328 NATURAL HISTOEY. 



563. There is a third order of the Gasteropods, in 

 which the gills are not in a covered cavity or chamber, 

 as they are in the second order, but they either stand out 

 on the back, or are more or less concealed at the sides in 

 folds of the mantle. Some of them have shells, but most 

 have not. I will give but a single example, the Glaucus, 

 Fig. 260, found in the Mediterranean and Indian Seas. 



The hues of these beautiful 

 animals are azure blue and sil- 

 ver. The gills form two or 

 three large tufts on each side, 

 which, besides being the 

 breathing apparatus of the an- 

 imal, are also its instruments 

 for swimming. 



564. We now come to the 

 Fig. m-Giaucus Atlantic. sec ond grand division of the 

 Mollusks the Acephalous or Headless Mollusks. These 

 may be divided into two groups: 1. Those which have 

 shells, called the Conchiferous, or shell-bearing. 2. Those 

 which are covered with a leathery or membranous tunic, 

 called the Tunicated. 



565. The shells of almost all the Conchifera are bivalve. 

 This group includes the Oysters, Clams, Mussels, Scal- 

 lops, etc. The shell is exuded or secreted from the man- 

 tle, and is in different layers, as may be seen in the shell 

 of the Oyster. The outermost layer is the smallest, and 

 as the animal grows, each layer is a little larger than the 

 one outside of it. The two parts or valves of the shell 

 are joined together by a hinge. Near this hinge is an 

 elastic ligament, which allows the valves to be a little 

 apart, which is their natural position, admitting the wa- 

 ter freely to the mouth of the gills. When the animal 

 wishes to shut the valves closely, it does so by means of 

 a muscle. Sometimes there are two muscles for this 

 purpose. 



566. That you may understand the plan of the organs 



