STRUCTURE OF THE MOLLUSCA. 199 



ORDEB YET -GASTEROPODA NIDIBRANCHIATA. 



All the Mollusca of this order are destitute of a shell, and 

 bear branchiae seated on some part of the back. They are 

 all marine, and have a general resemblance to slugs. Of the 

 families which belong to this order may be mentioned : the 

 JEoUdina, having the branchiae in the form of shreds, cirri, 

 or papillie, disposed along the sides, and two pairs of ten- 

 tacula; the Tritoniina, with the branchiae in the form of 

 arbuscules or papillae disposed along the sides, and two re- 

 tractile tentacula ; the Dorina, with four tentacula, the 

 branchiae tufted, and disposed in a circular form at the hind- 

 part of the body above. 



ORDER K -GASTEROPODA HETEROPODA. 



The Heteropoda are not formed for crawling, as all the 

 preceding orders are, but for swimming. Their foot is com- 

 pressed into a vertical membranous plate, which they use as 

 a fin. Their body is formed of a gelatinous, transparent 

 substance, and their branchiae are placed on the hind-part of 

 the back. Of this kind are the Carinarics, of which some 

 of the organs are covered by a thin, transparent, conical shell, 

 having the apex curved backwards ; and the Firola, which 

 have no shell, but otherwise resemble the Carinariae. 



ORDER X -GASTEROPODA CIRROBRANCHIATA. 



The Cirrobranchiate Gasteropoda have the respiratory 

 apparatus composed of tufts of tentacular filaments situated 

 on the sides of the neck. Such is the family of the Den- 



