392 
NOTES. 
The two subkingdoms of the earlier edition are thus divided into four. The 
Classes remain the same, except the Annelida. 
137 The most important genera are Terebratula , Rhynchonel la, Discina , 
Lingula , Orthis, Spirifer, and Productus. The first four have representa¬ 
tives in existing seas. Most naturalists now admit their affinity to the 
worms, although some still keep them in the subkingdom Mollusca. 
138 qq iere are some exceptions: the Oyster is unequivalved, and the Pecten 
equilateral. 
139 The chief impressions left on the shell are those made by the muscles— 
the dark spots called “ eyes ” by oyster-men; the pallial line made by the 
margin of the mantle; and the bend in the pallial line, called pallial sinus, 
which exists in those shells having retractile siphons, as the Clam. 
140 The Clam is the highest of Lamellibranchs, and the Oyster one of the 
lowest. The Mya arenaria , or “Soft Clam,” has its shell always open a 
little; while Venus mercenaria , or “ Hard Clam,” keeps its shell closed. 
141 The Slug has no shell to speak of, and the Chiton is covered with eight 
pieces. It may be remembered, as a rule, that all univalve shells in and 
around the United States are Gasteropods, and that all bivalves in our rivers 
and lakes, and along our sea-coasts (save a few Brachiopods), are Lamelli¬ 
branchs. 
142 Hold the shell with the apex up and the mouth towards the observer. 
If the mouth is on his right, the shell is right-handed or dextral, if on his 
left, sinistral. In other words, a right-handed shell is like a right-handed 
screw. 
143 Instead of a strong breathing-tube with a valve, answering for a force- 
pump and propeller, as in the Cuttle-fish, it has only an open gutter made by 
a fold in the mantle, like the siphons of the Gasteropods. The back cham¬ 
bers are filled with nitrogen gas. 
The common Poulpe has two thousand suckers, each a wonderful little air- 
pump, under the control of the animal’s will. 
144 The order of the classes is one of relation rather than of rank. They 
cannot be arranged serially. The Myriapods have a worm-like multiplica¬ 
tion of parts, degrading them, and their nervous system is simpler than that 
of Caterpillars; yet their heads show a close relationship to Insects. The 
Arachnids include some lower forms than Myriapods; on the other hand, 
for their wonderful instincts, Owen places them above the Insects. They are 
closely allied to Crustaceans, and stand more nearly between Crustaceans and 
Insects than between Myriapods and Insects. 
145 Certain Crabs live on dry land, but they manage to keep their gills 
wet. 
146 The student should remember that this threefold division is not equiva¬ 
lent to the like division of a vertebrate body. 
147 Each ring (called somite) is divisible in two arcs, a dorsal and ventral, 
and each arc consists of four pieces. 
148 The four pairs of legs in Arachnids answer to the third pair of maxillae 
and the three pairs of maxillipedes in the Lobster. The great claws of 
Scorpions are the first maxillae of the Lobster, as are the pedipalpi of 
Spiders. 
