470 



CHAPTER XVIII. 



MOLLUSCA CEPHALOPODA. 



" Monstrum horrendum, informe, ingens." — ViRGlL. 



The highest class of the MoUusca is the Cephalopoda, which has 

 been divided by Professor Owen into two orders, Tdrahranchiata, or 

 animals having four branchiae, and the Dibraiichiata, having two 

 branchice. The first order, Tetrabranchiata^ contains the family 

 Ammonitidcv, with the fossil genera Goniatites and Ammo?iifcs ; the 

 family Orthoceritidce, the fossil genera Gomphoceras and Ortlwceras ; 

 and the family A^aiitilidce, with the genus Nautilus. 



The name Cephalopoda, as already stated, is taken from the 

 position of their feet, or more properly their arms, which are inserted 

 in the anterior part of the head : in Greek Kecpd\.r], head, iroi"s-7roS^j, 

 foot 



The Cephalopodous Molluscs are indeed highly organised for 

 Molluscs, for they possess in a high degree the sense of sight, 

 hearing, and touch. They appear with the earlier animals which 

 present themselves on the earth, and they are numerous even now, 

 although they are far from playing the important part that was 

 assigned to them in the early ages of organic life upon our planet. 

 The Ammonites and Belemnites existed by thousands among the 

 beings which peopled the seas during the Secondary epoch in the 

 history of the globe. 



Tetrabranchiata. 



In this order the animal is creeping, protected with an external 

 shell ; the head is retractile within the mantle ; the eyes are peduncu- 

 lated ; the mandibles are calcareous; there are very numerous arms. 

 The shell is external, camerated, and provided with a siphuncle, or 

 membranous tube [Nautilus), with a thin nacreous investment. In 

 many fossil forms it consists of a number of funnel-shaped or bead- 

 like tubes. This order differs from that of the Dibranchiata chiefly 

 in their more numerous arms, which are quite tentaculiferous, in 



