XI 



PHYLUM MOLLUSCA 



291 



The organs of respiration in the majority of the aquatic 

 Gastropoda are in the form of gills or ctenidia usually 

 plume-shaped appendages consisting of a central stem 

 bearing two rows or a single row of compressed filaments 

 or lamellae. Two ctenidia may be present, or only one may 

 be developed : they are enclosed in the mantle-cavity. 



In the Nudibranchs true ctenidia are absent, and their 

 place as breathing organs is taken by a number of secondary 

 branchice, sometimes simple, sometimes branched or pinnate 



FIG. 166. Shell-bearing Pteropoda. f.f. fins ; /. liver ; o. ovary ; sh, shell. 

 (From Cooke, after Souleyet.) 



processes, which are distributed over the dorsal surface as 

 in EoliS) or as in Doris (Fig. 165), form a circlet surround- 

 ing the anus, or, again as in Pleurophyttidia, a row on each 

 side beneath the mantle-flap. 



In the Limpets (Patella and its allies) (Fig. 167) the 

 true ctenidia are represented only by a pair of vestiges, and 

 respiration is carried on by a number of secondary branchiae 

 (g. /.) in the form of lamellae situated between the short 

 lateral fold of the mantle and the foot. In the Pulmonata, 

 and in some members of other groups, ctenidia are absent, 

 and the mantle-cavity, completely closed except for a small 



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