134 ^ Guide to the Zoological Collections in the 



certainly swim forwards by the undulation of the side-flaps 

 of the mantle ; but the typical and characteristic motion 

 is by a series of jerks backwards, this being effected by 

 the forcible expulsion through the siphon— the mantle 

 being kept closely applied to the body so as to close the 

 mantle-chamber in front — of the water contained in the 

 mantle-chamber. 



Cuttle-fishes are highly rapacious, and some species 

 reach such an enormous size as to be dangerous even to 

 man. 



The curious bunches of eggs of Sepia are shown above 

 Case 170. The remarkable " hectocotylus arm", or arm 

 charged with packets of sperm-cells, which the male 

 sheds into the mantle-chamber of the female, is shown 

 ready for detachment in the model of Argonauta, in 

 Case 171. 



The siphonopodous Cephalopoda are divided into two 

 Orders, namely (i) the Tetrabranchiata in which there are 

 two pairs of gill-plumes, and (2) the Dibranchiata, in 

 which there is a single pair. The Tetrabranchiata are 

 represented by the single existing genus Nautilus 

 drawings and shells of which are shown in Case 170. 

 The shell of the Pearly Nautilus, as may be seen in the 

 vertical sections in the Case, consists of a plane spiral of 

 many chambers, of which only the last one is inhabited by 

 the animal, the others containing only air. The series of 

 chambers represent stages in the growth of the animal, 

 each chamber having been at one time inhabited, and 

 having been deserted and shut off as the animal outgrew it : 

 the whole series is connected, as can be seen in the sec- 

 tion, by a calcareous tube, known as the " siphuncle ", 

 which traverses the middle of each chamber and keeps the 

 smallest and earliest formed chamber in communication 

 with the largest and latest formed. The animal of Nautilus 

 differs from all other existing Siphonopoda in the following 

 among other particulars : — there are two pairs of gill- 

 plumes ; the processes of the fore-part of the foot are not 

 sucker-bearing " arms ", but are bunches of sheathed ten- 



