24 ARKIV FÖR ZOOLOGI. BAND 8. N:0 10. 



air-sac is emptied. These muscles have as to their position 

 the nature of integumental muscles, but are derived from 

 muscles of the body. They will be dealt with in connection 

 with the other muscles. I have mentioned them already 

 here, since they form one of the peculiar characters of the 

 skin of this family (Fig. 12 and 13). 



Diodontidae. 



The integument of this family agrees in all main charac- 

 ters with that of the Tetrodontids. The corium has the 

 same structure. A subcutaneous tissue is present, but not 

 so well developed as in the previous family. The ossifica- 

 tions have the same shape: a spine with a varying number 

 of roots, showing the same structure as described for the 

 Tetrodontids. Hertwig has described the spines of a Chilo- 

 mycterus. He found concentric zones. From the axis of the 

 spine striae radiate. I have seen the same in Biodon (Fig. 16). 

 In Diodon I have observed the same two muscular layers as 

 in the Tetrodontids. Darwin has observed that the skin of 

 the belly of Diodon secretes a red viscous fluid. I have not 

 found this to be the case in the specimens of D. hystrix and 

 holacanthus, which I have observed in a living state. 



General Remarks. 



The account of the integument of the Plectognaths given 

 above shows, that, though several peculiarities are found in 

 some families, the integument still has the same main struc- 

 ture in all families. The epidermis varies slightly. The 

 corium consists in all forms of two distinct strata, one ex- 

 teriör with plenty of cells but few fibres, one interiör very 

 rich on fibres. The fibres in the interiör layer are arranged 

 in bundles of a varying direction. In Mola, where this part 

 of the corium is enormously developed, the bundles have a 

 winding irregular loop. The corium becomes härd and tough. 

 In the Tetrodontids and Diodontids the bundles are also wind- 

 ing but not so irregular, making the skin extensible. The 

 families Ostraciontidae and Balistidae have the bundles 



