18 ARKIV FÖR ZOOLOGI. BAND 8. NIO 10. 



out by Joh. Muller. Before Hertwig Hollard and Wil- 

 liamson have described the scales of several Balistids. Re- 

 ferring for a detailed account to Hertwig's work I shall 

 give here merely a brief sketch of the integument. 



The epidermis is composed of a few layers of cells. As 

 in the previously described families of Plectognaths two layers 

 of the corium can be distinguished, both characterized in the 

 same manner as before. The interiör layer consists of bund- 

 les of fibres running in different horizontal directions as 

 well as vertically. The integumental ossifications have the 

 form of plates, which cover each other more or less, varying 

 in different species. The superior layer is homogeneous pro- 

 jecting in spines, which sometimes seem to pierce the epider- 

 mis (Fig. G). The inferior portion of the plates has a struc- 

 ture corresponding to that of the interiör stratum of the 

 corium. The plates are penetrated by canals, which include 

 vessels. In the homogeneous stratum they form a more or 

 less developed network. The form of the plate and the 

 number of the spines varies rather much in different forms, 

 as is shown by Hollard and Hertwig, but they can all be 

 derived from the type described. Of special interest is 

 the initial reduction of the plates in forms which are pro- 

 vided with an extended stomach, the origin of an air-sac. 

 Pigment occurs in the exteriör stratum of the corium along 

 the basal membrane and in the interiör layer along the lower 

 surface of the scales. The subcutaneous tissue is very slightly 

 developed. 



The families Triodontidae and Triacanthidae have scales 

 of a type which agrees in all main characters with that of the 

 Balistids and affords no special interest. 



Tetrodontidae. 



The integument of the Tetrodontids has undergone a 

 specialisation which is connected with the power of inflating 

 the belly, a peculiarity with which I have dealt in the second 

 part of this series of studies. 1 Before entering into a descrip- 

 tion of the integument a few facts about the air-sac may be 



1 Studies on the Plectognaths. 2. The Air-sac. Arkiv för Zoologi, 

 Bd 7. N:o 30. 



