MOLLUSCA. 



185 



living animal is of rare occurrence. One was taken, when 

 floating in the South Seas, and heing presented to the College 

 of Surgeons, London, was there dissected by Professor Owen, 

 who published an elaborate memoir on its structure, and its 

 relations to other families, both recent and extinct. We learn 

 from this source that it has four gills (Tetrdbranchiatd), in 

 which respect it differs from all other existing species of 

 Cuttle-fish, that it occupies the outer chamber of its shell, 

 and that it can rise to the surface or descend at pleasure. 

 Similar in structure and in powers were the Ammonites (Figs. 

 173, 174), which at former periods of the earth's history, 



AMMONITES. 



must have been living in its seas, though now known only as 

 fossil; and alike in general organization, though different in 

 form, are those large tapering chambered fossils 

 (Orthoceratites) which, in some parts of Ireland, 

 are so abundant in the limestone quarries. 



The other Cuttle-fishes (Dibranchiata) abound 

 in all seas, and are arranged in two divisions, 

 according as they have eight or ten arms. To 

 the latter group belong the Loligo or Calamary 

 (Fiy. 171) the common Sepia or Cuttle-fish 

 and the Loligopsis (Fig. 175), so remarkable for 

 the great length of one pair of its arms. All 

 possess a shell or internal skeleton differing in 

 form and structure in different species; all arc 

 furnished with a powerful horny beak for tearing 

 up their prey, and with an ink-bag, from which, 

 at pleasure, they can emit a fluid which darkens 

 the water and favours their escape from their ]7 ,, 



enemies. 



BKLLMMTE. 



