228 AN INTRODUCTION TO ZOOLOGY. 



the shell is external. The Pearly Nautilus has a spiral 

 shell which is alike on both sides, the outside edge of 

 the coil being occupied by the posterior surface of the 

 animal, with the funnel, while the head lies next the 

 coiled whorl : the animal in the live state therefore 

 carries this shell standing on its edge, and the shell is 

 straight, i.e. bilaterally symmetrical on each side of the 

 plane in which it is coiled, instead of having a twist, 

 as the shell of the Gasteropod mollusc has. Some 

 of the extinct genera, however, have a shell of the 

 ordinary spiral type, but sinistral (6g. 78a) . The shell 

 is divided into chambers, successively partitioned off by 

 walls of shell, called septa. The animal only lives in 

 the outer chamber, outside of the last-formed septum. 

 The use of these empty chambers is to enable the 

 animal to float on the surface of the sea. There is a 

 small hole in the middle of each septum, called the 

 sipliuncle ; through this hole passes a thin tube of 

 flesh which connects all the successive chambers. By 

 means of this the chambers can be filled with air. 



The DibrancMata have two gills and eight or ten 

 arms, which are arranged round the mouth, and are 

 provided with suckers. There are two sub-orders; 

 the Deeapoda include the common Cuttle-fish (Sepia) 

 and also the fossil form called Belemnites : the 

 Ootopoda include the Octopus, or Devil-fish, and the 

 Paper Nautilus, Argonauta Argo, so called from its 

 appearance when floating on the sea. The " bone " 

 of the Cuttle-fish is of cartilage, strengthened by cal- 

 careous additions : it is not present in the Ootopoda. 



