352 COMPARATIVE ANATOMY CHAP. 



part of the head immediately surrounding the eye, by its articula- 

 tion, and further growth into a stalk. Formerly the stalked eyes 

 were often erroneously considered to represent a pair of extremi- 

 ties. In the development of the Crustaceans the paired eyes always 

 appear much later than the unpaired eye, and we have reason for 

 assuming that the unpaired eye is phylogenetically older than the paired. 

 The following is a short review of the occurrence ar.d distribution of 

 the paired or principal eyes. 



Entomostraca. Phyllopoda : in the Estheridce, and Apusidce the paired eyes have 

 moved towards each other in the middle line. The Branchiopoda have movable 

 stalked eyes. The two principal eyes of the Cladocera have fused in the middle 

 line to form a trembling frontal eye, which, however, is wanting in Monospilus. 

 Ostracoda : the Cipridinidce possess besides the unpaired eye paired movable 

 lateral eyes. In the Cypridce and Cytheridce also paired eyes occur, which may fuse 

 together to form one unpaired eye. Whether these correspond with the paired eyes 

 of other Crustaceans is not known. Copepoda : the Carp-lice, which are nearly 

 related to the Copepoda, possess besides the unpaired eye 2 large lateral eyes. In the 

 true Copepoda the latter are generally wanting, but paired eyes do occur, e.g. in 

 Pontellidce, and these perhaps correspond with the lateral eyes of other Crustacea. 

 The paired eyes of the Corycaeidce on the contrary do not easily admit of such a com- 

 parison. It is, however, not improbable that the racial forms of the Copepoda 

 possessed paired compound eyes, which have been again lost. Cirripedia : in the 

 adult animals the paired eyes are wanting throughout, but on the other hand the 

 Cypris-like larvce of the attached forms are provided with large lateral eyes. 



Malacostraca. Lateral eyes occur everywhere. They are stalked in the 

 Leptostraca and all Thoracostraca except the Cumacea. In the last case the sessile eyes 

 are generally fused in the middle line ; they may, however, be altogether wanting. 

 In the Arthrostraca, which have also been called the Edriophthalmata (in contra- 

 distinction to the other Malacostraca, which are known as the Podophthalmata), the 

 eyes are sessile. The facts that movable stalked eyes occur in the Phyllopoda 

 (Branchipus), and that the eyes of Leptostraca (Nebalia), which in every way stand 

 nearest the racial form of the Malacostraca, are similarly stalked, make it appear 

 probable (other facts also being taken into consideration) that the paired eyes of 

 the Arthrostraca were once stalked. 



Among the Amphipoda the Phronimidce show peculiarities. They possess two 

 pairs of compound eyes, one frontal pair and one in the region of the cheek. The 

 two eyes of the same side, however, must have proceeded by division from the single 

 eye of the Amphipoda which in the Hyperidce is very large superficially. 



Structure of the Eyes. The unpaired eye was formerly described 

 as an x- shaped eye -spot, with or without a refractive body. On 

 account of its general distribution in the Entomostraca, it is also called 

 the Entomostracan eye. Its structure will be best illustrated by 

 means of an example. 



The frontal eye of Calanella mediterranea (Fig. 237), a free-living 

 Copepod, consists of 3 single eyes united together, 1 unpaired median 

 and ventral, and 2 lateral and dorsal. Each single eye is composed 

 of a pigment cup and a strongly refractive transparent " lens " laid in 

 and on it. The term "lens" is, however, not applicable. It is 

 composed of several cells, each of which is connected, whether at its 



