CHAPTER XI 



MOLLUSCA 



The Mollusca (Lat. mollis^ soft) are bilaterally symmetrical. 

 This symmetry is modified by dextral, sinistral, or frontal torsion in 

 the adult Gastropoda. While for the most part the Mollusca differ 

 from the Annelida and the Arthropoda in being unsegmented, the 

 Cephalopoda have certain segmented ducts, and the Amphineura 

 are segmented. The majority have an exoskeleton of calcium car- 

 bonate, the shell. In general we find that Mollusca are sluggish. 

 Many Mollusca are of economic importance (see page i6i). 



Classification 



Class 1. Pelecypoda (hatchet foot) or Lamellibranchiata (leafy 

 gills). Clams, oysters, mussels, scallops. Usually bilater- 

 ally symmetrical, with two-valved shells, and a two-lobed 

 mantle. Abundant in Cretaceous of America. Marine and 

 fresh water. 



Class 2. Amphineura (on both sides — a nerve). Chitons, with 

 bilateral symmetry, shell of eight transverse calcareous 

 plates and many pairs of gill filaments. Ordovician to 

 present. Marine. 



Class 3. Gastropoda (belly foot). Snails, slugs, whelks, with a 

 spirally coiled shell. Some dextral and others sinistral. 

 Cambrian to the present. Abundant since Ordovician. 

 Marine and fresh water. 



Class 4. Scaphopoda (boot foot). Tooth shells with tubular shell 

 and mantle. Cambrian to the present. Dentalium from the 

 Tertiary to the present. Marine. 



Class 5. Cephalopoda (head foot). Cuttle fishes, squids, octopi 

 and nautili. Bilaterally symmetrical foot divided into arms 

 with suckers. Nervous system is located in the head. 

 Nautiloids are first known from the Cambrian rocks; they 

 reach their maximum development in the Silurian and decline 

 to the Triassic. Marine. 



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