GENERAL NOTES. 3 



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. 



Nearly all Molluscs, except the Pelecypods, have a very remarkable Tlie 

 structure developed in the floor of their mouth-cavities ; on a basis 

 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 odontopliore, tongue, radula, or lingual ribbon (see fig. 3).* 



Eyes may be absent, as in nearly all the headless Pelecypods ; 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 Cephalopods are at 

 first pit-like, or pass through a stage which is permanent in Nautilus, 

 one of the geologically oldest types. 



Cephalic eyes have been noticed in Mytilus and the PteriidcB. 

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

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

 pods (e.g. Pecteri) ; 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 

 appearance in the form of an open pit, the mouth of which gradually ^^'^^o- 

 closes up, leaving only a narrow slit in communication with the 



* A framed series of photographs, illustrating different kinds of radulaj, is 

 placed on the east wall of the gallery. 



B 2 



