PHYLUM MOLLUSCA 



691 



cavity; but in the majority of the Streptoneura and bran'chiate 

 Euthyneura the primitively left (actually right) ctenidium alone is 

 retained. In those Gastropoda that possess two ctenidia, and in 

 many forms with only one, the axis of the ctenidium bears two 

 rows of compressed filaments, and is attached only towards its 

 base. But in the majority of those with one ctenidium there 

 is, as in Triton, only a single row of filaments retained, and the 

 organ is attached throughout its length. 



In the Nudibranchs true ctenidia are absent, but their place 

 as birthing organs is taken by a number of secondary branchiae, 

 sometimes simple, sometimes branched or pinnate processes, which 



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FIG. 594. Pleurophyllidia lineata. 



from the ventral surface, a. anus : 

 br. secondary branchiae ; m. mouth ; 

 .. o. sexual opening. (From the Cam- 

 bridge Natural History.) 



FIG. 595. Patella vulgata, seen from the ven- 

 tral side. /. foot ; <j. I. circlet of gill lamellse ; 

 m. e. edge of the mantte^ m u. attachment muscl4 ; 

 si. slits in the attachment muscle ; sh. shell ; 

 v. efferent branchial vessel ; v'. aorta ; ve. smaller 

 vessels. (From the Cambridge Natural History.) 





are- distributed over the dorsal surface, as in JEolis, or, as in 

 Doris (Fig. 585), form a circlet surrounding the anus, or, as in 

 Pleurophyllidia (Fig. 594), a row on each side beneath the 

 mantle-flap. 



In the limpets (Patella and its allies) (Fig. 595) the true 

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

 is carried on by a number of secondary branchiae (g. I.) 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 enclosed 

 except for a small rounded opening, has the function of a pul- 

 monary sac or lung (Fig. 596), its roof being richly supplied 



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