CRAYFISH 111 



eyes. The subesophageal ganglion gives off a number of nerves 

 which supply the mandibles, maxillae, and the first two pairs of 

 maxillipeds. The remainder of the ganglia present in the ventral 

 nerve cord give off branches which, in general, supply the append- 

 ages, muscles, and other organs lying near the region in which they 

 are situated. For the most part the branches arising in all these 

 ganglia, except the cerebral ganglion, are motor in function. 

 (W. f. 63.) 



Sense Organs. The surface of the body is entirely covered 

 with the hard, chitinous exoskeleton and is not provided with 

 scattered sensory nerve cells as is the case in the skin of the Earth- 

 worm. Thus the sensory tissue in the Crayfish is largely grouped 

 in the special sense organs situated in the anterior end of the body. 

 These may now be noted. 



(a) Tactile Organs. Scattered over the appendages as well as in 

 some other regions of the body are found many fine filaments, or 

 setae, which are believed to be tactile sense organs. Each of 

 these setae is directly connected with a tiny nerve fiber, and any- 

 thing which comes into contact with the setae sets up an impulse 

 along the attached nerve. 



(6) Olfactory Organs. It is believed that certain jointed fila- 

 ments, which are present in small groups on the under surface of the 

 exopodites of the antennules have an olfactory function. 



(c) Organs of Hearing or Position. Situated in the basal joint of 

 each antennule is a sac, the statocyst, which contains a great 

 number of very fine hairs distributed along two ridges. A nerve 

 runs into the base of the sac, and branches from it are distributed 

 to the neurons. Among these hairs, grains of sand (statoliths) 

 are commonly found, and these may be attached to the hairs. Al- 

 though this structure was originally thought to be a sound-receiving 

 organ, experimental work has apparently shown that its real func- 

 tion is that of equilibration. In other words, it gives the animal a 

 sense of position. It has been found, for example, that animals 

 from which the statoliths have been removed are unable to orient 

 themselves and, furthermore, if the sand grains are replaced by 

 fine iron filings, then the equilibrium of an animal can be influenced 

 by the use of a magnet. 



(d) Organs of Sight. The Crayfish possesses a pair of eyes, each 

 borne on a small stalk attached, just dorsal to the antennae, near 

 the anterior end of the head. The eyes are very highly developed, 



