BREATHING OF SNAILS 115 



edge, the border of the contracted siphons forms a 

 bay, and by it the bivalves with long siphons may 

 be distinguished when we have only the shell to 

 guide us. 



The univalves, or snails, have more varied breath- 

 ing organs. The majority of sea-snails carry a tube, 

 trunk-like, in front of their head, as they creep over 

 the shallows at low tide, and by this trunk they inhale 

 water which, after travelling over the gill hid within 

 the shell, goes out by a slit near the base of the trunk. 

 To support the trunk the snail-shell has a groove or 

 spout, and by it the presence of the breathing-tube 

 may be known even from the shell alone. Other sea- 

 snails have no such canal, but are, as it were, snub- 

 nosed, the nostrils being separated into an in-letting 

 and an out-letting one. A few, such as the ear-shell 

 and the key-hole limpets, have a slit, a row of holes, or 

 a single hole, through which the water makes its escape 

 after bathing the gills. 



Cuttlefish, squid, and nautilus have a short funnel, 

 or escape pipe, for the same purpose, and in them the 

 effort of breathing is convulsive ; the whole body 

 shakes with the effort and twists its gaping orifices 

 as it unceasingly sucks in and transmits the gulps of 

 water that flood its gills. The size, activity, and 

 growth of these fierce creatures demand larger quan- 

 tities of oxygen than are needed by other snails, and 

 as the blood courses through their gills, it changes 

 from a colourless to a blue fluid, a sign of the presence 

 of that coppery blood-pigment which holds oxygen 



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