V CRUSTACEA— SENSORY ORGANS 355 



body secreted by the crystal cells. The layer of the crystal cells of 

 the compound eye is covered by 



C. The transparent smooth ehitinous integument (eornea) with 

 subjacent hypodermis cells, a continuation of the general body 

 integument. 



The principal distinctions to be pointed out between the 

 stalked eye of the higher Crustacea and that of Branchipus are in the 

 cornea and the optic ganglion. In the former the cornea (ehitinous 

 cuticle) is somewhat thickened over each single eye, forming for each 

 a eopneal facet or corneal lens, which is convex either on its inner or 

 outer side or on both sides (Fig. 238, A, d). The cornea then appears, 

 when viewed from the surface, to be divided into regular polygonal 

 areas, each of which corresponds with a corneal lens of a single eye. 



Again, whereas the ganglion opticum is simple in Branchipus, in 

 the stalked eyes of the Malacostraca it falls into 3 ganglia also placed 

 in the eye stalk. 



There are many important variations in detail in the structure of the Crustacean 

 compound eye ; these chiefly concern the number of cells in a retinula, the number 

 of crystal cells in a single eye, the number of single eyes in the whole eye, and the 

 specific arrangement of the elements. The DecajioAa and Isopoda. possess 7 retinular 

 cells, the Branchiopoda and AmpMpoda 5, the Sdiizopoda 4. The single eyes of the 

 Cladocera have 5 crystal cells, the Decapoda and the Branchiopoda 4, the Isopoda, 

 AmpMpoda, and Schizopoda 2. In the Isopoda only a few single eyes are found 

 which are not closely packed (4 in Aselhcs, 20 in Porcellio). The isolated corneal 

 lenses (not flattened into polygonal facets) are here biconvex. 



Each of the paired eyes of the Oorycaeidce consists of one single eye, which in 

 Corycaeus is strikingly large and long. It is in many respects markedly different from 

 the ommatidia of the compound eyes of other Crustacea. 



In the EupluLusidm among the Schizopoda there are, besides the two 

 compound stalked eyes, other "accessory" eyes. These are found on 

 the basal joints of the second and penultimate pairs of thoracic feet, 

 and further, one in the ventral middle line of the abdomen between the 

 pleopoda of each of the 4 anterior segments. "Whether these organs 

 belong to the category of visual organs is very doubtful ; we only 

 know for certain that they are luminous. 



B. Other Sensory Organs. 



Among the other sensory organs of the Crustacea the most 

 widespread are the tactile, and what are generally supposed to be 

 olfactory, organs. In many Entomostraat we find, in addition, the 

 so-called frontal sensory organs of unknown physiological significance. 

 Auditory Organs occur in all the Decapoda, and have also been 

 observed in isolated cases in other divisions. Other structures which 

 have been described as sensory organs with unknown functions must 

 be passed over in silence, because of their sporadic occurrence and also 

 because too little is known about them. 



