VISUAL ORGANS OF AKTHKOPODA. 265 



crystalline cone of some size, in front of which a corresponding por- 

 tion of the cuticnlar layer of the integument forms a lens-like organ 

 (Corycasida)). The presence of several crystalline cones in each eye 

 forms an intermediate step to the more complicated form of eye. 

 As the integument, which is found over the simple pair of eyes, 

 becomes thickened into two facets corresponding with the crystal- 

 line cones, these structures resemble in their mode of formation the 

 cornea-lenses. 



In addition to the median eye, which is sometimes represented by 

 a mere speck of pigment, the Cladocera and Phyllopoda possess two 

 compound eyes, from which it may be concluded that the median 

 one, which corresponds to the eye of the Nauplius, is a special 

 structure, which does not become developed into the permanent 

 eye. This " larval eye " is probably a transmitted arrangement. 



By their power of movement, and their position immediately 

 below the chitinous carapace, the eyes of the Branchiopoda form an 

 intermediate step towards those in which the chitinous carapace takes 

 a more direct share in forming the optic organ. Further, the position 

 of the eye, on a stalk-like process (Artemia and Branchipus), presents 

 a point of affinity to the podophthalmate Malacostraca. 



Two types of eye are derived from these conditions, which arc 

 very common in the higher divisions of the Crustacea, and in the 

 Tracheata. According as the elements of the retina, which form 

 the perceptive apparatus, unite into a compact simple organ, or as 

 these organs are themselves part of a more complicated structure, the 

 optic organs are distinguished as simple eyes (Stemmata, Ocelli), 

 or as compound eyes. The cuticnlar layer of the integument 

 takes a greater or less share in the formation of this apparatus. 



The simple eye (Fig. 134) is commonly found in the larval 

 forms of the Insecta, and there are generally a number of them on 

 either side of the head. In the Thysanura this form appears to 

 persist. The chitinous layer forms a cornea-lens over each eye. In 

 many Insects we find these simple eyes in company with the compound 

 ones, between which they are placed, generally in twos or threes, 

 and on the frontal surface. They are distinguished from the 

 compound eyes by being composed of a large number of retinal 

 elements, which are covered by a simple cornea-lens. 



In the Myriapoda the eyes, which are placed in one or two rows, 

 vary in number (4-8). It seems as if we had the larval stage in the 

 eye of insects permanently represented, but wc have no exact 

 knowledge on the subject. 



In the Arachnida the arrangement is much the same. There 

 are numerous peculiarities both in the arrangement and number 

 of their eyes. Two large eyes are, in the Scorpions, approximated 

 to one another, and on each side there is a group (2-5) of smaller 

 eyes. In the Spiders and Phrynida there are, as a rule, 8, more 

 seldom 6, symmetrically distributed on the anterior part of the 

 cephalothorax ; they generally vary in size ; in the Opilionida there 

 ai-e only 3 or 1 in the same place, the largest of which are placed on 



