CRAYFISH 



255 



-Corneal 

 facet 



-Vitrei la 



Pigment 



Crystalline 

 cone 



coordination of all of the appendages used in connection with the process 

 of food taking. The sense organs of the crayfish consist of a pair of eyes 

 and a pair of statocysts. Tactile organs also are well developed in 

 different parts of the body, particularly upon such parts as the chelae of 

 the walking legs, the mouth parts, the ventral surface of the abdomen, 

 and the edge of the telson. There also seems to be a general distribution 

 of organs for the perception of chemical stimuli. 



Crayfishes are diecious. The vasa deferentia open on the median 

 side of the base of each last walking leg. The 

 openings of the oviducts are at the base of each third 

 walking leg. 



292. Eyes and Vision. — The crayfish possesses 

 compound eyes. Each eye is hemispherical in form 

 and is covered by a transparent cornea which repre- 

 sents a modified portion of the cuticula (Fig. 156). 

 The cornea is divided into rectangular facets, each 

 one of which is the outer end of a rodlike unit known 

 as an ommatidium. These ommatidia — of which 

 there are approximately 2500 — are radially arranged 

 rods tapering toward the base, which causes their 

 axes to converge toward a common center. From 

 the ommatidia lead nerve fibers which, together, 

 make up the optic nerve. When an ommatidium is 

 observed carefully it is seen to consist of a corneal 

 facet at the outer end (Fig. 157), within which is a 

 lenslike structure known as the vitrella. Back of 

 the vitrella is the crystalline cone, and back of the 

 crystalline cone the rhahdorn, on the outer side of 

 which are the sensory cells, making up a retinula. 

 The corneal facet, vitrella, crystalline cone, and 

 rhabdom are all transparent. Pigment cells are 

 scattered over the surface of the ommatidium. In ommSidium"frtmThe 

 bright light these cells are distributed throughout the eye of a crayfish. Some- 

 length of the ommatidium; in dim light, however, ^vhat diagrammatic, 

 they contract, leaving much of the wall without pigment. 



A compound eye sees just as many little images as there are omma- 

 tidia, and since these images together make up the whole of the picture 

 recei^'ed by the animal, the picture has been termed a mosaic image. 

 There is, however, some overlapping of the separate images. The pro- 

 duction of a separate image for each ommatidium results from the fact 

 that each of the ommatidia is long and slender, and since the pigment 

 along the sides absorbs all the rays which it receives, only those rays 

 reach the bottom and stimulate the retinula which are practically in line 

 with the axis of the ommatidium. In dim light the withdrawal of pig- 



- Rhabdom 



Retinular cells 



-Basement 

 membrane 



^ Optic nerve 

 fibres 



