270 AN INTRODUCTION TO THE STUDY OF FOSSILS 



The skeleton is entirely internal ; it consists principally of the 

 narrow pen of conchiolin (Fig. 121 D,E) passing the entire length 

 of the trunk and protecting the visceral organs dorsally. There 

 are, in addition, several somewhat horny cartilages which are 

 supporting and protective in function. The cranial cartilage pro- 

 tects the principal nerve centers and supports the eyes ; other 

 cartilages support the bases of the fins and the arm bases. 



Another protective organ is the ink sac. This is located in the 

 posterior part of the trunk and secretes a dark brown substance. 

 When the squid is startled, it discharges this fluid through the 

 duct opening within the anus, where, mixing with the water in 

 the mantle cavity, it is discharged through the funnel as a black 

 cloud under cover of which the animal escapes. 



Locomotion. — The squid can dart with great rapidity, partly 

 by the arms, partly by the muscular, lateral fins, but principally 

 by the rhythmical contraction of the mantle. The expansion of 

 the mantle draws water through the slit between the anterior 

 edge of the mantle and the body into the mantle cavity. 

 Muscles then close this opening and the contraction of the 

 mantle forces the water out through the funnel, causing the ani- 

 mal to move in the opposite direction. It can move in any direc- 

 tion by simply bending the funnel in the direction opposite to 

 that in which it wishes to move. A valve prevents the water 

 from entering the mantle cavity through the funnel. 



The a,mma,Vs food consists of moUusks, crustaceans, and fishes 

 which it catches with its arms. The arms are furnished on the 

 inside with many suckers. 



Each sucker is a shallow cup supported on a short stalk ; 

 its membranous lip is lined internally by a narrow, horny rim 

 and in the middle of the bottom of the cup is a piston-like plug 

 which, when drawn down by the contraction of muscles, pro- 

 duces, when held against a soUd object, a partial vacuum and 

 resulting adhesion. The prey stands little chance of escape 

 when surrounded by arms Hned by hundreds of such suckers. 



The eyes are much more perfect than those of Nautilus, and 

 are next in order of development to those of the vertebrate. 



