epithelium cells are numerous glandular looking cells, to 

 which I have never found any reference in descriptions of this 

 region. These cells are arranged in groups of five or six together, 

 and correspond in height to the whole thickness of the epithelium; 

 they are somewhat swollen in the middle and are covered super- 

 ficially by the small epithelium cells with the exception of a small 

 space above the centre of the group where their tips converge to- 

 gether at the surface. In some preparations a collection of deeply 

 stained granules can be seen at the outer ends of these cells, and 

 in others a stringy mucus-like secretion is issuing from them. It 

 is striking how on the branchial folds of the anterior wall of the 

 first gill-pouch the epithelium containing these cells predominates, 

 only the most internal folds appearing to have retained their respi- 

 ratory function. Clearly the abundance and the evident activity of 

 these pharyngeal glands is a sufficient histological reason why the 

 extract of the pharynx is so active in digestion. On the other 

 hand the epithelium of the narrow anterior intestine consists of a 

 single layer of tall columnar cells with a distinct cuticular border 

 and uniformly ciliated ; there is no histological evidence here of the 

 presence of any secreting cells. The epithelium of the rest of the 

 intestine is very uniform, and similar in character to that of the 

 anterior intestine, though only the anterior dilated portion is 

 ciliated ; and the ciliation even here is not uniform, but occurs in 

 patches. The whole structure of this region suggests an organ of 

 absorption rather than of digestion. 



