swimming- web, is at the margin of all the arms 

 except the two lowermost pair. 



I have just referred to the siphon. The cephalo- 

 pods are equipped with perhaps the most unique 

 mechanism in Nature for achieving locomotion. 

 The vital parts of the squid are enclosed within the 

 mantle which is attached along the back and which 

 except at the line of attachment forms a freely 

 hanging bag. From the mouth of this bag pro- 

 jects the head and tentacles, and underneath 

 these the short tubular organ known as the 

 siphon. With the expansion of the mantle-bag, 

 water is drawn to the interior through the slit- 

 opening around the sides of the neck, and passes 

 over the paired gill structures on the body within 

 the cavity. If the creature be quietly resting, the 

 water is expelled through the same orifice, and 

 this constitutes its normal method of breathing. 

 But if it essays to swim, the free edge of the mantle 

 engages with the outer wall of the siphon, in this 

 way closing the slit; thus the water is forced 

 through the siphon, propelling the squid in the 

 direction opposite to that in which the siphon is 

 pointed. This direction is usually backward so as 

 to avail itself of the advantage offered by the 



[336] 



