MOLLUSCA 



201 



Most bivalves, however, have numerovis oceUi arranged Hke a coronet 

 around the margin of the mantle (pallial eyes) ; these may be numbered in 

 hundreds and are probably to be looked upon as modified tentacles. In some 

 foi-ms, such as Lima, they are very primitive. This bivalve is provided with 

 30 simple cup-shaped depressions, 0-3 mm. in diameter, lined with sensory and 

 pigmented cells forming primitive cu2:)ulate eyes ; in others such as the fresh- 

 water mussel, Anodonta, eyes are completely absent. Most of these types are 

 relatively shiggish and quiescent, but in actively swimming forms the eyes may 

 be more elaborate. This development is well exemplified in svich bivalves as 

 the comnion scallop, Pecten, and Spondylus, both of which possess eyes 

 unique among IMolluscs. The pallial eyes are arranged in a single row around the 

 edge of the mantle ; when they are exposed as the 

 shell gapes they shine as brilliant emerald green or 

 purple spots, 0-6 to 0-8 mm. in diameter ; 28 to 46 

 have been counted in the upper half of the mantle, 

 15 to 36 in the lower, and each is borne on a con- 

 tractile pedicle (Fig. 190). These are of remarkable 

 complexity with a well-formed inverted retina 

 which appears to be much more elaborate than the 

 visual demands of the shell-fish would seem to 

 warrant (Fig. 123). Each is comiected by means of 

 its optic nerve with a large circumpallial nerve and 

 so with the branchial ganglion.^ An anomalous 

 occurrence in certain lamellibranch molluscs (the 

 Noah's-ark shell. Area ; Pectunculus), is that of 

 unicellular ocelli grouped together in a spherical 

 mass constituting an aggiegate eye which 

 bears a superficial resemblance to a compound eye - 

 (Carriere, 1885 ; Patten, 1886 : Hesse, 1900). 



Pearly 



Nautilus 



Fig. 191. — The 

 Nautilus, 

 pompilius. 



The animal is seen in 

 section. Above is the spiral 

 shell. E, the eye, which 

 opens to the exterior ; Si, 

 siphon ; T, tentacles (after 

 Owen). 



The CEPHALOPODS (cuttlefish, etc.) 

 usually exhibit the most elaborate visual 

 organs found among Molluscs, a characteristic 

 understandable in view of their active be- 

 haviour and carnivorous habits ; only one species living at abyssal 

 ocean depths is knoAMi to lack eyes, Cirrofhauma murrayi? They are 

 the most specialized of the molluscs and i:)resent considerable diversities 

 of type, but most of them are freely SAvimming and they all have a 

 \vell-develoj)ed head furnished with numerous "arms" bearing tentacles 

 or suckers and provided with eyes and other sensory structures. 



In the pearly nautilus of the seas of the Far East, the sole survivor of the 

 primitive and almost extinct tetrabranchiate Cephaloi^ods which were largely 

 Palaeozoic in distribution, the eye retains its ancestral simplicity and consists 

 merely of an epithelial depression with a tiny aperture 2 mm. in diameter 

 (Figs. 100 and 191) ; it is situated on a raised flat peduncle which is also provided 

 witli two " ocular tentacles "', probably olfactory in function. 



In the more recent and voraciously carnivorous dibranchiate 

 Cephalopods, however, such as the common cuttlefish, Sepia, the 



Ayiodonta 



Spondylus 



Sepia 



p. 1.51. 



J23. 



