MOLLUSC A. 271 



the gills of a fish. In the common terrestrial molluscs gills 

 are absent, but the inside of the mantle-chamber is lined 

 with a fine network of blood-vessels, so that the whole 

 organ resembles somewhat a lung, and has received that 

 name. 



In the flow of the blood there is a great difference be- 

 tween the mollusc and the fish. In the mollusc the blood 

 returns at once from the gill to the heart, and is then 

 forced by this organ to all parts of the body. The heart is 

 situated in a chamber or pericardium, and consists of one or 

 two (right and left) auricles which receive the blood, and 

 a ventricle which pumps it to the body. In the squid ac- 

 cessory or branchial hearts are added. These are placed at 

 the bases of the gills and force the blood through these 

 organs, from which they return to the other or systemic 

 heart, to go to all parts of the body. 



In all molluscs except the Acephala the region of the 

 mouth is provided with a lingual ribbon. This is a band 

 of horny material, bearing on its free surface rows of hard 

 and sharp teeth, so that the whole resembles a flexible file. 

 It is supported in such a way that it may be moved back 

 and forth, thus rasping the food. In some Gasteropods it 

 can even be used in boring holes in the shells of other 

 molluscs. This lingual ribbon is constantly growing at its 

 deeper end, so that the loss by wear in front is continually 

 made good. The teeth on the ribbon vary in number and 

 shape in different species. In some there are but three in 

 a transverse row, while in others there may be over one 

 hundred. 



In the ideal mollusc the alimentary canal goes straight 

 through the body from mouth to vent. In nature it 

 usually has some convolutions, increasing* the amount of 

 digestive surface. In the Cephalopods and in most Gas- 



