XIV. CLAM 



The Mollusca constitute a large important, and structurally 

 aberrant phylum of Invertebrate animals which includes such 

 commonly known forms as the Clam, Oyster, Snail, Slug, Squid, 

 and Octopus. These animals, although notably complex, exhibit 

 in their bizarre adult organization very few morphological features 

 by which they can be closely linked to any other group of animals. 

 Like the Sponges and the Echinoderms, they apparently portray 

 a comparatively isolated structural arrangement. Furthermore 

 the differences between various members of the phylum are unusu- 

 ally marked. This fact may be noted when one compares, for 

 example, the structure of the sedentary Clam with that of a 

 free-swimming Squid. All Molluscs, however, are triploblastic ; 

 they show no evidence of segmentation in the adult condition, and 

 the coelom is greatly reduced. Bilateral symmetry is present to a 

 greater or less degree. Two characteristic molluscan features are 

 present in some form or other in most species, namely, a muscular 

 structure, known as the foot, and a calcareous shell. 



The study of a representative of the Mollusca is important, 

 not only because of the unique structural features which are there 

 shown, but also because of their importance from the economic 

 standpoint. A great many species, such as the Oyster and Clam, 

 are edible. The shells of some species, such as the fresh-water 

 Mussel, are the basis of the pearl button industry, and true pearls 

 are also formed by Molluscs. Altogether the income from the 

 shellfish industries amounts to many millions of dollars annually. 



A, External Structure 



For our present study, either the common, marine Hard Clam, 

 or Quahaug (Venus), or the fresh-water Mussel (Unio or Anodonta) , 

 may be used. All these forms are very similar in general structure. 



The Clam is enclosed in a secreted shell, largely composed of 

 calcium carbonate. The shell is made up of a right and left half, 

 each of which is spoken of as a valve. The Clam and other Mol- 

 luscs which have two valves are commonly designated as ri- 



139 



