318 ZOOLOGY. 



student. We may just mention here that in the crab this concentra- 

 tion of ganglia has gone farther, and we have only two ganglionic 

 masses the cerebral and a large thoracic mass representing all the 

 rest fused together. 



The ganglia contain unipolar cells almost exclusively. From the 

 single process of the cell comes off both axis-cylinder and a series 

 of dendrons. The axis- cylinders convey efferent impulses only (in- 

 cluding those from one ganglion to another as well as to muscles). 

 It would seem that the fibres distributed to muscles include not 

 only motor fibres, but also inhibitory fibres, as is the case only with 

 the splanchnic muscles in Vertebrates. The sensory fibres seem to 

 originate, as in the earthworm, from cells in the sensory epithelium 

 itself. 



In function, the cerebral ganglia so far resemble the Vertebrate 

 brain that they control the reflex action of the other ganglia (as 

 has been proved by the actions of crayfish in which the cerebral 

 ganglia have been destroyed). Undoubtedly in its nervous system 

 the crayfish shows a great advance on any of our other Invertebrate 

 types an advance necessary to its more active existence. But there 

 is no satisfactory evidence that it has any such powers as memory, 

 association of ideas, etc., or any other of those mental powers without 

 which we can scarcely speak of consciousness as existing. 



23. Sense Organs. The Eyes are borne on short 

 movable stalks, and lie between the antennules and the 

 rostrum. In structure they differ from the Vertebrate eye 

 as completely as any two organs fulfilling the same function 

 very well could. In the first place the eye is a practically 

 solid organ, devoid of the large " chambers " of a Vertebrate 

 eye. Then it is compound, i.e. composed of a number of 



elements each physiologically com- 

 plete in itself; and lastly, its 

 sensory cells (rods and cones) are 

 nearer the light than the nerve- 

 fibres. 



The "elements" of the eye are 

 arranged in a radiating manner, 

 as shown in fig. 160. Each one 



(After Huxley ) lacet, sometimes called a lens, 



though its refractive power is 



doubtful : this is simply a polygonal area of the ordinary 

 cuticle, slightly thickened. (2) A crystalline cone, formed 

 by the secretion of a clear substance by four adjacent 



