290 



COLLEGE ZOOLOGY 



OJl 



Trochophore larva 



Veliger larva 



Figure 179. Two stages in the development of a mollusk; both are free-swimming larvae. 

 Both annelids and mollusks have a trochophore larval stage, and many zoologists interpret this 

 to mean that both groups of animals have developed from a common ancestor (Fig. 107). 



is internally segmented and not quite bi- 

 laterally symmetrical. The segmentation 

 violates one of the general criteria by which 

 mollusks are most readily known, that of an 

 unsegmented body plan. Neopilina (no 

 common name) is the generic name of this 

 new mollusk. 



The new mollusk has a single shell, and 

 several pairs of internal organs, such as 

 auricles, excretory organs, and nephridia— 

 all evidences of internal segmentation. 

 There may still be other internal organs 

 which are segmented, but further study of 

 the internal anatomy will be necessary to 

 determine this. There are 5 pairs of strong 

 double muscles on the inner surface of the 

 shell, and the animal has 5 pairs of gills 

 (Fig. 180). It possesses a well-developed 

 ladula. 



Neopilina apparently belongs to a very 

 ancient stock. Its early fossil ancestors are 

 already known, and further study should 

 throw additional light on our knowledge of 

 molluscan evolution. 



Chitons 



The coat-of-mail shells (Fig. 170) are pro- 

 tected by a shell of 8 transverse plates, which 

 are arched above and overlapping like shin- 

 gles on a roof. These plates are not evidence 

 of metamerism. When detached from the 

 rocks to which they cling, chitons roll up 

 like an armadillo, with the soft parts prac- 

 tically covered by the hard shell. They live 

 on rocky seashores mostly in water less than 

 25 fathoms in depth and cling tightly to 

 the rocks by means of the suction of the 

 foot. 



Gastropods 



Gastropods (Figs. 181 and 182) live in 

 fresh or salt water and on land. A few are 

 parasitic on other animals. Land snails must 

 protect themselves from drying at certain 

 seasons; they retire into their shell as far as 

 possible and secrete a parchmentlike wall 

 (epiphragm) across the opening, which pre- 



