XII 



PHYLUM MOLLUSCA 



091 



cavity; but in the majority of the Streptoneura and branchiate 

 Euthyneura the primitively left (actually right) ctenidium alone is 

 retained. In those Gastropoda that possess two ctenidi;i, 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 breathing organs is taken by a number of secondary brancliice, 

 sometimes simple, sometimes branched or pinnate processes, which 



TO- 



C. 594. Pleurophyllidia lineata, 



from the ventral surface. . anus : 

 br. secondary branchiae; in. mouth : 

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



In-, :l'!i Siitni'nl History.) 



ve 



sli 



-me' 



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

 tral side. /. foot ; ;/. /. circlet of gill laruellse ; 

 in. e. edge of the mantle; mu. attachment muscle ; 

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

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

 vessels. (From the Canibrii/rje Natural History.) 



are distributed over the dorsal surface, as in Eolis, 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 branchia3 (g. I.) in the form 

 of lamellas 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 



Y Y 2 



