446 MR FRANK E. BEDDARD ON THE STRUCTURE OF 



types is, according to Grenacher's figures, that in the Isopoda the vitreous body 

 is short, more or less oval in form, and is made up of two closely opposed halves 

 in correspondence with the two cells which secrete it ; the vitreous body is also 

 separated by a considerable interval from the chitinous product of the retinula 

 cells — the rhabdom. In the Decapoda, on the other hand, the " vitrella " is 

 composed of four cells, and the vitreous body is consequently separable into four 

 portions, each of which is a product of one of the four cells. The vitreous body 

 (or crystalline cone) is immensely elongated and conical in form, and is in actual 

 contact with, or at most separated by an extremely minute interval from, the 

 upper extremity of the rhabdom ; the latter is, like the corresponding structure 

 in the Isopod eye, formed as a secretion from a number of retinula cells. One 

 of the main points concerning which Dr Patten is at issue with Dr Grenacher 

 relates to the formation of the crystalline cone and the rhabdom in the Decapod 

 eye. Grenacher, as already stated, regards this structure as being made up of 

 two different parts ; the upper crystalline cone is secreted by the four cells of 

 the " vitrella ; " the lower dilated portion, known as the striated spindle or 

 rhabdom, is the product of the numerous cells of the retinula. 



Patten, on the contrary, brings forward reasons for believing that there is 

 no distinction between these two parts of the axial crystalline rod, and con- 

 siders that they form a continuous structure, both being formed by the cells of 

 the vitrella, which he terms retinophoral cells ; the retinula cells of Grenacher, 

 on this hypothesis, sink to the level of mere pigment cells which surround the 

 axial retinophorse. 



In a recent review of Patten's work this conclusion is considered to be 

 probably correct, but to require some further confirmation. # 



Dr Patten unfortunately did not direct his attention to the unravelling of 

 the structure of the Isopod eye, and this is to be regretted, as in his opinion 

 the eye in that group is in a primitive condition. 



One of the strongest reasons for accepting Grenacher's theory of the 

 Arthropod eye is its extreme simplicity and the exact correspondence which it 

 enables one to demonstrate between the structural features of different Crus- 

 tacean eyes. It is quite easy, for example, by an inspection of his figures to 

 see the essential similarity, masked only by some difference in detail, between 

 each element of the eye of Porcellio and the rather more complicated elements 

 of the Decapod eye. 



If we are to accept Dr Patten's views it is quite impossible to compare, 

 except in the most general way, the eye of an Isopod with that of a Decapod. 

 Dr Patten claims as a merit of his own views, " that they lead to the reduction 

 of the essential parts of all visual organs to one structural plan, which can be 

 followed through the whole animal kingdom from the lowest to the highest." 



* Quart. Jour, Micr. Sci, Oct. 1886. 



