208 PECTINIBRANCHIATA. 



presence in an aquarium is useful, inasmuch as they destroy voraciously the 

 green confervae, that otherwise are apt to accumulate on the sides of the glass. 



ORDER PECTINIBRANCHIATA.* 



The Comb-gilled G-asteropods constitute by far the most 

 numerous Order of the class. They are so called because they 

 breathe by means of gills disposed in the form of a comb, and 

 arranged in one or two rows suspended from the interior of a 

 chamber or cavity situated in the last-formed or most capacious 

 whorl of the shell, and communicating with the surrounding ele- 

 ment by means of a wide channel or tube called the siphon. The 



FIG. 215. THE WHELK, SHOWING ITS OP 



multitudinous species of marine Mollusks that inhabit spiral or 

 univalve shells belong to this Order. Many of them have a shelly 

 or horny plate attached to the hinder part of their body called 

 the opercnlum : this accurately fits trie orifice of the shell, to 

 which it serves as a door when the animal withdraws into its 

 habitation. 



All the Pectinibranchiate Mollusks have two tentacles and two 

 eyes, sometimes supported on special footstalks. Their mouth is 

 in the form of a tube or proboscis, capable of being protruded by 



* Pecten, a comb ; branchiae, gills: comb-gilled. 



