'II 



PHYLUM MOLLUSCA 



723 



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

 organ is attached throughout its length. 



ve 



Tn.e 



FIG. 642. Pleurophyllidia lineata, 



from the ventral surface, a. anus ; 

 br. secondary branchiae : m. mouth ; 

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

 bridge Natural History.) 



FIG. 643. Patella vulgata, seen from the 

 ventral side. /. foot ; g. /. circlet of gill- 

 lamellae ; m. e. edge of the mantle ; raw. attach- 

 ment-muscle si. slits in the attachment- 

 muscle : sh. shell ; v. efferent branchial vessel ; 

 '. aorta ; ve. smaller- vessels. (From the Cam- 

 bridge Natural History.) 



pul.v 



In the Nudibranchs true ctenidia are absent, but their place 

 as breathing organs is taken by a number of secondary branchice, 



sometimes simple, 

 sometimes branched 

 or pinnate processes, 

 which are distributed 

 over the dorsal sur- 

 face, as in Eolis ; or, 

 as in Doris (Fig. 633), 

 form a circlet sur- 

 rounding the anus ; or, 

 as in Pleurophyllidia 

 (Fig. 642), a row on 

 each side beneath the 

 mantle-flap. 



In the limpets 

 and its allies, 

 the true 



ctenidia are r e p r e - 

 sented only by a pair of vestiges, and respiration is carried 

 on by a number of secondary branchiae (g. (.) in the form of 

 lamella situated between the short lateral fold of the mantle 



vent 



ciorl 



eph 



FIG. H44. Pulmonary cavity and related parts in a slug 

 (Xiimax). aort. aorta ; aur. auricle ; neph. kidney ; 

 peric. pericardium, laid open ; pul. ap. pulmonary aperture ; 

 pu 7 . v. pulmonary vein with its ramifications ; rect. rectum; 

 ur. ureter ; rent, ventricle. (After Pelseneer.) 



