AMPHINEURA. 369 



becomes well developed ; these classes are therefore often placed 

 together as Glossophora 01 Odontophora, in contrast to the Lamelli- 

 branchiata (Lipocephala or Acephala), where the radula has disappeared, 

 and the head region remains undeveloped. As already seen, however, 

 the lowest LameUibranchs have a flattened creeping foot and simple 

 feathery gills, in these respects resembling Gasteropods. There is also 

 much reason to believe that the Scaphopoda arose from a stem common 

 to them and the lowest Gasteropods, which are central unspecialised 

 forms. The Cephalopoda are the most highly specialised of all the 

 Mollusca, and in their existing forms at least not nearly related to the 

 other classes. 



The Mollusca may therefore be classified in outline as follows ; but 

 the relation of LameUibranchs and Dentalium to Gasteropods should 

 be kept in mind. 



/ Class Amphineura. 



Order 1. Polyplacophora, e.g. Chiton. 



Order 2. Aplacophora or Solenogastres, e.g. Neomenia. 



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Class Gasteropoda. 



Order 1. Prosobranchia, including the pelagic Heteropoda. 



Order 2. Pulmonata. 



Order 3. Opisthobranchiata, including the pelagic Pteropoda 



Class Scaphopoda. 



Class Cephalopoda. 



Order I. Tetrabranchiata. 

 Order 2. Dibranchiata. 



Lipocephala. — Class Lamellibranchiata. 



The characters of the trochosphere larva which occurs in 

 many Molluscs, and many of the features of the simple 

 Amphineura, suggest that Molluscs arose from some worm 

 type, but beyond this all is hypothesis. 



Class I. Amphineura. 



Syn. — Gasteropoda Isopleura, e.g. Chiton. 



General Characters. — The Amphineura are marine 

 Molluscs, more or less elongated in form, with bilateral 

 symmetry. They are often ranked along with Gasteropods. 

 The mouth is anterior ; the anal and nephridial apertures are 

 posterior. The mantle, which bears cuticular spicules, covers 

 at least a great part of the body. The nervous system consists 

 of a cerebral commissure and two faired longitudinal cords, 

 24 



