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 {Nucula, &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 (Mytilus), 

 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 (Cuspidaria, &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. 



