738 ZOOLOGY SECT. 



larva sinking to the bottom ; and though still occasionally swimming with 

 the aid of the velum, coming to use the foot as a creeping organ. The shell 

 now increases in size step by step with the growth of the mantle, and bends 

 round the body of the larva until its edges meet and coalesce in the ventral 

 median line. Later it assumes the elongated conical form, curved towards 

 the dorsal side, characteristic of the adult. The foot at the same time 

 elongates and takes on the characteristic three-lobed shape. 



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. 662) 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, is 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 



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

 S. cultrata but the differences between the various species of the genus are 

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

 oth r species. 



