MOUTHS AND JAWS 109 



Sturgeon feeds largely on small invertebrates, rooting up the 

 mud or sand with its snout, feeling for the prey with its sensitive 

 barbels, and finally sucking them up by means of its funnel-like 

 mouth. Its diet not infrequently includes small fishes, and a 

 large Sturgeon will rapidly engulf large numbers of "minnows" 

 unsuspiciously engaged in depositing their spawn. The related 

 Paddle-fish or Spoonbill (Polyodon), found only in the rivers of 

 the southern states of North America, has a comparatively 

 wide mouth armed with small teeth. The rostrum, forming no 

 less than one-fourth of the entire length of the fish, is a thin, 

 flat, spoon-shaped blade, the outer surface of which is more 

 or less sensitive (Fig. 470) . This fish lives entirely on mud and 

 the minute organisms contained therein, the snout being used 

 for stirring purposes, whilst the food is strained from the water 

 by the exceptionally long and close-set gill-rakers. 



A mouth which is semicircular in outline and placed on the 

 under side of the head, is characteristic of a large number of 

 fishes habitually living in mountain streams or torrents {cf. 

 p. 239). In many Indian species of the Carp tribe {Cyprinidae) 

 the jaws are much strengthened, and their edges have become 

 sharp and cutting. One fish, which lives on very fine weeds 

 stripped from the rocks and stones of the river-beds, has the 

 jaws encased in a strong covering of horny substance. In 

 many of the Cat-fishes (Siluroidea) of these regions the mouth, 

 together with the much modified lips, forms a broad, flat 

 sucker by means of which the fish is able to cling to a stone or 

 other fixed object when resting. In the Mailed Cat-fishes 

 [Loricariidae) of South America the sucker-like form of the 

 mouth is well shown, the lips being greatly enlarged, reflected 

 outwards, spread in circular form round the mouth, and often 

 fringed with membranous tentacles of various sizes (Fig. 450). 

 The mouth itself is provided with small, weak jaws, and armed 

 with feeble teeth, the food consisting of putrifying organic 

 substance or the minute creatures contained therein. 



Apart from those cases in which they are adapted for taking 

 part in the formation of a sucking apparatus, the lips of fishes 

 do not exhibit many striking modifications. In the Wrasses 

 [Labridae) they are particularly thick, a feature from which 

 the German name of Lippenfische given to the members of this 

 family is derived. In certain species of Cichlids {Cichlidae) the 

 central portions of both upper and lower lips are prolonged to 

 form freely projecting fleshy lobes (Fig. 47D). It is of interest 

 to note that this peculiar modification, which may be con- 



