THE CRA YFISB 261 



each from a small ganglion on the course of the circumcesophageal 

 commissure. The second arises from the last abdominal ganglion. 



The eyes of the crayfish are compound, containing a 

 sen number of elements, known as ommatidia, 



each of which is a small complete eye. The 

 whole eye is black, owing to the presence of pigment in 

 some of its cells, and is covered with a colourless portion of 

 the cuticle known as the cornea, divided into a number of 

 square facets, each of which corresponds to an ommatidium. 

 The structure of the ommatidia is complex: each is an 

 elongated body consisting of a number of cells derived from 

 the epidermis with refractive bodies secreted by them. The 

 innermost cells form a group known as the retinula, whose 

 inner ends are continued into nerve fibres. The ommatidia 

 are separated by pigment cells and the retinular cells also 

 contain pigment. The way in which such eyes give rise to 

 vision has been the subject of various theories. It appears 

 that the pigment flows about within the cells, being retracted 

 in weak light and expanded in strong. When it is retracted 

 the eye gives a single image ; when it is expanded each 

 retinula gives a separate image, sharper but formed with a 

 greater loss of light than that given when the eye acts as a 

 whole. No doubt this mosaic of images is combined in the 

 nervous system to give a single impression. 



The statocysts are a pair of sacs, situated in the basal 

 joints of the antennules and provided with nerves. Each 

 has a cuticular lining beset with hairs, with which the nerve 

 fibres are in communication. Within it are grains of sand, 

 which are scattered over the opening of the sac by the 

 pincers and fall into it. It is probable that the principal 

 function of the organ is informing the animal of its position 

 by the movements of the sand grains against the hairs, and 

 thus enabling it to keep its equilibrium. If the statocysts 

 be removed, the crayfish loses its sense of position and will 

 often swim upside down. Experiments upon the prawn, 

 an animal related to the crayfish, illustrate the function of 

 the statocysts. A prawn that had lost the lining of its 

 statocysts with the sand grains by moulting was kept in 

 filtered water and supplied with finely powdered iron in 

 place of sand. When it had placed some of these in its 



