228 LIFE : OUTLINES OF GENERAL BIOLOGY 



organ is the post -anal body, which consists almost entirely of 

 strong W-shaiKxl blocks of muscle, dovetailed into one another, and 

 centred in the backbone, which is very flexible from side to side. 

 By an alternate bending and straightening of the posterior body, 

 masses of water are displaced to right and left; and thus the fish 

 is projx'llod forwards. In the case of Cetaceans the locomotion is 

 similar, but a complication is introduced by the adaptive shape 

 of the horizontally flattened flukes of the tail. In ordinary seals 

 (Pliocidie) the hind-limbs are permanently turned backwards and 

 bound up with the short tail, forming with it a unified functional 

 projx'ller, for alternate lateral displacement of the water. The 

 j)rinciplc is the same in cases like sea-snakes and swimming leeches, 

 where the gripping of the water and the using of it as a resistant 

 mass, against which to contract, are not localised posteriorly, but 

 extend over the whole length of the body. In sea-snakes there is 

 usually a side-to-side flattening of the posterior region of the body. 



(IV) Fourthly, the man in the boat may row, the principle being 

 the simultaneous exertion of pressure on each side. Thus the duck- 

 mole rows in the water with its webbed fore-feet, and the turtle 

 with its paddles. Rowing in the air is the essence of flight in birds, 

 bats, and insects, but the comparison with the boat must not be 

 pressed, since the boat floats on the water, whereas the lightest 

 bird has to expend part of its energy in keeping its body from sinking 

 in the air. Brittle-stars (Ophiuroids) sometimes strike the sand with 

 their posterior arms, and may be said to row themselves along on 

 the solid; and in the mole's rapid turning in the ground, the fore- 

 limbs are used like oars, as if the animal were rowing in the ground. 

 Similarly the insect known as the Water Boatman (Notonecta) 

 swims back-downward in the pool, using its long third pair of legs 

 as oars. In many birds of the auk family the wings are used as well 

 as the feet in swimming under water. 



(V) The useful analogy of the man in the boat would become 

 forced if applied to out-of-the-way modes of animal locomotion; 

 so of these a few examples may be given. A jellyfish or a medusoid 

 swims beautifully by alternately expanding and contracting the 

 disc-like or bell-like translucent body. The rapid contraction drives 

 the water out from the mouth of the bell, and the medusa or medu- 

 soid is pro})elled in the opposite direction. Cuttlefishes expand their 

 mantle cavity or gill-chamber, and having filled it with water 

 proceed to close it, by a remarkable hook-and-eye arrangement, so 

 that the water cannot leave by the way in which it entered, but is 

 forced, as the cavity contracts, through a narrow funnel. As the jet 

 comes out with considerable strength, the body of the squid is 

 driven rapidly through the water, with the head and the tentacles 

 in the wash. The foremost part of the body is the top of the visceral 

 hump. The same method of propulsion, by a posterior outgush of 



