THE OEPHALOBODA. 41 



developed optic und olfactory organs. That which essentially 

 characterises the Cephalopoda, in fact, is simply the manner in 

 which, in the course of development (as Kolliker long since 

 proved), the margins of the foot proper and the epipodia become 

 modified and change their relations. The margins of the foot 

 are produced into more or less numerous tentacular appendages, 

 often provided with singularly constructed suckers or acetabula ; 

 and the anterolateral parts of each side of the foot extend 

 forwards beyond the head, uniting with it and with one another ; 

 so that, at length, the mouth, from having been situated, as 

 usual, above the anterior margin of the foot, comes to be placed 

 in the midst of it. The two epipodia, on the other hand, unite 

 posteriorly above the foot, and where they coalesce, give rise 

 either to a folded muscular expansion, the edges of which are 

 simply in apposition, as in Nautilus ; or to an elongated flexible 

 tube, the apex of which projects beyond the margin of the 

 mantle (Fig. 16, /), called the "funnel' or " infundibulum," 

 as in the dibranchiate Cephalopoda. 



The Cephalopoda present a vast number of the most inte- 

 resting features, to which it would be necessary to devote much 

 attention if we were studying all the organic peculiarities mani- 

 fested by the class ; but it is in the characters of foot and of the 

 epipoclium that the definition of the class must be chiefly sought. 

 In addition, the flexure of the intestine is, in all Cephalopods, 

 neural ; and the mouth is always provided with a horny or more 

 or less calcified beak, like that of a parrot, composed of two 

 curved pieces, which move in the median antero-posterior plane 

 of the body ; and one of which, that on the neural side, is always 

 longer than the other. 



