50 



MOLLUSCA. 



of two parts placed side by side, (1) the penis with the continuation 

 of the ciliated groove ; and (2) the gland rod which encloses a longish 

 gland. The oviduct (fig. 90) is more complicated, inasmuch as a 

 large albumen gland and a receptaculum seminis open into it; its 

 dilated terminal part acts as a vagina. 



The Heteropoda are exclusively pelagic animals, and they are often 

 found in great numbers in the warmer seas. They are somewhat 

 clumsy in their movements, which are effected with the ventral 

 surface uppermost by oscillations of the whole body and the fin. 

 They are all carnivorous. When the tongue is protruded, the lateral 

 teeth fly apart from one another like the limbs of forceps, and when 

 retracted they again fall together. By means of these prehensile 

 movements small marine animals are seized and drawn into the 

 mouth. 



Fam. Pterotracheidse. Oirinnrla mcditerranea Lam., PterotraeJiea eoronata 

 Forsk. Mediterranean. 



Fain. Atlantidae. Atlanta Peronii Less., Mediterranean. 



Al 



FIG. 522. Arion empiricoriim (regne animal). Al, respiratory aperture. 



Order 3. PULMONATA.* 



Terrestrial and fresh-water Gastropods with lung which is placed in 

 front of the heart. Hermaphrodite. 



The roof of the mantle cavity, as in the Cyclostomidce, is provided 

 with a network of vessels for aerial respiration. The mantle (pul- 

 monary) cavity opens to the exterior on the right side by a respiratory 

 aperture (fig. 522.) The mantle cavity of the young of the fresh- 

 water Pulmoiiates is at first filled with water, and only later with 

 air. Some species of Planorbis and Limnceus retain, during the 

 whole time of their life, the ability to breathe both in air and water 

 (some Limnaius, with lungs full of water, have been dredged up at 



* L. Pfeiffer. ' Monographia Heliceorum viventium." Leipzig, 1848-1869; acti 

 ' : Monographia Auriculaceorum viventium." Cassel, 1856. 



A. Rossmassler, " Iconographie der Land-imd Susswassermolhisken Europas." 

 Leipzig, 1835-1859. 



Ferussac et Deshaj'es. " Histoire naturelle generate et particuliere des 

 Moliusques terrestres et fluviatiles." Paris, 1829-1851. 



