382 INVERTEBRATE MORPHOLOGY. 



rounded upon the outside by a delicate layer of protoplasm 

 placing the part of each cell above the cone in continuity with 

 the part lying below it. In the higher forms there occur, 

 partially surrounding the cone-cells, two pigmeuted cells 

 which seem to be sensory in function and are termed the 

 distal retinular cells (DR). They are, however, unrepresented 

 in the lower forms, in which the sensory portion or retiuula is 

 represented by a single circle of usually 5 cells (PR) lying 

 proximally to the cone-cells and surrounding a chitiuous rod 

 which is manufactured as a secretion from their approximated 

 surfaces, and is termed the rhabdom (Eli). These cells are 

 also pigmented and are prolonged below into nerve-fibres, 

 which, piercing the basement-membrane upon which the om- 

 niatidia rest, pass to the optic ganglia. In the higher Crus- 

 tacea, in which a distal retiuula is present, the rhabdom is 

 formed by a circle of eight cells (one of which is almost 

 aborted, so that there appear to be only seven). These con- 

 stitute the proximal retinula, and appear to correspond to the 

 single retinula of the simpler forms. Finally, a number of 

 accessory cells, usually pigmented, may surround each om- 

 matidium, separating it from its neighbors, but not appearing 

 to be essential constituents of the eye. 



The view according to which these lateral eyes are regarded as an ag- 

 gregation of a number of independent eyes has already been referred to. 

 It seems questionable, however, if this be the correct interpretation of 

 them in view of the occurrence of so-called compound eyes in the Mollusca 

 (Area) and the Polyehaetous Annelida. It seems more probable that, as 

 in these forms, the Crustacean eye is to be regarded as a separation into a 

 number of more or less isolated parts of an originally continuous retina, 

 a corresponding division of the originally simple refractive apparatus 

 also taking place. This view seems to harmonize most satisfactorily with 

 the facts of development. 



Occasional departures from the usual arrangement of the 

 eyes are to be found as for instance in Phronima, one of the 

 Amphipoda, in which two pairs of compound eyes occur on 

 the head. Mention may also be made here of the peculiar 

 eyelike structures occurring in Euphausia, one of the Schizo- 

 poda. They occur on the basal joints of the appendages of 

 certain of the thoracic metameres, as well as upon the ventral 



