xii PHYLUM MOLLUSCA 759 



Class V.-CEPHALOPODA. 



The Cephalopoda, including the Cuttle-fishes, Squids, Octopods, 

 and Nautili, are marine Mollusca of a high grade of organisation. 

 There is a very definitely-formed head, bearing a pair of highly- 

 developed eyes, and surrounded by the anterior portion of the foot, 

 modified into arms or tentacles. The body is bilaterally symmetrical. 

 The posterior part of the foot is modified to form a funnel leading- 

 out from the large mantle-cavity. A shell is sometimes present, 

 sometimes absent. When present it is usually internal, but 

 sometimes external, and in the Nautili is capable of containing 

 the body of the animal. 



1. EXAMPLES OF THE CLASS. 



i. THE CUTTLE-FISH (Sepia 1 ). 



Cuttle-fishes are marine Molluscs, which live usually at a depth 

 of a few fathoms, but often come into shallower water, and are 

 frequently caught in the trawl or the seine. The animal arrests 

 attention when compared with Unio or Triton by the strength, 

 and more particularly the rapidity, of its movements; by the 

 possession of a pair of eyes resembling in size and complexity 

 those of a Fish ; and by various other features, all pointing to 

 a higher grade of organisation than is attained by the members 

 of the classes of Mollusca dealt with in the preceding pages. 



External Features.- -The Cuttle-fish (Fig. 653) has a distinct 

 head, bearing ten long arms, and a pair of large, highly-developed 

 eyes. The head is connected with the body by a constricted 

 region or neck. The trunk is elongated and shield-shaped, the 

 base of the shield being towards the head. The long axes of 

 head and trunk are in line with one another. Not only the head, 

 but also the trunk, are completely equilateral, in which respect 

 there is a marked contrast to Triton ; and this symmetry extends 

 to most of the systems of internal organs. The free extremity of 

 the head bears the mouth, and is accordingly termed the oral 

 extremity, the opposite extremity, the apex of the shield-shaped 

 body, is the aboral end. The surfaces of the shield are anterior 

 or antero-dorsal and posterior or poster o-rentral, its borders right 

 and left. The anterior surface is to be distinguished by its darker 

 colour, and by the firmness of the body- wall, due to the presence 

 in this position of a hard internal shell. 



1 Most of the figures have reference to a common Australian species S. cnltrata 

 but the differences between the various species of the genus are slight and 

 unimportant, and the description given will apply fairly well to any other 

 species. 



