PELECYPODA 567 



the sand or mud or bore into clay, wood, or rock, with only the tips of 

 the siphons extending- into the water, and are then protected from their 

 enemies. Others protect themselves from predacious fish by fastening 

 themselves to some other object by means of their byssus, or cementing 

 the shell to a rock or stone. 



Bivalves are useful to man in a variety of ways. Many are impor- 

 tant articles of food, the oyster and clam being raised artificially for 

 this purpose. The shells of numerous species, especially the large fresh- 

 water mussels, are used for making buttons and other things, and pearls 

 are probably the most valuable single objects obtained from any animal 

 whatsoever. 



History. — The term bivalve was employed by Linnaeus, in whose 

 system of classification the Bivalvia were a subdivision of the Testacea, 

 one of the four divisions of Vermes. Cuvier called the group the Aceph- 

 ala, and included in it, besides the bivalves, tunicates, brachiopods, 

 and cirripeds. Blainville named bivalves the LameUibranchiata (1816), 

 a name which has had the greatest vogue of any of the numerous names 

 given to this group, down almost to the present time. In 1818 Lamarck 

 called them the ConcMfera, and in 1821 Goldfuss, the Pelecypoda. The 

 latter name was revived by Trj^on (1884) and Fischer (1885), and has in 

 the past few years almost entirely supplanted the others. 



The main subdivisions of the class, in the older books, were usually 

 based upon the presence and form of the siphons or the adductor muscles, 

 but at the present time the system proposed by Pelseneer (1888), the 

 basis of which is the structure of the gills, is universally adopted. 



The Pelecypoda contain about 11,000 species grouped in 5 orders, 

 of which a fifth inhabit fresh water ; about 15,000 fossil species are known. 



Key to the orders of Pelecypoda: 



Qi Gills not lamellar but pinnate (Fig. 890, A), and in the hinder part of the 

 mantle cavity : foot with a creeping surface ; pallial sinus present or 

 absent (Fig. 889, 10) 1. Protobranchiata 



02 Gills composed of rows of parallel filaments (Fig. 890, B) ; no pallial 



sinus ; byssus well developed 2. Filibranchiata 



Oj Gill filaments joined together ; no pallial sinus ; but one muscle. 



3. Pseudolamellibranchiata 



04 Gill filaments completely joined, forming a continuous surface (Fig. 



890, C) ; pallial sinus usually present (Fig. 889, 10) ; most 

 pelecypods 4. Eulamellibranchiata 



05 Gills absent ; pallial sinus present 5. Septibranchiata 



Order 1. PROTOBRANCHIATA. 



Gills (Fig. 890, A) consisting on each side of an axis, to which are 

 attached 2 rows of short, flat leaflets projecting backwards freely into the 

 mantle cavity ; foot more or less of a flat creeping sole often surrounded 



