ANIMAL BIOLOGY. [Part I. 



clefts opening into the branchial chambers. The tongue is 

 small, and cannot be protruded. Both upper and lower jaws are 

 armed with teeth ; and, in addition, there are teeth in the roof 

 of the mouth (vomerine teeth) , and in the pharynx (superior and 

 inferior pharyngeal teeth). There are no posterior nares opening 

 into the mouth. The second, third, and fourth median openings 

 are, in order, the anal (a.), genital (g.), and urinary (u.). 



Of the lateral apertures two pairs are nasal, situated close 

 together, and near the anterior end of the snout (e. na.). The 

 other lateral apertures are the openings of the gill-chambers. 

 Each lies behind a flap-like gill-cover, which is bony anteriorly 

 (operculim, op.), and softer posteriorly (branchiostegal membrane, 

 br. m.), containing bony rays the branchiostegal rays. On rais- 

 ing these gill-covers there will be seen four complete gills, each 

 supported on a bony branchial arch, and composed of a number 

 of free deep-red branchial filaments. There is a fifth more rudi- 

 mentary branchial arch which bears no gill. Attached to all 

 the branchial arches are gill-rakers, horny filaments which bound 

 the margins of the five clefts, and act as strainers. On the inner 

 side of the opercular flap is a red patch (pseudobranchia), which 

 is the rudiment of a fifth gill. 



The fish breathes by gulping in water through the mouth and 

 forcing it out backwards through the clefts, over the gills, and 

 beneath the gill-cover. To prevent the water passing out again 

 through the mouth there are two flaps of skin, one on each jaw, 

 which, as the water passes inwards, fold down against the jaw, 

 but as the water attempts to pass outwards are raised and, 

 coming into contact, bar the passage. Their action may be well 

 seen in the pike in an aquarium. 



The body is invested with an exoskeleton, consisting of over- 

 lapping scales, over which there is spread a thin layer of slimy 

 skin (epidermis) containing pigment cells. Each scale consists 

 of a thin oval plate, which under the microscope (low power) is 

 seen to be built up of concentric rings. The free border is 

 smooth and even (cycloid scale), and is not, as in the perch, 

 produced into a number of comb-like processes (ctenoid scale). 

 Along a definite line down each side of the fish, called the lateral 



