418 



MOLLUSCA. 



The normal swimming position for a Cephalopod (Fig. 333) is 

 horizontal, with the anterior surface of the visceral sac upward, 

 and the posterior, i.e., the surface on which lies the mantle-cavity 

 and the mantle -opening, downward. When creeping the animals 

 apply their arms to the substratum and the hump projects backward. 



FIG. 333. Octopus vulgaris (after Merculiano, from Lang), in swimming and sitting positions. 



Bearing in mind these facts and the fact already mentioned that 

 the antero-posterior axis is much shortened, the ordinary terminology, 

 which is often confusing, used in describing Cephalopoda will be 

 readily understood. The ventral surface is the region including the 

 mouth and the anus. The funnel is therefore a purely ventral 



