10 



Guide to the. Molhiscii. 



must not be taken to imply that their secretions are of exactly 

 the same nature as the secret ions of these organs in the Vertebrates. 



Horny jaw-plates (mandibles), which may be paired or single, 

 are found in all molluscs except Lamellibranchs and most of the 

 Amphineura. 



Behind these jaw -plates is the radula, which is generally present 

 except in the Lamellibranehia. This is a tongue-like organ 

 beset with numerous transverse rows of minute teeth. The form 

 and arrangement of the teeth are highly characteristic, and in the 

 Gastropoda they have been made the basis of the classification 

 of part of the group. 



In some Gastropoda and most Lamellibranehia a second 

 characteristic structure, the crystalline style, is found in the 

 stomach or in a sac at the junction of the stomach and the 

 intestine. It is a more or less elongated rod of semi-gelatinous 

 substance. It is believed to provide a digestive ferment. 





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insra 



Fig. 4. 



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Three rows of Teeth of the Radula of Siphonaria. 

 c. central ; /. lateral teeth. 



The circulatory system consists of a heart composed of a ventricle 

 and one or more auricles, blood-vessels, and blood sinuses. The 

 last are large, irregular spaces in the connective tissue between 

 the various organs and may possibly represent in part the 

 capillary system of other animals. The blood is usually colour- 

 less, but in certain cases it is red or blue owing to the presence 

 of haemoglobin or of haemocyanin. 



The respiratory organs of the Mollusca are usually gills, but in 

 certain cases these are replaced by other organs. The typical 

 gills (ctenidia) consist of a number of filaments or flattened plates 

 arranged along a main axis and suspended from the roof of the 

 mantle-cavity. Important modifications of the gills occur in the 

 Lamellibranchs. In many molluscs the gill is lost and the mantle- 

 cavity is adapted as a lung for breathing air ; in others the typical 

 gills are suppressed and ' false gills ' are developed from the 

 mantle, whilst in a few there are no gills at all and respiration is 

 carried on by the general surface of the body. 



The nervous system consists of several pairs of ganglia, nerve- 

 cords and sense-organs. The most important ganglia are the 

 cerebral, the pedal, the pleural, and the visceral. These are 

 joined in various ways by connecting cords (* commissures '). 



