NERVOUS SYSTEM 213 



A transverse commissure immediately behind the oesophagus 

 joins the two circumoesophageal commissures. It contains fibres 

 which take this roundabout course between the portions of the 

 brain which supply the antennae, thus indicating that these limbs 

 belong to the same series as those behind the mouth. That is 

 probably also true of the antennules, and the fact that the 

 antennules and antennae are innervated from the supraoesophageal 

 gangha must be connected with the position of the mouth, which, 

 as a result of cephahsation (p. 193) to a high degree is farther 

 back than in the earthworm, where it Hes in front of the first 

 somite. The alimentary canal is suppHed by two visceral nerves. 

 The first has a three-fold origin, being formed by the junction 

 of a nerve from the cerebral ganglion with two nerves which arise 

 each from a small ganghon on the course of the circumoesophageal 

 commissure. The second arises from the last abdominal ganghon. 



SENSE ORGANS 



The eyes of the crayfish are compound, containing a number 

 of elements, known as ommatidia, each of which is capable of 

 forming a separate image. 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 an ommatidium is shown in Fig. 

 145. The inner ends of the visual cells are continued into fibres 

 which pass into an optic ganglion in the eyestalk, and from this 

 arises the optic nerve. 



In strong light the pigment is spread through the cells so that 

 each ommatidium is isolated, and its corneal facet and refractive 

 bodies combine to form a small image of a portion of the field of 

 view. The separate images are combined to form a mosaic image, 

 which is necessarily erect, of the whole field. In weak hght the 

 pigment is retracted, and a single diffuse image is formed by the 

 whole eye. In prawns, and presumably in the crayfish, the move- 

 ment is controlled by hormones. When separate images are 

 formed a compound eye presumably gives very accurate directional 

 vision, especially to a small point of light. It is this which helps 

 the fixation by which the moth flies towards the candle, turning 

 always so that the same ommatidium is brightly illuminated. 



The statocysts (Fig. 146) are pair of sacs, situated in the basal 



