INTRODUCTION. 1l 
Class 3. Pteropoda. 
In this class the organs of locomotion are a pair of fleshy, wing-like fins 
at the sides of the neck. It includes but one order. Pl. 74, fig. 22, Clio. 
Class 4. Cephalopoda. 
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. 7A, jig. 23, Octopus. Pl. 76, jigs. 16, 17, 75,76, TT. 
Division LI. Arricunata. 
Class 1. Annelida. 
Worms with cold red 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 Zwmbricus (earth-worm) is, however, an exception. 
Some live in tubes which they form in the bottom, some form them out of 
agelutinated particles of sand, and others secrete a kind of shell. Some of 
these, on account of the shell, have been thoughtlessly classified with the 
Mollusca. 
Orper 1. ABRANCHIATA. Pl. 74, jig. 24, Hirudo (leech). 
“¢ 2. Dorstprancuiata. «25, Aphrodite. 
Ss. oes’ EOBECOLA. ee “26, Amphitrite. 
Class 2. Currhopoda. 
This class was formerly included in the Mollusca, 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. 
Pl. 74, fig. 27, and pl. 76, fig. 52, Anatifa. Pl. 76, fig. 54, Balanus. 
Class 8. Crustacea. 
This class includes the articulata with articulate feet, which breathe by 
means of gills. Their circulation is double. There are two pair of 
antennz, and never less than five pair of feet, and there are frequently 
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