DIBRANCHIATA 



443 



like a chimney. For this purpose the mantle cavity must, of course, be 

 tightly closed, which is effected by the edge of the mantle being firmly 

 apposed to the body, whilst two stud-like prominences of the inner 

 mantle surface are pressed into corresponding depressions of the body. 



3. Locomotion. — When water is violently driven through the funnel, 

 the effect of the recoil thus produced drives the animal rapidly baclnvards. 

 (Why is such a connection between 



respiration and locomotion only 

 possible in the case of aquatic 

 animals ? Compare larva of dragon 

 fly.) This backward movement is 

 further assisted by the animal 

 alternately expanding and rapidly 

 striking together its arms, whereby 

 a second forwardly-directed current 

 of water is produced. By this action 

 of the arms alone, too, the animal 

 is able rapidly to swim backwards 

 (see Section 5). Its forivard move- 

 ments, on the other hand, are slow, 

 and are effected by undulatory 

 (screw-like) motions of the fins (see 

 Section A), or by rowing with the 

 help of the four lower arms. 



4. Food.— The cuttle-fish is of 

 predaceous habits, its food consist- 

 ing chiefly of fishes and crustaceans. 

 It is interesting to inquire how it 

 can capture these, for the most part, 

 rapidly-moving animals, seeing that 

 it can only slowly move in a forward 

 direction. (Why is its rapid back- 

 ward motion of no importance in 

 this respect ? See crayfish.) 



(a) The cuttle-fish when in 

 search of prey lurks motionless at 

 the bottom of the sea, to which it 



can adapt the colour of its body (see p. 253) to such a degree that fre- 

 quently it is impossible to discover the animal even in an aquarium. (In 

 connection with this capacity is also the peculiar play of colours noticed 

 in this and other cephalopods. When the animals are irritated or 

 excited in any other way, yellow, red, blue, and other colour tints pass 



Stktjotube of a Cuttle-Fish (Diagram- 

 matic). 



Ko., Head ; Ar., arms ; M., mouth ; R., body ; 

 Fl., a portion of the fin ; Mt., mantle ; 

 Seh., shell (cuttle-bone) ; F., perforated foot, 

 or funnel ; Mh., mantle cavity ; K., branchia ; 

 the course of the inflowing and outflowing 

 water is indicated by arrows ; D., intestine ; 

 Ma., stomach ; A., anus ; T., ink-bag. 



