13-f OYSTERS, AND ALL ABOUT THEM. 



THE CONCHIFERA. 



The Conchifera are mainly characterised by their 

 breathing organs. They have no fringed arms stretching 

 away from the sides of their mouth ; and their mantle not 

 being sufficiently effective as an organ of respiration, they 

 have, developed from the body, and lying between the 

 mantle-lined shells and the body mass, two sheets of 

 membrane on each side. The relation of these breathing 

 plates is best seen in an illustration, where there is a 

 section given of the animal as it would appear if it were 

 cut across so as to divide both shells. These gill-plates 

 secure that the blood shall be well aerated, not only by 

 exposing as large an amount of surface as possible, but also 

 by having gill-tubes, which run through the plates from one 

 edge to the other, through which the water passes. As is 

 usually the case with breathing surfaces in marine animals, 

 the plates are covered with cilia, whose motion secures a 

 constant change in the water. The gill-plates are very 

 variouslv modified in the different families of Lamellibran- 



j 



chiata ; but they are constant throughout the class. 



In some, as the oyster, the mantle simply lines the 

 shells and ends at their edges, so that the water has free 

 entrance from all sides. In other families the mantle of 

 one side passes across the aperture of the shell to be 

 united at certain points, or along almost its whole length, 

 to the mantle of the opposite shell. In others the animal 

 is not only almost entirely walled in by the union of the 

 two lobes of the mantle, but part of this mantle is drawn 

 out into two long tubes, one of which communicates with 

 the chamber in which the gills lie, and the other with the 

 smaller lined chamber into which the anus opens, and into 

 which, also, the gill tubes discharge the water. 



