2I g TEXT-BOOK OF ZOOLOGY 



2. The body being broad and flat, thus presenting a broad surface to 

 the water, the bird is supported more steadily than if its body were 

 cylindrical. (What might easily happen in the latter case ?) 



ii. Its Manner of Progression by Swimming. 



1. In its feet, the long front-toes of which are united by webs, the 

 duck possesses oars (compare with otter) which are moved in alternation 

 (natatorial feet). The expansion of the webs in the backward stroke 

 and their folding up in the forward movement are carried out involun- 

 tarily, as in the toes of a bird settling on a branch (see p. 143). This 

 may be easily observed in the bird while walking, since its walking and 

 swimming are essentially similar movements. 



2. Inasmuch as only the feet project from the body, and the leg (tarso- 

 metatarsus) is short, these oars can execute powerful strokes against the 

 resistance of the water (see seal). 



3. The sharp anterior edge of the leg (tarso-metatarsus) cuts the water 

 with ease. 



4. The legs are moved by powerful muscles. 



5. The legs perform the part not only of oars, but of rudder. The 

 body of a duck may therefore be likened to a boat which is propelled, 

 and at the same time steered, by an oarsman with two oars. Now, as 

 the point on which such a boat turns is located in the seat of the oars- 

 man, the latter, instead of placing himself in the middle of the boat, sits 

 as far back as possible ; for in this case the fore-end of the boat is made 

 to describe a wider deviation i.e., the steering has a greater effect. 

 Accordingly, in the duck, too, the oars are articulated to the body far 

 behind. For the same reason, in those swimming-birds whose life is 

 passed to a still larger extent on or below the water, the legs are articu- 

 lated still further behind (e.g., swans), or even quite at the posterior end 

 of the body (e.g., the great crested grebe and penguin). 



Though a skilful swimmer, the duck's progress on land is awkward 

 and ungainly. The tibia lies completely within the body, and hence 

 only slightly shares in the motion of the legs (compare, on the other 

 hand, the ostrich and stork) ; the tarsus, moreover, is short ; all these 

 arrangements impart a certain stiffness to the gait of the bird. (In 

 what other birds is this the case ? Name the different joints at which 

 the human leg is bent in walking. Endeavour to walk without bending 

 the knee-joint.) Further, the articulation of the legs far apart from 

 each other causes the duck to waddle. (Compare, on the other hand, 

 the stork and ostrich.) The webs of the feet also impede walking. 



