398 PHYLUM MOLLUSC A. 



From primitive forms, related perhaps to Chiton, Mollusca have 

 diverged in two directions. In Gasteropoda, Scaphopoda, and 

 Cephalopoda, the radula present in the primitive Isopleura is 

 retained, and the head region becomes well developed ; these three 

 classes are therefore often placed together as Glossophora or Odonto- 

 phora, in contrast to the Lamellibranchiata (Lipocephala or Acephala), 

 where the radula has disappeared, and the head region remains un- 

 developed. As already seen, however, the lowest Lamellibranchs 

 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 Gastero- 

 pods, 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. 



Class I. GASTEROPODA. 



Molluscs with a usually well-developed head region with 

 tentacles and odontophore. The foot is usually a fiat median 

 sole on which the animal creeps ; it is often divided into pro-, 

 meso-, and metapodium. Most are unsymmetrical, but there 

 is a primitive bilateral symmetry in Isopleura and a secondary 

 superficial bilateral symmetry in some pelagic forms, such as 

 Heteropods. The mantle or covering of the visceral sac usually 

 forms a well-marked fold or flap where the visceral sac joins 

 the head and foot, and thus encloses a mantle cavity. In most 

 cases the shell is a single piece ; in Chitons there are eight 

 pieces ; in many cases the shell is rudimentary or absent. 

 There is usually a trochosphere and veliger larva, except in 

 terrestrial forms. 



Sub-class I. GASTEROPODA ISOPLEURA. 



The Isopleura are marine Gasteropods, more or less elongated 

 inform, with bilateral symmetry. The symmetry is not only 

 seen in the form of the body, but in the numerous ctenidia, the 

 paired nephridia, auricles and genital ducts. The shell con- 

 sists of eight pieces. The mouth is anterior ; the anal and 

 nephridial apertures are posterior. The ma?itle, which bears 

 cuticular spicules, covers at least a great part of the body. 

 The nervous system consists of a cerebral commissure and 

 two paired longitudinal cords (pedal and visceral], with 

 ganglionic cells but at most very slightly developed ganglia, 

 which run the whole length of the body. Of these faired cords 



