o-'S 



MOLLUSCA 



iHis lavcr. contains flic piijmcnl and ma\- lie covered by a horny cnticlc. Tn rare 

 cases mantle and shell are lacking;, or the mantle is present lint the shell is rtidi- 

 mentarv and not xisible externally because the mantle folds ha\e ^jroun over it. 

 In these cases the \isceral sac is not prominent. Since the shellless forms 

 possess a mantle and shell in the young, the adult conditions are explained by 

 degeneration. 



Only a few gastcropods arc bilaterally symmetrical. I'stially the spi-al 



twist of the visceral sac has resulted in a torsion of otJicr parts from left to 



right, ill wliich alimcutary tract, tiephridia, gills, 



heart, ami iier\ous s_\-stem take part i^lig .^^j). Tiio 



intestine is bent in tliis wa\', the anus opening into 



the mantle chamber oil the right side (H) or the 



twisting mav be continued so far as to tlouble the 



,. . intestine on itself, l.he anus being in the middle line 

 ric. ^u. — ^inis- ^ 



tral shelf of Lii!ii.<i,\i in front, near the licad [O. Xepihridia, gills (willi 



Ciirmaius Uroni [hem the osphradia) , and Iteart wamler in company, 

 Lcunis). 



so that the organs pnmitiwdv belonging on the lelt 



side may be transferred to the right and ^■ice versa. With this there is 



a tendency to asymmetry and the loss of the organs (usuallv of the 



primitively left side). \Mien the nervous system takes part in the twist- 



FlG. 3;,j. — Three diaLjraiiis ilkKtraling llie torsion of the bodv and the twi^^tiiit; of 

 the nervous system in u;astero|.o,ls lafler l.an.L;). .1, liilateral; /^, asvmnielrieal; 

 C", slrc|il(mrnrous coii.liiioii. 'Pile icferciice Irllers are ]ilaeed ii|Hm the ortjans of the 

 primiti\-e Irfl si.le.^ ,j, anus-, r, cevebral uan..;linii; ^', eleni.liuni; /, auricle; '"/», mouth; 

 II. ncphriilial openinL;-; e. .K|iliradiuni ; p,i. pariiMai s^annlioii ; />,-, pedal itans^lion; pi, 

 pleura! t^anL^diuii; e, \cnlriclc. 



ing a crossing of the ccrebroxisceral commisstires lakes place, known as 

 slrrploiicurv or tiiiaslonairv {('), 



The alimentary canal begins with ;l inuscuhir region \vhich in some 

 groups is de\eloi)ed inio a large protrtisible ]>roboscis (lig. 3:;0. The 



