38 



Returning now to the primitive Cephalophora, after the 

 ancestors of the Scaphopoda had diverged, we must trace the 

 line leading up to the Cephalopoda and the Pteropoda. In 

 this series the primitive bilateral symmetry has been retained, 

 and the visceral mass has been greatly enlarged, so as to 

 produce a great dorsal projection, but no twisting of this 

 region takes place, the anus remaining in its primitive 

 posterior position. The foot has been greatly modified, in 

 accordance with the free-swimming habits of the animals. 

 Its front part has grown upwards and forward, so as to 

 surround the head, the result being that the mouth is 

 placed in the anterior portion of the foot, and opens ven- 

 trally. In most cases this region of the foot is drawn out 

 radially into paired processes, which may be provided with 

 suckers. The median part behind the head is developed 

 into a pair of flaps (the epipodia), which may be used as 

 swimming organs, or modified to form a posteriorly placed 

 funnel. The remainder of the foot is undeveloped. A shell, 

 if present, is light and fragile. 



Such an ancestral form as this is most nearly represented 

 at the present day by the Pteropoda, a group which diverged 

 far back, and probably have undergone comparatively little 

 modification. Some of them are probably degenerate. The 

 epipodia have remained in their primitive condition as large 

 muscular flaps, and are used as swimming paddles. The 

 other parts of the foot may remain in a rudimentary con- 

 dition. The ctenidia have been lost, and in one section of 

 the group (the Thecosomata), where a mantle and shell are 

 present, the walls of the large subpallial cavity act as organs 

 of respiration. In the Gymnosomata the mantle skirt is 

 absent, and there is no shell. 



The ancestral Cephalopoda, after the Pteropoda had 

 diverged, must have had their epipodia modified, so as to 

 become more or less completely united in the middle line 



