20 



HISTORICAL GEOLOGY 



VII. Mollusks, like Molluscoids, lack segmentation, but they 

 are more highly organized with more or less distinctly developed 

 locomotive organs and head. Nearly all have shells, generally ex- 

 ternal, and gills for respiration. 1. Pelecypods (e.g. Oysters and 

 Clams) are alwaj^s supplied with a pair of external shells nearly 

 alike and hence they are bivalves, but they differ from the Brachi- 



opods (bivalves) in the 

 absence of bilateral sym- 

 metry. The head is less 

 distinct than in the other 

 Mollusks. 2. Gastropods 

 have distinct head with 

 eyes and one or two pairs 

 of tentacles, and they are 

 almost invariably covered 

 by a one-chambered shell 

 (e.g. Snails). 3. Cepha- 

 lopods have well-defined 

 foot; head armed with 

 tentacles; and large com- 

 plex eyes. They propel 

 themselves rapidly by 

 forcible ejection of water. 

 Tetrabranchs (e.g. modern 

 Pearly Xautilus) are the 

 so-called chambered Ce- 

 phalopods because the 

 external shell, straight or 

 coiled, is divided into 

 compartments. They are 

 Dibranchs (e.g. so-called 



Foot 



Fig. 12 

 Gastropods: A and B, marine forms; C, 

 land Snail. (From Schuchert's "His- 

 torical Geology," permission of John 

 Wiley and Sons.) 



four-gilled and with numerous tentacles. 

 Cuttle-fishes) are two-gilled; with either eight or ten tentacles; 

 bag for secreting an inky fluid; and almost invariably without 

 external shell. Usually there is a sort of cigar-shaped internal 

 shell. Mollusks of all classes have been abundantly preserved in 

 rocks of all but the earliest geologic ages. 



VIII. Arthropods are highly characterized by longitudinal 

 body segmentation; jointed appendages (usually a pair from each 

 segment); and usually by a pair of nerve centres in each segment. 

 1. Crustaceans (e.g. Lobsters and Crabs) are water animals breath- 



