312 MOLLUSCA 



From the foregoing it will be seen that the mantle must exert an in- 

 fluence on the shell and on the respiratory organs. Paired mantle folds 

 form two valves, right and left, to the shell; a right and left branchial 

 chamber, and right and left gills. With an unpaired mantle the shell 

 is always unpaired, while the gills may retain their primitive paired 

 condition. 



The gills in the mantle cavity are called ctenidia, from their resemblance to 

 combs with two rows of teeth. Each consists of an axis (back of the comb), con- 

 taining the chief blood-vessels, and two rows of branchial leaves. The whole 

 is united to the wall of the branchial cavity by the axis (fig. 351). Many aquatic 

 forms lack ctenidia, and then the respiration is either by the skin or by accessory 

 gills which differ from ctenidia in structure and position (usually outside the 

 mantle cavity). 



Those parts of the surface which are not covered by the shell have a 

 columnar epithelium which is frequently ciliated and which contains 

 large unicellular mucus glands (fig. 29), especially abundant on the edge 

 of the mantle. These give these animals the soft slippery skin which is 

 implied in the name Mollusca (moll is, soft). Although head, foot, and 

 mantle are very characteristic of the molluscs, they are not always present. 

 In the Acephala there is no distinct head region; many gasteropods lack the 

 mantle and hence the shell and mantle cavity; in the Cephalopoda the 

 foot is converted into the siphon and arms. 



r r. 



f\r ^=-^_ i""-/ " * r u>\ 



jm w 



B 



FIG. 312. Nervous systems of Molluscs. A, most gasteropods; B, acephals; C, 

 cephalopods and pulmonates. c, cerebral; pa, parietal, pe, pedal, pi, pleural, and 

 v, visceral ganglia. 



The nervous system has some highly characteristic features. As a 

 rule it consists of three pairs of ganglia associated with important sense 

 organs and connected by nerve cords. One pair lies dorsal to the oesoph- 

 agus and corresponds to the supracesophageal ganglion of the worms; it 

 is the brain (cerebrum] and supplies the tentacles and eyes. A second 

 pair lies ventral to the alimentary tract on the front part of the muscle 

 mass of the foot ; these are the pedal ganglia with which the statocysts are 

 connected. The third pair, the visceral ganglia, are also ventral, and near 

 them are the third sense organs, widely distributed through the Mollusca, 

 and from position and structure are regarded as organs of smell (osphra- 

 dia). They are thickened patches of ciliated epithelium in the mantle 



