GENERAL NOTES. 



and so give rise to plates; these, when well developed, are best 

 seen in the division to which the Oyster and the Mussel belong, 

 and which, therefore, has been called the division of the plate- 

 gilled Mollusks, or Lamellibranchiata, but now called Pelecypoda. 

 Where the body is coiled or twisted on itself, as so often happens, 

 the gills of one side may be altogether lost. Sometimes, as in 

 Phyllirhoe, when the body is small and its wall thin, the gills 

 disappear altogether, and there is no special breathing-organ ; in 

 others the loss of the gill is compensated by the formation by the 

 mantle of a lung ; this is most often seen in the forms that live 

 on land. 



But these so-called gills may have other functions : in the Pele- 

 cypods, where there is no head and no special means by which the 

 creature can obtain food, the delicate waving filaments or cilia with 

 which they are covered cause currents in the surrounding water, by 

 means of which minute organisms are brought to the mouth. 



All Mollusks, except the Pelecypods, have a very remarkable Theodon- 

 structure developed in the floor of their mouth-cavities ; on a °P ore ' 

 basis of cartilage, which may be moved backwards and forwards by 

 muscles, there is developed a stout horny plate, which may be of 

 considerable length, and which has its upper surface covered with a 

 number of more or less fine, flattened, or spiny outgrowths, which 

 are known as teeth. This is the odontophore, tongue, or radula 

 (see fig. 12). 



Eyes may be absent, as in the headless Pelecypods ; but in the The eyes. 

 rest they are generally present, and may be more or less well 

 developed. An instructive series of stages is exhibited by the 

 Cephalopoda. In Nautilus the eye remains an open pit ; in 

 Ommatostrephes two chambers appear, the anterior of which is 

 bounded posteriorly by the lens, and is open to the exterior, so 

 that sea-water enters it; in Sepia, finally, the anterior chamber 

 becomes closed in front. We may observe that the eyes of all 

 Cephalopods are at first pit-like, or pass through a stage which is 

 permanent in Nautilus, one of the geologically oldest types. 



Eyes of a more complicated structure, which are modified ten- 

 tacles, are sometimes found on the edges of the mantle in Pele- 

 cypods (e. g. Pecten) ; these eyes resemble those of Vertebrates, 

 and differ from those of most invertebrate animals in having the 



