MOLLUSCA. 



185 



livin!^ animal is of rare occurrence. One was talcen, when 

 floating in the South Seas, and being 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 {Tetrahranchiata), in 

 which respect it differs from all other existing species of 

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

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

 Similar in structure and in powerswere the Ammonites {Figs. 

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



m&l- I 



rig. 173. 



Fig. 174. 



Ammo.'sites. 



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 

 [Orthoccratites) 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 Calaraary 

 [Fig. 171) — the common Sepia or Cuttle-fish — 

 and the Luligopsis [Fig. 175), so remarkable for 

 the great length of one pair of its arms. All 

 possess a shell or internal skeleton difi'ering in 

 form and structure in different species; all are 

 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 Avater and favours their escape from their ^. 

 enemies. Belem.ih*. 



