226 AN INTRODUCTION TO ZOOLOGY. 



The above is the old-fashioned classification of the 

 Gasteropoda. The classificatioa framed by Ray 

 Lankester is founded on the amount o£ twisting un- 

 dergone by the animalj and especially by its nervous 

 system, in consequence of the presence of the shell. 



The Pteropoda (wing-footed molluscs) receive their 

 name from the wing-like expansions of the foot 

 {epipodia). They are animals that float, anterior end 

 uppermost, near the surface of the water, and are 

 sometimes called sea-butterflies from the pretty ap- 

 pearance of their " wings." The body is straight, or 

 nearly so. Some have a shell [Tliecosomata) , while 

 others are without {Gymnosomata) . The Pteropods 

 are sometimes classed with the Gasteropods, but are 

 included with the Cephalopods by Lankester. 



The Cephalopoda are the most highly developed 

 ■of the Mollusca. They have a distinct head, with 

 .a, circle of sucker-bearing arms round the mouth, 

 which represent the anterior part of the "foot" of 

 other Mollusca, while the rest of the foot is modi- 

 fied into a tubular form called the funnel, which 

 serves as an excretory and exhalant siphon. The 

 nervous system is highly developed, and there are 

 very large eyes. The order Tetrabranchiata attained 

 its greatest development in past times, and is 

 nearly extinct : while the fossil genera are many 

 and large, including Orthoceras, Goniatites, Ammonites, 

 Geratif.es, Turrilites, etc., there is only one living 

 genus, the Nautilus. (This nautilus is the Pearly 

 Nautilus : the Paper Nautilus belongs to the next 



