MOLLUSC A. 455 



instances) and forms an area upon which the shell begins to develope, 

 while its edges thicken and extend, and eventually form the edges of 

 the mantle : the foot also enlarges in a manner characteristic of the group 

 to which the Veliger belongs. It generally bears an operculum upon 

 the dorsal aspect of its hinder extremity in Pteropoda and Gastropoda, 

 whilst in Lamellibranchiata a byssus gland appears as an epiblastic in- 

 volution at its edge. Both operculum and byssus gland may persist or 

 disappear. Individual peculiarities occur in different groups e. g. Chiton, 

 Dentalium, and many Lamellibranchiata possess a flagellum or tuft of 

 cilia projecting from the centre of the velar area ; a posterior tuft of 

 cilia may also occur. The normal course of development may be abbre- 

 viated or modified in freshwater, terrestrial or viviparous forms. The 

 Cephalopoda have a modified development owing to the presence of food- 

 yolk. The yolk-sac appears to occupy the position of the foot, and the 

 velum is either not represented at all, or only in a very rudimentary 

 manner. The edge of the blastoderm is ciliated in some instances. The 

 shell-sac is formed by two dorsal folds, and does not appear to be homo- 

 logous with the shell-gland which is therefore not represented. 



The great majority of Mollusca are water-breathers, and marine in 

 habitat : some Gastropoda and Lamellibranchiata are fluviatile and lacus- 

 trine : and a few Gastropoda are terrestrial and air-breathers. The 

 remains of Cephalopoda, Pteropoda, Gastropoda and Lamellibranchiata occur 

 in the lower Silurian strata. 



The Mollusca may be grouped in two main divisions, the Glosso- 

 phora and Lipocephala of Ray Lankester, sometimes also termed Cephalo- 

 phora and Acephala. The division Lipocephala comprises the single class 

 Lamellibranchiata ; the division Glossophora the four classes Cephalopoda, 

 Scaphopoda, Pteropoda and Gastropoda. 



The Pteropoda and Cephalopoda are associated together by Professor Lankester 

 as Cephalopoda the class here called Cephalopoda receiving the name Siphonopoda. 

 This arrangement has not been followed because the Pteropoda seem to be a group 

 which are really asymmetrical and have acquired an external symmetry. External 

 asymmetry is preserved in the Limadnidae with their spirally twisted visceral dome 

 and shell ; and in the spiral shell of the larval Cymbulia and Tiedemannia. The 

 occasional presence of an operculum in the adult and larva; the asymmetry 

 observable in the osphradium, and nephridium, and in the position of the anus ; 

 the structure of the genital organs are points in which the members of the class 

 strongly resemble the Gastropoda, as they do also in development. The cephalic 

 appendages of the Pteropoda have probably nothing to do with the arms of a 

 Cephalopod. The latter are supplied by the pedal nerve, whilst the buccal cones 

 of Clio, the only Pteropod very accurately monographed, are supplied by the 

 cerebral ganglia. It is possible that the arms of the Cephalopod represent the epi- 

 podium. In this case they may be compared with the epipodia of Thecosomatous 

 Pteropoda which have grown round the mouth. And in the three lobes of the 



