GASTEROPODOUS MOLLUSCA. 141 



eyes. The back of the creature opens with two wide 

 lobes, which can be expanded or closed over the opening 

 at the animal's will. When open, they expose to view, 

 on the right side, the finely-fringed and lobed branchiae, 

 seated in a deep hollow beneath a fold of the mantle. 



Next stand the Pectinibranchiata, the most numerous 

 order of Gasteropoda, comprising all the spiral univalve 

 shells. In these the gills are pectinated, or shaped 

 like the teeth of a comb, and placed in a large hollow 

 chamber in the animal, communicating with the sur- 

 face by a wide slit, through which the water finds 

 free access to the gills. This type is obviously ana- 

 logous to the Pulmoniferous order, except that in 

 these the medium is water, and not air. Closely allied 

 to this order are the Scutibranchiata, which have pec- 

 tinated gills, similarly placed in a special chamber ; but 

 in these the shell is wide, and cup, or shield-shaped, in- 

 stead of being spiral. Such 

 is the Common Limpet (Pa- 

 tella], which may be taken as 

 the type of the order. And, 

 lastly I shall mention the 

 Cydobrancldata, in which 

 the gills form a fringe round 

 the margin of the body, be- 

 tween the edge of the mantle 



and the foot. To this belongs the Chiton, the only mul- 

 tivalve shell among the Gasteropoda. There are two or 

 three common species, which may be found adhering 

 to stones near low-water-mark. They are Slugs, coated 

 with eight transverse, shelly plates, resembling the plates 



