PHYLOGENY 



originally posterior have moved to an anterior position, and vice versa. 

 The class Scaphopoda, or tooth shells, is a small group which is very much 

 specialized for digging. All species are marine, and there are only a few 

 of them. The class Lamellibranchiata includes the bivalves, the familiar 

 clams, mussels, and their allies. They are compressed bilaterally, and en- 

 closed within a two-valved shell, hinged dorsally, and secreted by the 

 two lobes of the mantle. The gills are immensely enlarged to form broad 

 ciliated tracts which produce a current for feeding upon plankton and 

 detritus. The head is very much reduced. The foot is generally wedge- 

 shaped, unlike the broad, flat foot of the more primitive molluscs, but it 

 can be protruded between the valves to serve a locomotor function. In 

 some species, it is specialized for burrowing. 



The class Cephalopoda includes the octopi, the squids, the chambered 

 Nautilus, and its extinct allies. They are the most complex of mollusca, 

 comparing favorably with the most advanced insects and vertebrates in 

 degree of complexity. The visceral hump, like that of the gasteropods, is 

 much enlarged, but it is still symmetrical. The mouth is surrounded by 

 a ring of tentacles for seizing food. The head and foot are fused so com- 

 pletely that there is no agreement among specialists as to which these 

 tentacles are derived from. There is a siphon by which water may be 

 forcibly ejected from the mantle cavity. The coelom is better developed 

 than in most molluscs. The nervous system is highly centralized, and 

 efficient eyes, superficially quite similar to those of vertebrates, are pres- 

 ent. The more primitive members of the class have a chambered shell of 

 which the animal always lives in the most recently secreted chamber. 

 Each chamber corresponds to a stage in the growth of the animal, like 

 the successive moults of an arthropod. The shelled cephalopods are today 

 represented only by a single genus. Nautilus, but this is the last remnant 

 of a once dominant group in the oceans of the world. The nautiloids 

 appear in the fossil record in the late Cambrian, and rapidly assumed a 

 dominant position in the marine fauna. They reached their peak of de- 

 velopment in the Silurian, and then gradually declined. During the De- 

 vonian, they gave rise to another suborder, the Ammonoidea, with which 

 they competed unsuccessfully for a long period of the earth's history. The 

 ammonites were the dominant marine molluscs throughout most of the 

 Mesozoic Era, yet they dwindled and became extinct near the end of 

 the Cretaceous. Meanwhile the nautiloids survived in small numbers, and 

 are known from a single genus at the beginning of the Cenozoic Era. No 

 longer being in competition with the ammonites, the nautiloids underwent 

 a rapid evolution at the beginning of the Cenozoic, for seven new genera 

 appear during the Paleocene epoch. But only one of these. Nautilus, has 

 survived to the present time. Meantime, forms with the shell much re- 

 duced and internal, the octopi and squids, have become the principal 

 cephalopods. 



Phylum Annelida. The second main branch of the protostome line is 

 the annelid-arthropod branch. The phylum Annelida is best known by the 

 common earthworms, but it includes a wide variety of worms arranged in 

 four classes. All annelids are segmented, that is, the functional and struc- 



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