THE COASTS OF SICILY. 181 



whose extraordinary metamorphoses have already 

 modified in so many respects the views which were 

 formerly entertained in reference to the propagation 

 of animal species, the Firolae*, and the Diphyes f, 



confirmed the general conclusions to be deduced from his work, viz. 

 that these pretended masses of living jelly are by no means homo- 

 geneous, as some naturalists still suppose, and that the transparency 

 of their tissues was the only reason which could have led observers 

 into this error. The reproduction of the Medusae which has been 

 studied in more recent times by MM. Sars, Siebold, Van Beneden, 

 Dujardin, &c., exhibits the most marvellous phenomena, to which 

 we shall have occasion to refer in our next chapter on the Gulf of 

 Castellamare. [The Medusae had been long previously injected by 

 John Hunter.] 



* The Firolae ( Pterotrach&a) are Gasteropodous Molluscs. This 

 genus consists exclusively of marine species, all of which swim in 

 the open sea instead of creeping over a solid surface. It will readily 

 be understood that the mode of life which differs so essentially from 

 that of most other gasteropods must have induced great modifica- 

 tions in the organisation of the Firolae. Their external forms are, 

 indeed, very singular, for in these animals we find that the fleshy 

 disc, which serves as a foot in Snails and Slugs has been changed 

 into a broad and vertical swimming organ, the extremity of the body 

 being, moreover, lengthened and flattened to fulfil analogous func- 

 tions. The whole body is perfectly transparent, excepting only the 

 mass of the liver, which possesses a certain degree of opacity, and 

 thus constitutes a sort of coloured nucleus. The Firolse have no 

 shells, in which respect they differ from the Carinariae, which, 

 although they resemble them in most particulars, possess a shell 

 which, however, only covers the mass of the liver, and which in the 

 Mediterranean species ( C. Cymbivm) constitutes a beautiful object 

 much prized in our Conchological collections on account of its trans- 

 parency and extreme delicacy. 



f The Diphyes, which are placed by Cuvier in the class of 

 Acalephse, approach very nearly to the Stephanomias, to which we 

 shall presently refer. The organisation of these animals, which 

 for a long time continued to be very imperfectly understood, has 

 hitely been made the object of two very important memoirs by 

 Kolliker and Vogt. 



N 3 



