252 '^"^ ANIMAL KINGDOM 



without iliMli.iigiiii; ilu> tuMn.iUx \s(n. In the Ntoniadi. ilir iuMna(«H\sts arc 

 ^lly^c^lcHl liiT. aiul alinwaul air pitkotl up b\ anichnui <<1K ami (ai 

 iinl to the onulnuus. whoic tlu\ piotot the sea slug m imu h the same 

 wax that tlu-\ were Nupposcii to protect ihc- ludroid. 



Pulmonata. I he thuil laige order ol gastropods is the Pulmonata, 

 or air hieathers. This iiuludes most ol the teiiestrial ami liesh waiei 

 speties. 1 he\ show lull toisiou and ha\e a single nei)hridium and 

 autiile. No gill is pieseni. Aii is tudilaied through the mantle eavity, 

 whith is lineil with a ii(hly vascular epidermis that serves as the respira 

 ior\ surlaie. Iheve aie the lamiliar gardei\ snails (Kig. 11.8 /)). In 

 iludeil also are the slugs ^ig. I l.S C), in which the shell is reduced 

 to Iragmeius hmieil m the n\antle oi is (ompleteK absent. The mantle is 

 still piesent. |)H)\itling a taxiiy loi Kspivaticm. 



Most ol the prosobranchs ha\e opercula, horny lids botne on the 

 upper surlace ol the posterioi jiaii o[ the loot, by whidi the opening 

 to the shell is tight Iv closcil when the annual is withdr.iwn inside the 

 shell. In the pulmonates. whith are the most susieptilile to ilesiccaiion. 

 the operculum is latkmg. When the envitotuneni becomes dry. i)id- 

 tuonates bui\ themsehes iti the soil ai\cl seciete a thick mucus in the 

 shell opening that hardens to lorm .m etlcctixe seal. When rain leiurns 

 moisture to the soil the sc.il soltens and the snails become active again. 



Most piosobtaiuhs have sei)arate sexes, but most ot the opistho- 

 btamhs and pidmcMiates are henn.iphroditic. Cross lertili/alion is the 

 rule, l-.ithei imlixiduals are temjioiaidx active as males or as females, 

 or simultaneous cross lertili/ation. as iji tlie llatworms, will occur. vSclt- 

 lertili/alion is known to occur in .i lew species. 



109. Class Pelecypoda: General Features 



The pelecypods include all ol the bixalves. In these forms the foot is 

 compres-sed to form a muscular spade lor digging and the head is 

 greatly reduced, lying xvell within the mantle cavity. In most species the 

 >hell. composed of two valves hinged together cUnsallv, can completelv en- 

 dose the boch. .Strong muscles, the adductors (lig. 1 1. 10"), can hold the 

 shells liglulv shut against enemies. 



The mantle cavity and gills are elaborated to serve both respira- 

 tion and feeding. Typically the edges of the mantle around the free 

 margins of the valves are kept together, forming a closed cavity, except 

 postericMh where ihev separate to Unin twc> opei^ingN. a \entral Incur- 

 rent siphon and a dorsal excurrent siphon. Ihese openings mav be c\ 

 tended .is .i Umg double tube which cm be projected up into the water 

 while the dam lies buried in the sand. 1 he mantle ca\it\ extends all 

 aroutul the bc>d\. Ihe two gills im each side are large and are attacheci 

 to the body along the whole length oi each side and around the poste- 

 rior end. The inner gill of each side extends mediallv against the lcxit. 

 and each outer gill extends laterally against the mantle, thus dixiding 

 the mantle cavity into upper and lower chambers that connect with 

 the excmrent and incinrent siphons respectively. The lining of the 

 cavity and the surfaces of the gills are ciliated. The beating of the cilia 



