N. ROSÉN, STUDIES ON THE PLECTOGNATHS. 21 



body wall. The skin of this part of the body becomes ex- 

 tended. With increasing inflation the skin of the whole body 

 extends indirectly. As already pointed out in the inquiry 

 on the air-sac, several adaptations for this purpose are to be 

 found. My studies are chiefly based on Spheroides testudi- 

 neus L., of which I have had both young specimens and 

 full grown ones at my disposal. 



The skin is thicker on the ventral side of the body, than 

 on the dorsal and lateral ones. This part of the skin is, as 

 will be understood, more extended than the rest. The sur- 

 face of the skin presents feeble furrows with a longitudinal 

 direction (Fig. 11). Transverse sections through the skin 

 show them very clearly. The folds between the furrows are 

 chiefly composed of the epidermis. Owing to these folds, 

 the possibility of extension is increased. An examination of 

 the skin shows, that the folds are mos t developed on the 

 ventral side. 



The epidermis consists of a few layers of cells. Between 

 the indifferent epithelial cells there are some of a clear appea- 

 rance, the nature of which I have not studied. They are 

 not stained with indigocarmine or Bismarckbrown. 



Corium. In adult specimens the corium seems at the 

 first glance to be of about the same structure both in the 

 exteriör and interiör part. The bundles of fibres are very 

 numerous. They are not arranged in regular lamellae but 

 have a winding loop, though not in quite the same irregular 

 mode as in Mola. This arrangement of the fibres allows a 

 greater extension than if they had a straight vertical or hori- 

 zontal course, forming lamellae. A more careful study shows, 

 however, that, as in the previous Plectognaths, two layers 

 of the corium can be distinguished. The exteriör layer has 

 the fibres less highly developed than in the interiör. In 

 young specimens this difference is more distinct. The exte- 

 riör layer is in early stages cellulär with few fibres, the in- 

 teriör strongly fibrillated. This fact is of importance as 

 regards the question of the development of the spines. Pig- 

 ment occurs along the basal membrane. 



Integumental ossifications. The skin is in almost all 

 the species of Tetrodontids covered with small spines, which 

 in full grown specimens pierce the epidermis with their tips. 

 The spine has the shape of a long narrow cone, the base of 



