THE EYE OF PECTEN. 
453 
portance to animals which are nocturnal or aquatic in habit, 
has hitherto been described only in the Vertebrata. That 
Pecten possesses a tapetum as highly developed as any 
found amongst the Vertebrata is anatomically a point of 
considerable interest; but it also indicates to a certain 
extent the physiological capability of the eye. 
The chief interest, however, lies in the relative positions of 
the optic nerve, the retina, and the pigment. In the eyes of 
the Cephalopods the pigment layer is situated in front of the 
rods, and the nerve-libres enter the rods from behind. In 
the eyes of the Gasteropoda, the Crustacea, &c., down to the 
simplest form of eye, such as that of the Rotifera, the same 
relationship of these parts holds good. In the Vertebrata, 
however, their relative positions are reversed ; the optic nerve 
pierces the retina, and distributes itself over the front of the 
retina, whilst the pigment’is situated behind it. In Pecten 
the relationship of these parts is the same as that in the 
Vertebrata; the nerve passing up the side of the eye-cup 
bends over, and spreads itself over the anterior surface of the 
retina. The pigment also is situated behind the retina. 
Pecten is not*, however, the only Invertebrate whose eyes 
are built up on this type. Semper^ has recently pointed out 
that on the backs of certain slugs (Onchidium) a number of 
eyes are found, and that in these the nerves pass to the front 
of the retina before being distributed. On account of this 
distribution of the optic nerve he says that they belong to 
the Vertebrate type of eye (typus der Wirbelthieraugen), so 
that two animals are now known, each belonging to a large 
and important class of In vertebrata (Gasteropoda and Lamel- 
libranchiata respectively) that possess eyes which are built 
up on this type. The eyes of Pecten are even more deserving 
of the name of Wirbelthieraugen than those of Onchidium, 
for they are much more highly developed, and possess, in 
addition to this relationship of the nerve and retina, other 
Vetebrate peculiarities. The lens is multicellular, a character 
which, although not unknown amongst the Invertebrates, is 
much more characteristic of the Vertebrata. The tapetum, 
too, a stru(:ture which doubtfully exists in any other Inverte- . 
brata is found in Pecten and some Vertebrates. But, although 
the application of this word Wirbelthieraugen to these eyes 
is convenient for the adult condition, it must be carefully 
remembered that the development of these eyes is essentially 
* Semper, ‘ Uber sehorgane vom Typus der Wirbelthieraugen auf dcm 
Riicken der Schnecken.’ Wiesbaden, 1877. 
