312 



^lOLLUSCA 



From tlic forcgoin;:; it will be seen that the mantle must exert an in- 

 lluenee or the sltell and on the respiratory organs. Paired mantle lolds 

 form two I'dli'fs. right and left, to the shell; a right and left branclual 

 chamber, and right and left gills, ^^'ith an unpaired mantle the shell 

 is alwavs unpaired, wlule the gills may retain tlteir printiti\-e jiaired 

 condition. 



The gills in the mantle cavitv are ealled itriiiJij. from their reseniblanee lo 

 combs widi two rows of teeUi. Each consists of an axis (baek ol the com I A. eoii- 

 taining the chief bUnHl-vessels, and two rows of hraneliial lea\es. The whole 

 is united to die wall of the branchial ca\ilv by the axis (fig. 351b Many ai|ualie 

 forms lack clenidia, and then the respiration is cither b\' the skin or by accessory 

 gills which differ from ctenidia in structure and po*;ilion ^usually outside the 

 mantle caviiyj. 



Those parts of the surface wliich are iiot covered by the sltell have a 

 columnar epithelium wliich is frequeiitlv ciliated and wdiich contains 

 large unicellular mucus glands (fig. 20), especially abundant on the edge 

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

 implied in the name !Mollusca [mollis, soft). Although head, foot, and 

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

 In the .\cephala 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 iitto the siphon and arms. 



-1 B C 



Fig. 312. — Xcr^"ous ?^'?teIrls of ^Molluscs. A, most gasteropods; 7^, accphals; C, 

 cephalopods and piilmonatcs. c, cerebral; /id, parietal, />(', ]ieilal, />/, jilcural, and 

 V, visceral ganglia. 



The ner\ous system has some highly characteristic features. .\s a 

 rule it consists of three p;iirs of ganglia associated with important sense 

 organs aitd connected by iter\e cords. One pair lies dors;d lo the esoph- 

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

 is the brain [icrchrum) and supplies the tentacles and e\'es. A second 

 pair lies \entral to the alimentary tract on the front part of the muscle 

 mass of the foot; these are the [^cdal t^aiii^lia with which the statocysts are 

 connected. The third p;Hr, the visfcral i^aiii^lia, are ;ilso \entral, :ind near 

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

 ;Mid from position and structure are regarded :i.s organs of smell (os/^l/ni- 

 dij). They are lliickcned patches of ciUatcd epithelium in the mantle 



