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CHAPTER XXIII. 



LAMELL1BRANCHIATA. 



THE Lamellibranchiata, Pelecypoda, or Bivalve Shell-fish, are 

 distinguished by the fact that the head is not distinct, and 

 the mouth is destitute of any apparatus o teeth. The body is 

 more or less completely protected by a bivalve shell, composed of 

 two, usually symmetrical, pieces or valves. There are generally 

 two leaf -like lamellar gills upon each side of the body. 



The Lamellibranchs include all the ordinary Bivalve Shell- 

 fish, such as Oysters, Cockles, Mussels, and the like, and they 

 are all either marine or inhabitants of fresh water. 



Though they agree with the Brachiopoda in possessing a 

 shell which is composed of two pieces or valves, there are, 

 nevertheless, many points in which the shell of a Lamelli- 

 branch is distinguishable from that of a Brachiopod, irrespec- 

 tive of the great difference in the structure of the animal in 

 each. The shell in the Brachiopoda, as we have seen, is 

 rarely or never quite equivalve, and always has its two sides 

 equally developed (equilateral) ; whilst the valves are placed 

 antero-posteriorly as regards the animal, one in front and one 

 behind, so that they are " dorsal " and " ventral." In the Lam- 

 ellibranchiata, on the other hand, the two valves are usually of 

 nearly equal size (equivalve), and are more developed on one 

 side than on the other (inequilateral) ; whilst their position as 

 regards the animal is always lateral, so that they are pro- 

 perly termed "right" and "left" valves, instead of "ventral" 

 and " dorsal." 



It is to be remembered, however, that many of the Bi- 



