ORGANIZATION AND HABITS OF SALPvE. 463 



whilst swimming in their native element, indeed, 

 such is their transparency, that, without profiting by 

 every change of light, their very existence might 

 easily escape the notice of an incautious observer, 

 and it is only after long and most careful study, that 

 their beautiful structure reveals itself to the asto- 

 nished naturalist : attentive examination, however, 

 shows that their delicate bodies are translucent 

 cylinders, open at both ends, each cylinder being 

 made up of two membranes, whereof the outer one is 

 tough, and possessed of little vitality, while the in- 

 ternal, of softer consistence, is eminently contractile. 

 The apertures, situated at each extremity of the body, 

 allow free passage to the surrounding element, which 

 permeates the animal from end to end ; and as one 

 of the terminal orifices is provided with a valve, that, 

 while it allows free admission to the entering water, 

 prevents its return through the same channel, it is, 

 by the contractions of the muscular walls of the 

 body, ejected in forcible streams out of the hinder 

 orifice, thus giving an impulse whereby the creature 

 is enabled to propel itself through the water. In the 

 long, chained forms that swim through the calm 

 water with regular undulatory movements, the in- 

 dividuals of which they are composed appear to 

 contract and expand simultaneously, keeping time 

 like a regiment of soldiers marching; each chain 

 seems consequently to be a single animal, and hence 

 the sailors often speak of them familiarly as sea- 

 serpents. 



We have already alluded to the crystalline trans- 

 parency which renders the Salpse almost undistin- 



