i8 



CHBL^ROID FISHES AND THEIR DEVELOPMENT. 



The mode of breathing of the fish is somewhat 

 remarkable. The mouth is very small, and its rims are 

 motionless or almost motionless, scarcely parted in breath- 

 ing; so nearly closed, in fact, that the movement of 

 the breathing-valve can hardly be seen. A portion, 

 probably a large portion, of the water— as in the case 

 also of dipnoans— is breathed through the prominent 

 nasal openings (fig. 4 a and c), whose cartilaginous marginal 

 iiap is specialized to this end ; and since the mouth is motion- 

 less, it follows that the branchio-opercular muscles are the 

 efficient means of introducing water to the gills. In point 

 of fact, in the living fish one readily observes an extensive 

 dilation and contraction of the opercular flaps. In spite, 

 however, of this extensive movement, the excurrent open- 

 ing is remarkably small, and at this point the opercular 

 fold puffs out conspicuously, like an opened valve, a small 

 one at that, forming a slit about three-sixteenths of an 

 inch in diameter. The rhythmic opening and closing of 

 this slit gives a further suggestion of its valvular nature. 

 The breathing, moreover, as in the case of other fishes, 

 is rendered more effective by the presence of oral breath- 

 ing-valves, operating so as to close not merely the open- 

 ing of the mouth, but the nasal passage also. The respi- 

 ratory movements are rajud, at least in captive fish. In 

 such specimens there are counted as many as 100 respi- 

 rations a minute, a number evidently abnormal. (3cca- 



sionally, when the fish is swimming, the mouth will open 



two or three times spasmodically. This occurs too rarely, 



however, to be of especial respiratory value ; and it is 



also to be observed, if the fish is a male, that the 



frontal clasping spine will at the same time be elevated 



and depressed. 



This correlated movement of clasping spine and jaw 



has already been suggested by Reis on anatomical grounds. 



In this connection it was once observed that both mixip- 



terygia were suddenly dropped from their position close to 



the trunk, rotating downward together from their bases, 



their tips rotating through an arc of 90°, just as rigid fingers 



mi^'ht be bent downward from the plane of the hand, but from females ; j L, from males. 



no details of this process were seen, for they at once rotated backward into their 



closed position. 



Fig. 5. — Mandibular dental plates 

 Chimaera colliei, shown in outer lat- 

 eral aspect, and indicating variation 

 in these structures. A 1. specimens 

 from females ; J L, 



