MOLLUSCS WHICH BREATHE IN WATER 399 



may therefore correlate the large size and great complexity of 

 the gills of freshwater mussels, &c., with the fact that they do 

 double duty, like the pharynx of a Lancelet or an Ascidian 

 (see p. 389). 



The plate-like gills of a Freshwater Mussel are remarkably 

 complex, constituting the last term of a series to which arrange- 

 ments found in other bivalves lead up. The simplest case is 

 that of certain forms [Niicula, &c. ), where either gill consists 

 of an axis bearing a double series of little flattened outgrowths. 

 An advance upon this is seen in the Saltwater Mussel {Mytilns), 

 for here the little outgrowths are represented by long threads, 

 which turn up at their ends owing to the limited size of the 

 gill -cavity. Adjacent threads are connected together by the 

 interlacing of long cilia. Each gill of a Freshwater Mussel 

 (fig. 530) consists of an outer plate and an inner plate. These 

 have arisen by fusion of the outer and inner sets of filaments, 

 which simply interlock in a Saltwater Mussel. 



A few bivalves {Czispidaria, &c.) have modified in another 

 way. The gill on either side is only represented by a horizontal 

 perforated plate of muscular nature, which divides the gill-cavity 

 into upper and lower compartments. In these forms the mantle- 

 lobes must do most of the breathing work. 



