1883.] Structure and Functions of the Eyes of Arthropoda. 141 



vitreous, is attached around its periphery to a structure which bears a 

 strong resemblance to a ciliary muscle. This is enclosed in a ring-like 

 sinus, which surrounds the ocellus almost as the canal of Petit 

 runs round the lens of a Vertebrate. The vitreous is composed of a 

 single layer of cuboid or prismatic cells, each with a nucleus near its 

 inner extremity. These cells extend from the inner surface of the 

 corneal lens to the outer surface of the fibrous membrane. 



The Compound eye has a lenticular cornea beneath which the 

 crystalline cones and great rods are placed. These are separated from 

 the deeper nervous structures by a membrane comparable with the 

 fibrous membrane of the Simple Ocellus ; I have named this membrane 

 the Membrana Basilaris. 



The Membrana Basilaris is usually attached to the Cornea by an 

 inflected ring of integument, the Scleral Ring, so that the Crystalline 

 Cones , and the Great Rods are entirely enclosed in a case. I have 

 called all these structures the Dioptron, and have come to the 

 conclusion that they are all Dioptric in function. They apparently 

 correspond to the Cornea, Vitreous and Fibrous membrane of the 

 Simple Ocellus. 



The Membrana Basilaris, like the fibrous membrane, has a sinus 

 around its periphery, and is connected with the inflected integu- 

 mentary ring by fibres, which have a disposition similar to those of a 

 ciliary muscle. 



The Dioptron is nourished by Lymph Sinuses, which carry the 

 circulating fluid from the Aorta* into the interior of the Dioptron and 

 permit its exit into the common lymph spaces of the head. 



Beneath the Dioptron is a nervous structure of great complexity; 

 this I have named the Neuron. 



The Neuron consists of a Retina, an Optic Nerve, and an Optic 

 Ganglion. 



The Retina is essentially a layer of rod-like bodies, Bacilla, sup- 

 ported by a delicate Neuroglia. The Bacilla are similar to the rods 

 and cones of a Vertebrate in size, in form, and in structure, each has 

 an outer highly refractive, and an inner protoplasmic segment. In 

 some cases the outer segment is double, like that of the twin cones of 

 fishes. In other specimens I have detected a Lenticulus between the 

 segments. As in the Simple eye the highly refractive segments are 

 turned towards the Dioptric media. 



A layer of cells has been also demonstrated between the Basilar 

 membrane and Bacilla ; these in the majority of insects send pigmented 

 fringes inward, between the outer segments of the Bacilla. The 

 fringes are wanting in the diurnal flies, they represent the pigment 

 layer of the Vertebrate retina. 



I have spoken of the parts which underlie a single corneal 

 * I have so designated the anterior extremity of the dorsal vessel. 



