82 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



symmetrical ; in theAzygol>raiicliiata,such as Palu- 

 dina, Dolium, the cowry (Cyproea), and the whelk 

 (Buccinum), the left gill and excretory organ become 

 aborted ; some members of this division, the so-called 

 Heteropocla, become modified to a free-swimming 

 life, as Atlanta or Firuloides. 



II. The ancient group of the Scaphopoda 

 exhibits some primitive characters, but is specially 

 remarkable for its elongated elephant- 

 tooth-like shell (Dentalium) which is 

 open at either end (Fig. 39). 



III. The Pteropoda closely ap- 

 proach in many important characters 

 the next succeeding group, but they 

 are most conveniently kept separated 

 from them. The anterior portion of the 

 foot (propodium) surrounds the head, 

 and the median part (mesopodiuiii) 

 Fig. 39. Shell i g converted into a pair of napping fin- 

 ofDentaiinm like organs by means of which these 



elephant!- -,. v, . J , -,-,-, 



num. ordinarily minute creatures are enabled 



to swim about on the surface of the 



ocean. According as they have or have not a shell, 



they are called Thecosomata (Hyalea, Cymbulia), 



or Oymnosomata (Clione, Pneumodermon). 



IV. The last and highest division of the Mollusca 

 is formed by the Cephalopoda ; the propodium is 

 here produced into a number of long tentacular pro- 

 cesses or arms, on which suckers are not unfrequently 

 developed ; the mesopodium of either side unites with 

 its fellow to form an incompletely or completely 

 closed siphonal tube, which serves as the chief organ 

 of locomotion. The shell is external or internal, 

 coiled or simple, or completely absent. 



a. The ancient group of the Tetrabrancliiata, to 

 which many fossil forms belong, is represented to-day 

 by a single genus, Nautilus ; they receive their 



