292 MANUAL OF ZOOLOGY SECT. 



In the limpets (Patella and its allies 1 ) (Fig. 179) 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 Pul- 

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

 absent, and the mantle-cavity, completely enclosed except 



sh 



TUB- 



FIG. 179. Patella vulgata, seen from the ventral side. /, foot;^. /, circlet of 

 gill lamellae; m. e, edge of the mantle; mu, attachment muscle; si, slits in the 

 attachment muscle; sh, shell; v, efferent branchial vessel; v' t aorta; ve, smaller 

 vessels. (From the Cambridge Natural History.} 



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

 monary sac or Jung (Fig. 180), its roof being richly sup- 

 plied with blood-vessels; in the aquatic forms its function 

 is apparently as much hydrostatic as respiratory. In some 

 of the Pulmonata there is a return to a completely aquatic 

 mode of respiration accompanied by the development of 



1 Our common eastern American limpet is Acmaa testudinalis. 



