MOLLUSCA. 353 



always unpaired, while the gills may retain their primitive paired 

 condition. 



The gills in the mantle cavity are called ctenidia, from their resem- 

 blance to combs with two rows of teeth. Each consists of an axial portion 

 (back of the comb), containing 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. 385). In many aquatic forms the ctenidia are lacking, 

 and then the respiration is either diffuse by the skin or by accessory gills 

 which by structure (usually outside the mantle cavity) are distinguished 

 from the ctenidia. 



Those parts of the surface of the mollusc which are not covered 

 by the shell have a columnar epithelium which is frequently ciliated 

 and which contains unicellular mucus glands, especially abundant 

 on the edge of the mantle. These give these animals the soft slip- 

 pery skin which is implied in the name Mollusca (mollis, soft). 

 Many-celled glands, like the byssus gland of the Acephala, the 

 pedal gland of many snails, occur. 



Although the existence of head, foot, and mantle is very char- 

 acteristic 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; in the Cephalopoda the foot is 

 converted into other appendages, the siphon and arms. These 

 modifications are to be explained by degeneration and evolution. 

 In the nervous system are also 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 oesophagus and corresponds to the supraoesophageal 

 ganglion of the worms; it is the brain (cerebrum) and supplies 



V^- 



Fia. 341. Nervous systems of Molluscs. A, most gasteropods; J9, acephals; C, cepha- 

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

 visceral ganglia. 



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 which are connected with the otocysts. The 

 third pair, the visceral ganglia, are also ventral, and near them 

 are the third sense organs, which are widely distributeed through 

 the Mollusca, and which from position and structure are regarded 



