HOW ANIMALS BREATHE. 



457 



a given direction as long as the cilia upon its surface continue their 

 mysterious activity." * 



In the oysters and scallops, the mantle is open, exposing the gills ; 

 while the mussels and giant Tridacna have the mantle closed but 

 pierced with two apertures for the reception and expulsion of water. 

 The remaining lamellibranchs, which include the common hard clam, 

 razor-shell, pholades, and teredo, have long inhalent and exhalent 

 tubes, called siphons, which enable the creature to burrow in the sand 

 and still obtain water. The two siphons may be separate or united, 

 and are retractile. The presence of such tubes can be determined by 

 examination of a dead shell ; as the muscles which retract the siphons 

 produce an indentation or " sinus " in the " pallial line." 



Perhaps the most conspicuous and beautiful gills are those pos- 

 sessed by the naked sea-snails or " sea-slugs." They are situated on 

 the back and sides of the body, and are frequently retractile. In the 

 Doris they have a flower-like or star-shaped arrangement. In the Eolis 

 they are papilliform and tufted, along the sides of the animal, or they 

 may be tree-like as in the Triton, or feathery in other forms. The 

 Aphysia, or sea-hare, has the gills placed on the back and protected by 

 a fold of the mantle. The Phyllidia have them as a fringe along each 

 side of the body, and covered by a projecting fold of the skin ; while 

 the Limpet and Chiton have leafy gills forming quite an entire circle 

 about the body, and also covered by the mantle. 



Fig. 13. Aik-Pipb of Fly. 



The sea-snails having shells carry their gills in cavities in the side 

 of the neck, and may or may not have siphons. The possession of 

 the latter is shown by a notch in the aperture of the shell. 



All the mollusks mentioned thus far are sluggish animals, with 

 little need of rapid respiratory changes of the blood. But the highest 

 of mollusks, the cephalopods, or devil-fishes, are active and muscular 

 creatures the most powerful of invertebrates. They have accessory 

 hearts to accelerate the flow of blood through the gills, which are 

 large and plumose, and contained in a cavity. Water is freely sup- 

 * T. Rymer Jones, "The Animal Creation," page 194. 



