196 INVERTEBRATE ZOOLOGY 



the nervous system crossed or twisted into the form of a figure 

 eight (Fig. 92 C). The sexes are separate. A shell is almost 

 always present and is usually provided with an operculum. 



I. Order Aspidobranchia 



The order Aspidobranchia includes prosobranchs with but 

 little concentration of the nervous system. The limpets (sub- 

 order Docoglossa) include marine forms with a non-spiral shell 

 and bearing either a single true ctenidium or a secondarily 

 developed pallial gill beneath the mantle margin or with both 

 true and secondary gills. Patella and Acmaea are representa- 

 tive genera. The abalones (Haliotis) and the genera Fissurella 

 and Trochus exemplify the suborder Rhipidoglossa in which 

 both limpetlike and spiral shells occur. 



II. Order Ctenobranchia 



This order includes large numbers of marine, fresh-water, 

 and terrestrial Streptoneura with the shell usually coiled in a 

 more or less elevated spiral. The heart has but one auricle. 

 Due to torsion, the primitively right gill is shifted to the left side 

 of the body. Both shelled and naked forms are included within 

 this order. The members of the suborder Heteropoda are free- 

 swimming and pelagic marine molluscs in which the foot is 

 modified to form a vertical fin. Carinaria, with its delicate, 

 glassy shell, and Atlanta are examples. In the suborder Platy- 

 poda the foot is flattened ventrally. Littorina, Crepidula, 

 Natica, Strombus, Pleurocera, Murex, and Terebra are examples 

 of this highly diversified suborder. 



Subclass Euthyneura 



The cerebrovisceral commissures are not crossed in the 

 members of this subclass but form a single loop. The indi- 

 viduals are hermaphroditic. The shell, which is typically spiral 

 or flattened, is frequently vestigial or wanting. In the order 

 Opisthobranchia are included marine forms of which the tecti- 

 branchs (Aplysiidae or sea hares and pteropods) and nudi- 

 branchs are examples. 



The order Pulmonata comprises chiefly terrestrial and fresh- 

 water forms. The walls of the mantle cavity are modified to 

 form a lung into which air is taken in respiration. Most of the 



