CHAPTER 25 



THE PHYLUM MOLLUSCA AND SOME 

 MINOR EUCOELOMATE PHYLA 



No region of the habitable earth is without its moUuscan popula- 

 tion. On land they may be found from the moist forest to the driest 

 deserts, and from areas below sea level to the highest mountains. All 

 bodies of fresh water have scores of molluscan inhabitants; the ocean 

 boasts an even greater number and variety of forms. From the ex- 

 tensive fossil record, it is clear that this is true not only for present 

 times, but has been the condition from very early geological periods. 



Like most major phyla, the classes of the Mollusca differ greatly 

 in appearance. At first it seems difficult to reconcile the placing of 

 the octopus and the clam in the same phylum ; nevertheless these two 

 seemingly unlike forms have important characters in common. Among 

 these is the soft body from which the phylum name is derived and the 

 presence of the glandular mantle which secretes a shell. The shell of 

 the clam is external, while the shell of the octopus is an internal sup- 

 porting structure. Also sharing these important characters are the 

 snails, the chitons, the tooth shells, and the squids. 



Like the annelids, the molluscs have an organ grade of construction 

 and have all major organ systems well developed. Their success as a 

 group, however, is traceable to other specializations. They are ex- 

 perts at protection : the snails and clams rely upon their heavy, limy outer 

 shells, while the octopus and squid rely upon such complex acquisitions 

 as jet propulsion, smoke screens, speed, and extraordinarily well-de- 

 veloped nervous systems and sense organs. 



Since most of the marine molluscs have a modified trochophore 

 larva known as a veliger, it may be safely assumed that both the an- 

 nelids and molluscs arose from some common unknown ancestral form. 



Characteristics of the Phylum. — Like all higher animals, the 

 molluscs possess three germ layers and an organ grade of construc- 



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