KEARf-C 



HE creatures to be described 

 this and some following 

 chapters differ from the fore- 

 going in the liigher develop- 

 ment of their respiratory 

 organs. In the Mussel family 

 we showed that the gill-filaments had got so long 

 that they had to be folded over so that the gills 

 appeared to be fourfold on each side of the visceral 

 mass. In the Oyster and Scallop family advance 

 had been made upon this development by the ends 

 of the turned-over o^ills unitino; with the foot and 

 the mantle respectively. The present chapter deals 

 with moUusks in which the two folds of each gill- 

 plate have been partially united by trusses which 

 leave internal spaces and increase the respiratory 

 capacity. There are usually two muscles for closing 

 the shell, and the mantle-edges are united at one or 

 more points. The group includes both marine and 



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