THE EYE OF PECTEN. 11 



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-tibres 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 Invertebrata (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 structure 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 

 different from that of the Vertebrate eye. The Vertebrate 

 eye is formed in the embryo from a hollow process given off 

 from the brain, and the future eye-cup is formed by an in- 

 vagination of this process. It is impossible for the eyes of 

 Pecten or Onchidium to be formed by any process similar 

 to this. Thus, in the young state these eyes are essentially 

 different from those of the Vertebrata, and the resemblance 



* Semper, ' Uber sehorgane vom Typus der Wirbelthieraugen auf dem 

 Riicken der Schnecken.' Wiesbaden, 1877. 



