1891.] Physiology of Protoptems annectens. 551 



folded epithelium of the oral cavity. The lips contain no muscles. 

 The tongue, as well as the palate, is covered with blunt conical 

 papillae, on which the taste-buds are situated. Beneath the epi- 

 thelium the tongue is composed anteriorly to the hyoid of a simple 

 connective tissue, while posteriorly to the hyoid it is made up of 

 extrinsic muscles, the main mass of which is continuous with the 

 ventral musculature of the trunk. A horny cap is developed over 

 each tooth from the overlying epithelium, which apparently becomes 

 cut through by the sharp edges and points of the teeth, and which 

 probably corresponds to the cuticula deniis. The thyroid is a small 

 bilobed organ imbedded in the tongue just above the hyoid symphysis, 

 and has the characteristic structure. The thymus consists of 

 lymphoid tissue, and is situated dorsally and posterior to the 

 branchial arches, surrounding the blood-vessels of the external 

 gills. 



The alimentary canal extends almost in a straight line from the 

 mouth to the vent. A ventral, as well as a fenestrated dorsal, 

 mesentery is present supporting the intestine. The so-called urinary 

 bladder (" cloacal caecum ") opens into the cloaca dorsally to the 

 intestine ; the author compares it with the " processus digitiformis " 

 of Elasmobranchs. A spleen and pancreas are present, imbedded 

 in the thin walls of the stomach, and extending on to the proximal 

 part of the intestine ; they are covered externally by sparse muscular 

 fibres as well as by the peritoneum. The relations of the pancreas 

 therefore most nearly resemble those met with in Ganoids and 

 certain Teleosteans. The pancreas is deeply pigmented, and its 

 ducts open into the bile-duct. The pigmented walls of the intestine 

 and the spiral valve are very thick, owing to the abundance of 

 lymphoid tissue contained within them. With the exception of the 

 bursa entiana, the internal walls of which are raised up into a number 

 of deeply pigmented oblique folds, the whole of the mucous membrane 

 of the stomach and intestine is perfectly smooth, and there is no 

 indication of any differentiated gastric or intestinal glands. 



Cilia are present on the epithelium throughout the stomach and 

 intestine. The epithelium is columnar and stratified, and branched 

 pigment cells extend into it in the greater part of the intestine. 

 Small leucocytes can be recognised among the epithelial cells here 

 and there. A layer of small-celled lymphoid tissue directly under- 

 lies the epithelium. In the spleen and lymphoid organs of the 

 intestine two kinds of tissue are present: (1) a large-celled tissue, 

 which forms the greater part of these organs, and which somewhat 

 resembles embryonic connective tissue ; and (2) a smaller-celled 

 tissue, similar to that lying directly beneath the epithelium, and 

 resembling that of ordinary lymphoid follicles. Large migratory 

 cells are present in both kinds of tissue, many of which enclose 



