MECHANICS OF RESPIRATION 



are half raised up, bowed upon their long legs, the 

 abdomen bent below, the large antennae carried up- 

 wards, the little antennas stretched forward like two 

 parallel horns, the bulging eyes sticking out under the 

 big spike which acts as an eyebrow. In this state, 

 with the head almost upright, most of the parts which 

 frame the mouth, the mandibles, and jaws, move and 

 oscillate in a continual to-and-fro motion, like a sort 

 of mumbling. The purpose of these movements is 

 to send a continually fresh supply of water to the gills 

 in a more sheltered position further back. The 

 lobsters, with their huge pincers, the crabs with their 

 stubby bodies, act in the same way, and in the fresh- 

 water tanks, the cray-fish, which have a similar structure, 

 have an identical respiratory action. 



Another tank has cuttle-fish swimming about in it. 

 I have already mentioned these molluscs of complex 

 structure, their two methods of swimming, the inner 

 shell which supports their body like a bone, the casting 

 out of their ink. Their usual method of swimming 

 is with the body kept horizontal. Their heads are 

 pushed forward, with large, fine black eyes, partly 

 covered by eyelids bordered with a thread of gold. 

 Their flexible, supple tentacles — one might call their 

 touch caressing, so delicately does the animal use 

 them — hang like probosces, curiously resembling an 

 elephant's head. The oval-shaped body, circled by its 

 marginal fin, is alive with changing hues, iridescence 

 running over the skin like a cloud, sometimes light, 

 sometimes dark and dense. As it swims the creature 

 breathes, and it is possible to follow the movements of 

 its respiration. The water enters through the sides 

 of the opening of the sac which surrounds the body, 

 reaches the gills within the sac, gives them oxygen 

 and then leaves by the median siphon. During in- 

 spiration the sides remain open to let the water pass 

 in; the sac fills and enlarges. During expiration, 

 it closes up and empties, and the sides come together 

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