422 



AIOLLUSCA: ACEPHALA. 



numerous, and widely varied from one another. Some 

 kinds have much of regularity in their outline (Figs. 

 620, 622). Others arrest our attention by their great 

 irregularity of form and the peculiarities of their surface 

 (Figs. 623, 621, 628, etc.). 



Most kinds have the shell well developed, but some 

 kinds, as Teredo and Aspergillum, have the shell exceed- 

 ingly small in comparison with the size of the animal. 



FIG. 625. 



'' Ship- worm," Teredo norvegica, removed from its burrow. 



The structure of the Acephals or Lamellibranchiates, 

 will be better understood by referring to Figs. 618, 626-7. 



Internal structure of the Oyster. 



t>, one valve of the shell; hg. hinge; TO, one of the lobos of the mantle, and m', por 

 ti-in of the other lobe turned upward ; ms, muscles of the shell: br, branchiae or gills; 

 m/i. mouth; t, labial tentacles; I, liver; i, intestine; a, anus; ht, heart. 



