170 THE MICROSCOPIST. 



2. Clavelinidce. Bodies distinct, but connected by a 

 common root thread. 



3. Ascidiadce. Bodies unconnected. 



B. Free ; mantle and test united throughout. 



4. Pelonceadce. Orifices near together. 



5. Sal pad (B. Orifices at opposite ends. 



XI. CONCHIFERA. This class consists of bivalve mol- 

 luscs, and is chiefly interesting to the microscopist from 

 the ciliary motion on their gills and the structure of the 

 shell. 



The ciliary motion may be observed in the oyster or 

 mussel, by detaching a small piece of one of the bands 

 which run parallel with the edge of the open shell, placing 

 it on a glass slide in a drop of the liquid from the shell, 

 separating the bars with needles, and covering it with 

 thin glass ; or the fragment may be placed in the live box 

 and submitted to pressure. The peculiar movement of 

 each cilium requires a high magnifying power. It appears 

 to serve the double purpose of aeration of the blood and 

 the production of a current for the supply of aliment. 



Dr. Carpenter has shown that the shells of molluscs 

 possess definite structure. In the Margaritaceaz the exter- 

 nal layer is prismatic, and the internal nacreous. The 

 nacreous or iridescent lustre depends on a series of grooved 

 lines produced by laminae more or less oblique to the plane 

 of the surface. The shells of Terebratidce are marked by 

 perforations, which pass from one surface to another. 

 The rudimentary shell of the cuttle-fish (of the class 

 Cephalopoda], or " cuttle-fish bone," is a beautiful object 

 either opaque or in the polariscope. Sections may be 

 made in various directions with a sharp knife, and 

 mounted as opaque objects or in balsam. 



XII. GASTEROPODA. These molluscs are either naked, 

 as the slug, or have univalve shells, as the snail, the lim- 

 pet, or the whelk. As in the other classes referred to, 

 the details of anatomical structure are full of interest ; 



