434 



MOTION. 



organs with sufficient force to propel itself 

 through the surrounding medium. The pro- 

 gression of the Sal pa cristata is effected by 

 drawing water into its body, at an opening si- 

 tuated in the posterior segment of the mantle, 

 where a valve (Jig. 228 r) is placed to pre- 



Fig. 228. 



Salpa cristata. 



vent its returning by the same aperture ; the 

 mantle having been distended by the water, 

 contracts upon it, by which action it is ex- 

 pelled at an opening situated at the side of the 

 mouth (); its progression is retrograde, or in 

 the direction of a b, opposite to that of the 

 fluid in b a. 



The larvae of some Dragon Flies, such as the 

 Ashna and Libellula, draw in and expel the 

 water alternately at the anus, which they occa- 

 sionally lift out of the water, and project a 

 small stream at a distance above the level of 

 its surface : the hydrodynamic effect of these 

 actions is to give a locomotive impulse to the 

 centre of gravity in a direction opposite to that 

 of the ejected fluid. The reaction of the water, 

 which is equal to the action of the ejected 

 stream, is not only sufficient to overcome the 

 resistance of the surrounding medium and the 

 inertia of the body of the animal, but also to 

 drive it along. The velocity of the Syringo- 

 grades is accelerated during the expulsion of 

 the water, and retarded during its reception; 

 consequently the motion is never uniform. 



The Vermiform animals are for the most part 

 destitute of distinct locomotive organs, yet, owing 

 to the flexibility of their lengthened and usually 

 cylindrical bodies, they swim with great faci- 

 lity. They glide by a series of lateral undu- 

 latory movements of the body, with which they 

 strike the water obliquely backwards, and with 

 equal force on each side of the axis of motion, so 

 that the force impressed on the water is trans- 

 lated to the centre of gravity of the animal in 

 an opposite direction forwards ; the compo- 

 sition of all the forces giving a resultant the di- 



rection of which coincides with the axis of motion . 

 Many serpents whose habits are chiefly terres- 

 trial, swim with the head elevated above the 

 surface of the water ; others glide entirely be- 

 neath it. Some of the Entozoa, as the Tani<e, 

 and, among the Annelides, the Planarise, when 

 immersed in warm water, swim by similar un- 

 dulations of the body; the latter, however, with 

 the ventral aspect upwards. In the Ophidian 

 Hydrophyli, or water-snakes, the tail is flat- 

 tened ; and its planes being directed vertically, 

 give it the properties of a powerful oar, in 

 striking the water by lateral oscillations. In 

 many chetopod Annelides, the setae and cirrhi 

 form numerous and complex external organs of 

 progression. The Terebetla has four rows of 

 setee in tufts; the dorsal row projecting in the 

 horizontal, the ventral in the vertical plane. 

 They extend along the whole of the elongated 

 subquadrangular-shaped body. In the Eunice 

 Gigantea, which grows to the length of more 

 than ten feet, each ring is furnished with two 

 lateral packets of bristles, and two cirrhi. In 

 the Nereis nuntia the locomotive organs are 

 complex and greatly multiplied. The cirrhi 

 and setse may propel the body forwards, in- 

 dependently of those undulatory movements 

 which are indispensable for the progression 

 of the apodous Annelides. 



Aquatic insects. The perfect insects swim 

 like quadrupeds and birds, by the alternate 

 flexion and extension of their legs. Amongst 

 the aquatic insects the Dytiscus is one of the 

 best organized for swimming ; its figure resem- 

 bling that of a boat, being calculated to glide 

 through the water with little resistance. The 

 posterior legs are greatly developed, and they are 

 moved by powerful muscles performing the 

 office of oars, and are the principal instruments 

 used in swimming. Their movements forwards 

 are made in a plane nearly horizontal. The 

 haunches being fixed to the thorax, give firm- 

 ness and precision to these legs, which do not 

 differ materially from those of other Coleoptera, 

 with the exception of the tarsi, which are much 

 flattened, and present their broad surfaces to 

 the water. They are furnished with rows of 

 stiff' hairs, which bend when the leg is carried 

 forwards, and become straight when its move- 

 ment is vigorously reversed, thus increasing the 



Fig. 229. 



The Dytiscus, from Straus-Durckkeim, showing the 

 various positions which the posterior leys take in 

 swimming. 



