ROSÉN, STUDIES ON THE PLECTOGNATHS. 7 



Monacanthus to Tetrodon and Spheroides, but if Thilo's 

 observation is not quite correct, that is, if he has overlooked 

 the connection between the air-sac and the body wall, there 

 will be found a gap between these two groups, a gap which 

 then may be filled up by the Diodons, which I shall describe 

 below. SÖRENSEN has described the air-sac in Tetrodon fahaka 

 Hasselqutst as very large, communicating through a rather 

 wide opening with the oral cavity between the pharynx and 

 the pharyngeal teeth, while Cuvier^ mentions the air-sac in 

 »Tetrodon» as »une grande dilatation». The remarks made 

 by these two authors without any figures are not sufficient 

 to give a correct idea of the shape of the air-sac, and the 

 same is the case with all earlier writers (cp. v. Baer, p. 47). 



Spheroides. The species I have studied is S. testudineus 

 L. (Fig. 5 and 7; Pl. I — II). The air-sac is enormously developed 

 and united with the ventral body wall. The oesophagus is 

 funnel-shaped, opening into the rather wide stomach, from 

 which the air-sac is marked off by a circular fold containing 

 a sphinctermuscle {sph). On account of the coalescense with 

 the ventral body wall the peritoneal layer has disappeared 

 on the ventral and anterior surface. The peritoneum on the 

 posterior wall of the air-sac continues directly backwards into 

 the layer on the ventral body wall (Fig. 5; Pl. II Fig 1). 

 The mucous membrane of the oesophagus forms longitiidinal 

 folds. The epithelium consists of goblet cells. Underneath 

 the epithelium there is a feltwork of connective tissue, which 

 is more developed in the dorsal wall than in the ventral and 

 lateral. In this mäss of connective tissue there is a layer of 

 longitudinal non-striped muscle-fibres. Muscles radiating from 

 the lumen are wanting, but the periphery of the wall of the 

 oesophagus is occupied by a thick stratum of circular striped 

 muscular bundles. The stomach (s. str.) shows irregular folds 

 on the inner surface. The epithelial cells are all high and 

 cylindrical. The stratum of connective tissue that surrounds 

 the epithelium is very thin and contains a very feeble träns- 

 verse layer of non-striped muscle-fibres. Near to the fold that 

 forms the boundary between the true stomach and the air- 

 sac some muscle-fibres of a longitudinal direction will be 

 found. The fold contains a sphincter muscle, as already men- 



^ Le(j:ons d'anatomie comparée. Tom. IV, 2; p. 160. 



