146 METAMOKPHOSES OF MAN 



ceded it. Chamisso detected the mode of reproduction 

 of the biphoras, and coined the expression " alternation 

 of generations." 



The Salpse are marine mollusks of a very pecu- 

 liar form, to give a general description of which 

 is exceedingly difficult. However, we may describe 

 them as being like an irregular cylinder of perfectly 

 transparent crystal, in the centre of which a pro- 

 portionally small mass of opaque, brilliantly coloured 

 matter, called the nucleus, is suspended. This nucleus 

 is composed of the principal viscera grouped together. 

 The cylinder represents the mantle and shell of other 

 mollusks, and is perforated at both extremities. The 

 water which is required for respiration is introduced 

 through one of these orifices, and by the contractions 

 of the mantle is as quickly expelled from the other 5 

 this action taking place with rapidity, the animal is 

 caused, as it were, to rebound; and thus by these 

 respiratory contractions the biphora is propelled 

 through the water. 



For a long period the attention of travelling 

 naturalists had been attracted by these animals, whose 

 phosphorescence was distinctly marked even in the 

 luminous waters of the intertropical ocean. They had 

 been seen sometimes in the isolated condition, and 

 at others in the form of long ribands composed of 

 exactly similar individuals. There were, moreover, 

 very considerable and well-marked differences between 

 the chained and isolated Salpge. The existence of 

 these two conditions seemed a sufficient reason for 

 the division of this singular group into two distinct 

 sections ; consequently, it was supposed by our cele- 

 brated traveller, Pcron, that the Salpce, which were 



