238 ON THE CLASSES OF THE 



the little known at present of the Cy dob ranches and Sen- 

 tibranches. Less however remains to be done in Cuvier's 

 family of Nudibranches, consisting of those beautiful ani- 

 mals, the simplicity of whose organs of digestion and re- 

 production, with the peculiar nature of the nervous system, 

 prepares us for the singular family of Pteropoda. Until 

 however the internal construction of such genera as Glaucus 

 and Eolida be better known, it would perhaps be as well to 

 follow the example of Peron, and to place these animals 

 with the Pteropoda ; at all events the principles on which 

 these Mollusca might be thus united into one group would 

 be infinitely more natural than those on which the family 

 of Gasteropoda was originally formed. The genus Clio, 

 which may be taken as the type of the Pteropoda, presents 

 several points of construction to the observer which are 

 well worthy of his most attentive consideration. 



In calm weather, says Cuvier, those northern seas which 

 like the land between the tropics astonish us so much by 

 their fecundity of life, are seen to swarm with minute gelati- 

 nous Mollusques of the species Clio Borealis. They come 

 by myriads to the surface of the water as if to respire, but 

 the least touch is sufficient to make them sink towards the 

 bottom and disappear. The nervous system of these little 

 animals, which are supposed to form the principal food of 

 the whale, is remarkably similar to that of the Aplysia, 

 the brain being composed of two distinct lobes, from each 

 of which runs a nervous thread to a ganglion which joins 

 its fellow under the oesophagus by another nervous chord. 

 In Clio as in dplysia the salivary glands are long and nar- 

 row, floating at the sides of the oesophagus. But the genus 

 Tethys perhaps will best show the affinity that exists be- 

 tween the Gasteropoda and Clio. Both these animals 



