FISHES. 347 



The extent of surface presented by the gills of a fish, to 

 enable the blood to come in contact with the air in the 

 water, is much greater than one would, without attentive 

 consideration, be led to suppose. Dr Monro calculated, 



myxine, the apodal chondropterygious fishes, and the least perfect in the sys- 

 tern. " In the lamprey (he says) the organs of respiration have seven ex- 

 ternal openings on each side of the animal ; these lead into the same num- 

 ber of separate oval bags, placed horizontally, the inner membrane of which 

 is constructed like that of the gills in fishes. There is an equal number of 

 internal openings leading into a tube, the lower end of which is closed, and 

 the upper terminates by a fringed edge in the oesophagus. These bags are 

 contained m separate cavities, and enclosed in a thorax resembling that of 

 land animals, only composed of cartilages instead of ribs, and the pericar- 

 dium, which is also cartilaginous, is fitted to its lower extremity like a dia- 

 phragm." Phil. Trans. 1815, p. 257. In the myxine, the external open- 

 ings are two in number, but there are six lateral bags on each side, placed 

 perpendicularly, to which there are six tubes from each of the openings, and 

 close to the left external opening there is one which passes directly into the 

 oesophagus. 



" In the lamprey, the water is received by the lateral openings of the 

 animal into the bags which perform the office of gills, and passes out by the 

 same opening ; the form of the cavities being fitted to allow the water to go 

 in at one side, pass round the projecting parts, and out at the other. A 

 part of the water escapes into the middle tube, and from thence, either 

 passes into the other bags, or out of the upper end into the oesophagus. 

 There is a common opinion, that the water is thrown out at the nostril ; 

 this, however, is unfounded, as the nostril has no communication with the 

 mouth." — " In the myxine, the elasticity of the two tubes, and the bags into 

 which they open, admits of the water being received ; and the pressure pro- 

 duced by the action of the external muscles, forces it into the oesophagus, 

 from whence it is thrown out by the opening at the lower end of that tube." 

 Ibid. 



The means here stated, as employed to bring the water to the surface of 

 the giUs, is probably, in the case of lamprey, only used during the action of 

 the sucker, as the gill cavities seem but ill fitted for the continued absorp- 

 tion, circulation, and ejection of the water by the same orifice, such as our 

 author supposes. Analogy, too, forbids us to conjecture, liiat the gills of 

 the lamprey shall be watered by motions the reverse of those which arc exe- 

 cuted bv other fishes. 



