MOLLUSCA TUNICATA. 



125 



cage, and do not rise again for many hours." * By what 

 process the animal rises to the surface I have not seen 

 distinctly stated anywhere ; but on the surface it certainly 

 neither sails nor rows, "but its arms quit the shell still less 

 than in creeping, because, being turned upside down, it 

 would the more easily be separated from it ; and thus loco- 

 motion takes place, as in the other animals of this class, 

 by the alternate dilatation and contraction of the mantle or 

 covering, drawing in and throwing out the water in which 

 the animal is immersed : it then swims backwards, like the 

 cuttlefish and calamaries." -f- 



Among the Mollusca tunicata there are several which, 

 though they more properly float than swim, may be conve- 

 niently noticed here. The Salpae or Biphores, as the 

 French usually call them, abounding in the seas of warm 

 climates, translucid as their native waters, and often united 

 in chains, after a pattern pecviliar to each species, are driven 

 along the surface with considerable quickness by alternate 

 contractions and expansions, and by the propulsion they 

 receive from a current of water, which is made continually 

 to traverse the long diameter of the body, sucked in by 

 the posterior aperture and issuing in a stream through 

 that on the side of the mouth. Hence the body is always 

 pushed backwards — a circumstance that has misled some 



Fiff. 20. 



naturalists to describe the posterior aperture for the true 

 mouth. I The Pyrosomae (Fig. 20) are a still more singular 

 family of the same order. Each seeming individual of this 



* Charlesw. Mag. N. Hist. iii. 106. t Sander-Rang in ib. i. 401. 



X " These long chains swim through the tranquil water with regular 

 serpentine movements, for the creatures of which they are composed con- 

 tract and expand simultaneously, keeping time, as it were, like a regiment 

 of soldiers upon parade. Each chain seems consequently to be a single 

 being, acting through the influence of an unique will ; thence sailors often 

 look upon it as a reptile, and in many seas the salpa-chains are called sea- 

 serpents." — Forbes and Hanlf.y, B?it. Mollusca, i. 48. 



