140 



Mr. B. T. LoAvne. On the 



[Apr. 12, 



Of the four distinguishing characters of Thaumatocrwus, therefore, 

 one appears in one or perhaps in two genera of Comatulce ; another is 

 not to be met with in any Comatida, though occurring in certain 

 stalked Crinoids ; while the two remaining characters are limited to 

 one family of the Paleeocrinoids, one of them being peculiar to 

 one, or at most two genera, which are confined to the Lower Silurian 

 rocks. 



Their reappearance in such a specialized type as a recent Comatula 

 is, therefore, all the more striking. 



IV. " On the Structure and Functions of the Eyes of Arth.ro- 

 poda." By B. THOMPSON Lowne, F.R.C.S., Lecturer on Phy- 

 siology in the Middlesex Hospital Medical School, Examiner 

 in Physiology in the Royal College of Surgeons, formerly 

 Arris and Gale Lecturer on Anatomy and Physiology in 

 the Royal College of Surgeons. Communicated by Professor 

 Flower, F.R.S. Received March 30, 1883. 



(Abstract.) 



Three distinct forms of eye exist in the Arthropoda ; the Compound 

 eye. the Simple Ocellus, and the less known Compound Ocellus, 

 common in larval insects, first described by Dr. Landois. 



The relationship of the Compound eye to the Simple Ocellus is shown 

 to be very distant, although I believe that these two types have been 

 evolved from a common but very rudimentary primitive type. On the 

 other hand, that between the Compound eye and the Compound 

 Ocellus of a larval insect, is Yery close, the Compound eye being 

 merely an aggregation of a great number of these ocelli, variously 

 modified in the more highly differentiated Insects and Crustaceans. A 

 fourth form of eye exists, in which the Ocelli are less closely united ; 

 this forms a connecting link between the compound eye and the 

 compound ocellus. It is found in the Isopods, and may be conveniently 

 termed the Aggregate eye. 



The Simple Ocellus consists essentially of a pigmented capsule, 

 behind a convex corneal lens, containing a cellular vitreous, which 

 is separated from the retina by a fine fibrous membrane. The 

 retina itself is a layer of Bacilla, comparable with those of Jacob's 

 membrane in the Vertebrate, except that the highly refractive outer 

 segments of the rods are turned towards and not away from the 

 refractive media. The fibrous membrane, between the rods and the 



group only. I cannot help suspecting that a better knowledge of this type will lead 

 to its absorption into Reteoc sinus.— P. H. C. 



