CHAPTER XXIX 

 THE MOLLUSCA 



By BRYANT WALKER, Detroit, Mich. 



For the purposes of this treatise and in order to differentiate the 

 group from the other great phyla of the animal kingdom repre- 

 sented in our fresh-water fauna, the Mollusca may be defined as 

 animals with a soft body encased in a hard shell, which may be 

 either in one piece (univalve) or in two pieces (bivalve). In the 

 univalve mollusca or Gastropoda, commonly called snails, the shell 

 may be coiled either obliquely or horizontally or may be a simple 

 uncoiled, conical, tent-shaped secretion on the back of the animal. 

 In this class, the animal possesses a distinct head, with a pair of 

 contractile tentacles, at the base of which are placed the eyes. 

 On the lower side of the head, between the tentacles, is the mouth, 

 which, on its inner, upper margin, is usually provided with a chi- 

 tinous Jaw of from one to three pieces. In the lower part of the 

 mouth is the radula, a muscular ribbon covered with minute teeth. 



The fresh-water Gastropoda are divided into two main groups or 

 subclasses according as they breathe the free air by means of a 

 lung or are provided with a gill for subaqueous respiration. As 

 the name impHes, these animals progress by crawling on the ven- 

 tral surface of the body, which is modified to form a flat, muscular 

 disk called the foot. 



The bivalve mollusca or Lamellibranchia, usually known as clams 

 or mussels, have the body protected by two S3mimetrical, opposing 

 valves, which are united above by an elastic tissue called the 

 ligament. They have no head and have, accordingly, been called 

 the Acephala. They have no tentacles, eyes. Jaws, or radula. 

 The mouth is simply an orifice at the anterior end of the body, on 

 each side of which is a flap or palpus, which assists in guiding the 

 food to the mouth. The foot is an axe-shaped mass of muscular 

 tissue (hence the name of Pelecj^oda often used for the class), 

 which may be extended from the anterior portion of the shell and, 



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