434 KEPRODUCTION OF THE SALPJE. 



bright sun-light ; and then they present splendid iridescent hues. 

 They have all one dark spot, however ; generally of a brownish- 

 yellow colour. This, which has been termed the nucleus, is 

 occasioned by the opacity of the liver, and its coloured secretion. 

 The branchial orifice possesses no tentacula ; but these are 

 replaced by a kind of valve, which permits the entrance of water 

 and prevents its return. By the constant action of the respi- 

 ratory apparatus, a considerable degree of locomotive power is 

 obtained for these animals, without any special apparatus for the 

 purpose. There do not appear, however, to be any sensory or- 

 gans by which the animals can be directed towards their food ; 

 and we can scarcely suppose, therefore, that their movements are 

 governed by an intelligent will. 



1067. One of the most curious circumstances in the economy 

 of the Salpte, is the singular alternation of generations which 

 was first observed in these animals by Chamisso, and since con- 

 firmed by other observers. All the species present themselves in 

 two different forms ; a race of isolated animals, and another of 

 aggregated masses composed of numerous individuals. These 

 last are so unlike the individuals of the separate race, that they 

 might be regarded as distinct species. Nevertheless it is found 

 that each of the associated individuals produces a single egg, 

 which becomes developed whilst still attached to its parent ; 

 and it is remarkable that in the connection between the latter and 

 its offspring, we find perhaps the nearest approach amongst the 

 lower animals to the peculiar mode of reproduction in the Mam- 

 malia, the highest of the Vertebrata. The young animals thus 

 produced are solitary Salpce ; and the long chains of aggregated 

 individuals are formed in the interior of these by a peculiar pro- 

 cess of gemmation, upon a double blood vessel, which is found 

 in the immediate vicinity of the heart. Thus, a Salpa which 

 differs equally from its parent and its own progeny, resembles 

 its grand-parent, its grand-children, and its own brethren. 



1068. The Sal pee are most abundant in tropical climates. 

 Messrs. Quoy and Gaimard relate having seen on one occasion, not 

 far from the Cape of Good Hope, long zones of a reddish-brown 

 colour traversing the surface of the ocean, as far as the eye could 



