GENERAL NOTES. 3 
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 Lamelli- 
branchs, 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 Molluscs, except the Lamellibranchs, have a very remarkable The 
structure developed in the floor of their mouth-cavities ; on a basis radula. 
of cartilage, which may be moved backwards and forwards by muscles, 
there is developed a 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, radula, or lingual ribbon (see fig. 3).* 
Eyes may be absent, as in nearly all the headless Lamellibranchs ; The eyes, 
but in other Molluscs 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 Omma- 
tostrephes 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 Oephalopods are at 
first pit-like, or pass through a stage which is permanent in Nautilus, 
one of the geologically oldest types. 
Cephalic eyes have recently been noticed in Mytilus and Avicula. 
Eyes of a more complicated structure, which are modified ten- 
tacles, are sometimes found on the edges of the mantle in Lamelli- 
branchs (e.g. Pecten) ; these eyes resemble those of Vertebrates, 
and differ from those of most invertebrate animals in having the 
fibres of the optic nerve entering the distal and not the proximal 
ends of the retinal cells. Eyes of a similar construction are to be 
found on the back of the shell-less Oncidium, and may be about 
one hundred in number. 
Eyes of a remarkable character on the shells of some of the 
Chitons appear to be modified from tactile organs, and are in- 
nervated like the ordinary molluscan eye ; they sometimes occur in 
enormous numbers, more than ten thousand being present on one 
animal (see wax-model, Case 2). 
In Cephalopods the ear, like the eye, is known to make its first Organ of 
* A framed series of photographs, illustrating different kinds of radulse, is earm S* 
placed on the east wall of the gallery. 
B 2 
