16 ARKIV FÖR ZOOLOGI. BAND 7. N:0 30. 



would not allow the belly to be inflated even in the case 

 of the presence of an air-sac. Thilo has shown that the 

 Triacanthids and the Monacanthus by erecting the dorsal 

 spine together with the ventral spine which is fixed on the 

 pelvic bone are able to keep themselves in erevices in the 

 rocks when the tide falls or rises. In Balistes only the ven- 

 tral spine, not the pelvic bone may be erected, but in several 

 species of Monacanthus even this latter bone is movable, in 

 which case the skin behind the spine is drawn out to a flap, 

 of greater or less size. When the spine is not erected the 

 flap is folded together. A series of different stages of spe- 

 cialization in this peculiarity is to be found amongthe Mona- 

 canthids. The first condition necessary for the development 

 of an air-sac that may be able to inflate the belly is given 

 by this movable pelvic bone and ventral skin flap. But all 

 these arrangements are, of course, not developed with any 

 relation to the air-sac. In Monacanthus there is, it is true, 

 an air-sac^ present, but this is, as shown above not large 

 enough to inflate the belly. I think the best proof of this 

 view Avill be given by a study of Monacanthus occidentalis, 

 of which I have given a figure above. In this species there 

 is a large movable skin flap which can be extended when the 

 pelvic bone is erected, but the body cavity does not enter 

 into it, and, what is most important, there is no dilatation 

 at all of the stomach. But, on the other hand, the movable 

 pelvic bone and skin flap have made it possible for a deve- 

 lopment of an air-sac of the large size, which we find in the 

 Diodontids and Tetrodontids. Already in some species of 

 Monacanthus the pelvic bone has become reduced and this 

 reduction has led to the absolute disappearance of this bone 

 in the Diodontids and Tetrodontids. This latter stage may, 

 perhaps, be directly caused by the development of the air-sac, 

 that is able to inflate the belly. Ribs are wanting in all the 

 Plectognaths which are provided with an air-sac, but as this 

 is the case even in some species, in which no such a sac is 

 developed, e. g. Balistes and some Monacanthi, this cannot be 

 considered as an adaption to the inflating power. 



Also in the integument we meet with direct adaptations 

 to the inflating power. It is evident that the skin must be 

 able to resist the great extension, which is caused by inflating 

 the body in the Diodontids and Tetrodontids. In these forms 



