336 



MOLLUSCA 



near the head, with the result that the auricle is forwards, the aorta behind. 

 The Testacellidye have the lungs at the posterior end of the body. Occasionally 

 streptoneurous conditions occur (Chilina). The lung, the most characteristic 

 feature of the order, has already been mentioned (p. 326). 



Many pulmonates are aquatic, but since they have no gills they must occa- 

 sionally come to the surface to fill the lung with air, but some, which live at 

 great depths in the Swiss lakes and consequently cannot reach the surface, 

 use the skin and to some extent the lung for water-breathing. Several genera 

 (Planorbis, Pulmobranchia, Siphonaria) have formed secondary gills. 



Sub Order I. STYLOMMATOPHORA. Four retractile tentacles, eyes at 

 the tips of the second pair. The HELICID^, a well-developed shell. Helix,* 

 many hundred species. Pupa* Acliatina, Bulimus, many tropical species. 

 LiMACiDyE. Shell reduced, completely concealed in the mantle. Li max,* 

 Arion* Sub Order II. BASSOMATOPHORA. Only one pair of non- 

 retractile tentacles, eyes at their base. LIMN^EHXE, pond snails. Limncea,* 

 Planorbis.* 



Class V. Cephalopoda. 



The Cephalopoda are distinguished among the molluscs by their size 

 and high organization. The majority measure, including the arms, from 



FIG. 347. FIG. 348. 



FIG. 347. Octopus tonganus from the side (after Hoyle). Funnel and mantle fold 

 to the right; back and eyes on the left. 



FIG. 348. Loligo kobiensis, ventral view (after Hoyle). 



eight inches to three feet in length, a few are smaller (two to seven inches), 

 while especially rare are the giants, some of which may be over forty feet 

 long. For a long time these large species were only known from the tales 

 of sailors. In the last half-century some of these forms, belonging to the 

 genus Architeuthis, have been stranded on the coasts of Newfoundland 



