INTRODUCTION. H 



Class 3. Pteropoda. 



In this class the organs of locomotion are a pair of flesh j, wing-like fins 

 at the sides of the neck. It includes but one order. PI. 74, fig. 22, Clio. 



Xllass 4. Cej^Tialopoda. 



The body is formed like a sack, the head is surrounded with long arms, 

 used for prehension and. locomotion ; the eyes are large, and the mouth 

 armed with a strong beak. There is but one order in the class. 

 PI. U.fig. 23, Octo2)us. PI. TG, figs. 16, 17, 75, 76, 77. 



Division III. Articulata. 



Class 1. Annelida. . 



"Worms with cold ^;ed blood, the body lengthened and divided into rings, 

 of which the first forms the head. Articulated feet are never present, but 

 many of the genera are provided with stiff movable bristles. Nearly all 

 live in water; the Limibria.is (earth-worm) is, however, an exception. 

 Some live in tubes which they form in the bottom, some form them out of 

 agglutinated particles of sand, and others secrete a kind of shell. Some of 

 these, on account of the shell, have been thoughtlessly classified with the 

 MoUusca. 



Okder 1. Abkanchiata. PL 7-1, fig. 2-i, Hirudo (leech). 



" 2. DoEsiBRANCHiATA. " " 25, Aplirodite. 



" 3. TuBicoLA. " " 26, AmjyUtrite. 



Class 2. Cirrho])oda. 



This class was formerly included in the MoUusca, on account of the shell ; 

 its aflinities are, however, with the Crustacea. They are attached to stones, 

 corals, crabs, shells, the bottoms of ships, whales, and marine tortoises. 

 PI. 74, fig. 27, and jpl. 76, fig. 52, Anatifa. PI. 76, fi^. 54, Balanus. 



Class 3. Crustacea. 



This class includes the articulata with articulate feet, which breathe by 

 means of gills. Their circulation is double. There are two pair of 

 antennesj and never less than five pair of feet, and there are frequently 



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