41 8 MANUAL OF ZOOLOGY. 



upon small Crustaceans and other diminutive animals. Though 

 all the living forms are small, geology leads us to believe that 

 there formerly existed comparatively gigantic representatives of 

 this class of the Mollusca. 



DISTRIBUTION OF PTEROPODA IN TIME. The Pteropods 

 are not largely represented in fossiliferous deposits, but they 

 have a wide range in time, extending from the Upper Cambrian 

 rocks up to the present day. The Theca and Conularia of the 

 Palaeozoic period, if truly Pteropods, are of comparatively 

 gigantic size. Both commence their existence in the Silurian 

 or Upper Cambrian, and the former is entirely Palaeozoic. 

 The genus Conularia, however, extends into the Mesozoic 

 period, and is found in the Liassic rocks. The Silurian fossils 

 which form the genus Tentaculites, though often referred to the 

 Tubicolar Annelides, appear to belong without doubt to the 

 Pteropoda. The recent genera Hyalea (fig. 223), Cleodora, and 

 Cuvieria are represented in the Tertiary period. 



CHAPTER XLVIII. 



CEPHALOPODA. 



CLASS IV. CEPHALOPODA. The members of this class are 

 defined by the possession of eight or more " arms " placed in a 

 a circle round the mouth ; the body is enclosed in a muscular 

 mantle-sac, and there are two or four plume-like gills within the 

 mantle. There is an anterior tubular orifice (the " infundi- 

 bulum" or "funnel") through which the effete water of respi- 

 ration is expelled. The flexure of the intestine is neural. 



The Cephalopoda, comprising the Cuttle-fishes, Squids, 

 Pearly Nautilus, &c., constitute the most highly organised of 

 the classes of the Mollusca. They are all marine and carnivo- 

 rous, and are possessed of considerable locomotive powers. 

 At the bottom of the sea they can walk about, head down- 

 wards, by means of the arms which surround the mouth, and 

 which are usually provided with numerous suckers or " aceta- 

 bula." They are also enabled to swim, partly by means of 

 lateral expansions of the integument or fins (not always pres- 

 ent), and partly by means of the forcible expulsion of water 

 through the tubular " funnel," the reaction of which causes 

 the animal to move in the opposite direction. 



The majority of the living Cephalopods are naked, possess- 



