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 Terebr alula, Rhynchonetta, Discina, Lin- 

 gula, Orthis, Spirifer, and Productus. The first four have representatives in 

 existing seas. Most naturalists now admit their affinity to the worms, although 

 some still keep them in the subkingdom Mollusca. 



138 There 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 deztral, 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 multiplication 

 of parts, degrading them, and their nervous system is simpler than that of 

 Caterpillars ; yet their heads show a close relationship to Insects. The Arach- 

 nids 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 into two arcs, a dorsal and ventral, 

 and each arc consists of four pieces. 



147a The eye-stalks were formerly considered to be appendages, but are no 

 longer so regarded. 



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 Scor- 

 pions are the first maxillae of the Lobster, as are the pedipalpi of Spiders. 



