THE CRAYFISH 137 



with a number of delicate and peculiarly shaped setae, the 

 auditory hairs. The cavity of the sac is filled with particles 

 of sand and other foreign matter to which the name " otoliths " 

 has been given. 



The eyes of the crayfish are compound, like the paired eyes 

 of Apus ; there is no trace of a median unpaired eye. Viewed 

 with a lens the eye appears as a convex oval area at the 

 extremity of the eyestalk. It is covered with a soft colourless 

 cuticle, the cornea, divided into a great number of quad- 

 rangular facets, by two sets of fine lines crossing each other 

 nearly at right angles. The eye looks black, because of the 

 large quantity of pigment associated with its inner parts and 

 seen through the cornea. A vertical section shows that the 

 eye is made up of a large number of elongated visual elements, 

 one corresponding to each facet of the cornea. These ele- 

 ments converge from the convex cornea towards a ganglionic 

 mass situated in the eyestalk, the optic ganglion, and from this 

 the optic nerve runs back to the cerebral ganglion. The paired 

 eyes of Apus are formed on the same principle. 



It is not possible to enter into a long description of the 

 compound eyes of Arthropods, still less to consider the 

 different theories that have been put forward to account for 

 their origin. But it may be stated shortly that every compound 

 eye is made up of a large number of visual rods called 

 ommatidea, and each ommatideum is an eye more or less 

 perfect in itself, and isolated from its fellows by a sheath of 

 dark pigment contained in special pigment-bearing cells. All 

 the different parts of the ommatideum are formed by the 

 multiplication and differentiation of the layer of columnar 

 ectoderm which underlies the cuticle, a layer which in the 

 Arthropoda is known as the hypodermis. Over the general 

 surface of the body the hypodermis is but one cell thick, but 

 at the spot where the eyes are formed it becomes several 

 layers thick, and its cells are arranged in groups about a 

 number of ideal lines converging from the surface of the eye 

 to the optic nerve beneath. Every such group gives rise to 

 an ommatideum consisting essentially of an outer group of 

 cells, the vitrellse, which form a refracting body, the crystalline 

 cone, and an inner group of cells, the retinula, which give rise 

 to a peculiar rod-like structure, the rhabdom. The inner ends 

 of the retinular cells are continued into the fibres of the optic 



