MECHANICS OF RESPIRATION 



is no sign of movement other than the mechanical, 

 regular, and continuous movements of respiration. In 

 large specimens these movements average from 30 to 

 32 a minute; in small specimens about 40. 



The skate and the shark belong to another category. 

 They are provided with air-holes. In them the main 

 inspiration, instead of being carried on by the mouth, 

 is effected by the two symmetrical special orifices called 

 spiracles or " air-holes " which are found in the head 

 behind the eyes. These openings give direct access 

 to the hinder mouth, and the water from outside 

 penetrates through them, then passes to the gills and 

 flows out by the gill-slits, which here are distinct and 

 not grouped beneath a gill-cover. The principal 

 inspiratory action is carried out through the air-holes, 

 and we notice movement in their neighbourhood. 

 Behind them and the eyes, the upper region of the 

 head, above the large hinder mouth which acts as a 

 respiratory vestibule, rises and falls alternately, so as 

 to enlarge or decrease in height the extent of this 

 space. In certain cases the water is brought from 

 outside through the air-holes, and sent back over the 

 gills. In order to prevent the water from flowing out 

 the same way as it came in, these air-holes are fitted 

 with a small valve like a horizontal flap, which closes 

 them at each expiration. It is interesting to watch 

 a skate breathing as it lies motionless upon the bottom, 

 We see it palpitate at the upper part of the head, 

 as the eels palpitate at the neck or the gurnard at 

 the throat, and as it does so, the flap of the valve 

 opens or closes the hole. The number of inspira- 

 tions per minute varies, according to individuals, from 

 40 to 50. 



It is not only the fishes, of all the dwellers in our 

 aquarium, who display such signs of respiration. The 

 large crustaceans, crabs, lobsters and rock-lobsters, 

 have their own type of breathing which they exhibit 

 continuously. The rock-lobsters, in their usual position, 

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