ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 231 



and twenty. Though it might be supposed that their organization 

 was primitive and simple, their anatomy is found to be exceedingly 

 complicated, exhibiting all the most important structural elements of 

 the eyes of the higher Vertebrata. Mr. S. J. Hickson, believing that 

 these eyes deserve more mention than is usually made of them, 

 describes in some detail the anatomy of the various parts which 

 compose the eyes of Pecten maximus, P. jacobceus, and P. opercularis, 

 illustrated with two plates of eleven figures. Under the head of 

 " General Considerations," the author points out some of the inter- 

 esting morphological peculiarities of these eyes. It is, in itself, a 

 remarkable thing to find a large and variable number of eyes situated 

 on an area at some considerable distance from any central nerve- 

 ganglion ; and, when it is remembered that the class and even family 

 (with one other exception — Spondylus) to which the genus belongs, 

 possess no organs of vision at all in the adult condition, it is alto- 

 gether surprising that they should be of such extraordinary com- 

 plexity as they have proved to be. The high structural development 

 that this eye has attained is, however, not so remarkable as the fact 

 that in many ways it differs from the ordinary Invertebrate eye, and 

 resembles that of the Vertebrata. 



In the first place, the lens is built up of a large number of distinct 

 nucleated cells, which undergo a flattening at its circumference very 

 similar to that found in the eye of the Vertebrata. Whether the lens 

 is developed from the cells of the epiblast, as in the Vertebrata, or 

 from the mesoblast, must at present be left unsettled, but it will pro- 

 bably be found, when the development of the eye is studied, that in 

 this respect also it resembles the eyes of the Vertebrata. The tapetum, 

 a structure which is of considerable importance 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 consider- 

 able 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 Cepha- 

 lopods the pigment-layer is situated in front of the rods, and the 

 nerve-fibres enter the rods from behind. In the eyes of the Gastero- 

 poda, the Crustacea, &c., down to the simplest form of eye, such as 

 that of the Kotifera, 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 



