AND ITS INHABITANTS 115 



stabilizing fins that may be used for slow forward swimming 

 by means of wave-like undulations. The body is enclosed in 

 a muscular fold of the body-wall known as the mantle, but 

 does not fill it, leaving a space beneath known as the mantle 

 cavity. There is on the under side of the well-developed 

 head, with its huge eyes and long tentacles, a tubular funnel 

 (Fig. 15, /) whose larger end connects with the mantle 



FIG. 15. Squid, Loligo sp., capturing a fish: f, funnel. After Doflein, from 

 Lull's "Organic Evolution," published by the Macmillan Company. 



cavity, the smaller one pointing forward beneath the head. 

 Relaxation of the mantle permits the ingress of water into the 

 cavity; closure of the muscular margin around the animal's 

 neck, on the other hand, prohibits its egress except through 

 the funnel. A forcible contraction of the entire mantle forces 

 the water out in a sharp jet which impinges against the sur- 

 rounding water and thus drives the creature backward at a 

 very rapid rate. As compared with the easy undulatory move- 

 ments of an aquatic vertebrate, however, this jet propulsion 

 mode is highly costly of effort, and its adoption by mankind 

 for steam-propelled craft was to meet peculiar conditions of 

 short choppy seas and a disastrously "racing" screw propeller 

 rather than because of any inherent virtue in the basic idea. 

 Its rejection from ordinary usage is due largely to its extreme 

 lack of economy. 



The cephalopods, being predaceous forms, must overtake 

 their prey, and in turn must flee from devouring Cetacea for 

 which they form a principal dietary staple, hence their urgent 

 need of locomotion; but for the great host of invertebrate 

 forms their static habitat places no premium upon locomotive 



