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CHAPTER XXXVIII. 



DIVISIONS OF THE GASTROPODA. 



Sub-Class I. Branchiogastropoda. 



This primary division of the Gastropods is distinguished, speaking 

 generally, by the fact that the animals included in it are water- 

 breathers, respiration being effected by the thin walls of the mantle- 

 cavity, by external branchial tufts, or by pectinated or plume-like 

 gills contained in a more or less complete branchial chamber. The 

 groups of the Cyclostomidce, Aciculidce, and Helicinidce, from the 

 general characters of their organisation, must, however, be placed in 

 this division ; although the animal in these is terrestrial, and is fur- 

 nished with a pulmonary chamber adapted for breathing air directly. 

 This sub-class comprises the four orders of the Prosobranchiata, 

 Opisthobranchiata, Pteropoda, and Heteropoda. 



Order I. Prosobranchiata. 



The great majority of the members of this order are aquatic in 

 habit, and possess gills situated in front of the heart, the auricle of the 

 heart being thus placed in front of the ventricle. The Cyclostomidce, 

 Aciculidce, and Helicinidce. alone possess a pulmonary chamber. The 

 gills are, typically, plume-like, and are lodged in a branchial chamber 

 formed by a fold of the mantle, which may or may not be so dis- 

 posed as to form an anterior and posterior tube or " siphon," for 

 the entrance and escape of water from the branchial chamber. The 

 foot in the Prosobranchiates is large and adapted for creeping, and 

 the sexes are distinct. Those members of the order which possess 

 an anterior siphon for the admission of water to the branchial cavity, 

 have the mouth of the shell notched in front, or produced into a 

 canal in which the siphon is lodged (fig. 642) ; while a similar notch 

 or canal may exist at the posterior end of the shell-aperture (fig. 

 643). The shell in these forms is said to be " siphonostomatous." 



