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



II. The functioii of tlie Air-sae. 



It is a fact known for a long time that many of the 

 Plectognaths are capable of inflating the belly by means of 

 filling the air-sac with air. Some writers, e. g. Oken, con- 

 sider this to be possible even by filling it with water, but no 

 observations have been made that support this theory. What 

 is the advantage of this inflating power? As regards Diodon 

 this question seems to be rather easy to answer. In this fish 

 the whole body is covered, as is well known, with large spines 

 which become erected when the body is inflated. The Diod ons 

 are, when inflated, turned över on the dorsal side and are 

 driven along by the streams and waves. That they in this 

 inflated state, altered as they are into a very spinous ball 

 are very well protected from their enemies is quite evident. In 

 the Tetrodontids the spines are very small, but it is probable 

 that the inflating power for them also is a means of defence. 

 But is this the only function of the air-sac? In order to 

 answer this question we will take the development of the 

 air-sac into consideration. We have seen above that the first 

 stage in the development of this sac is represented by a slight 

 dilatation of the stomach which then increases until it reaches 

 the large form in Diodons and Tetrodons. Thilo has given 

 an account of this primitive form of the air-sac and its 

 function. As I have not made any observations myself about 

 these forms I refer to his paper, the chief contents of which 

 I give here in a few q notations. He says, that Monacanthus 

 which lives among rocks takes air into the stomach and con- 

 tinues (p. 76): »Die Aufnahme von Luft ist fiir Monacanthus 

 besonders wichtig, da in den Spalten oft nur wenig Wasser 

 vorhanden sein känn und der Sauerstoff in demselben bald 

 verbraucht wird. Hat sich der Fisch mit Luft gefiillt, so 

 braucht er nur immer ganz kleine Mengen von Luft in die 

 wassergefiillte Kiemenhöhle streichen zu lassen. Er känn dann 

 leben wie in lufthaltigen Wasser. Diese Art der Atmung wird 

 dem Monacanthus ganz besonders durch seine engen Kiemen- 

 spalten erleichtert.» I think it is probable that this secondary 

 respiratory function of the air-sac is maintained also in the 

 Diodontids and the Tetrodontids, though in these forms to this 

 function is added another, that of defence. The Diodontids 



