MOLLUSCS. 



195 



restrlal 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 course of the blood there is a great difference 

 between a mollusc and a fish. In the mollusc the blood 

 returns at once from the gills 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 and cuttlefish accessory or branchial hearts are 

 added. These are placed at the bases of the gills and 

 force the blood through these organs, from which it re- 

 turns 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 (fig. 35, called 

 also radula and odontophore) . 

 This is a band of horny ma- 

 terial, bearing on its free sur- 

 face rows of hard and sharp 

 teeth, so that the whole resem- 

 bles a flexible file. It is sup- 

 ported 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. 



* This is the ccelom, greatly reduced in size. 



FIG. 35. A bit of the lingual 

 ribbon of a gasteropod 

 (Eburna). After Cooke. 

 Enlarged. 



