110 MANUAL OF ANIMAL BIOLOGY 



When the young embryo has reached a certain stage of develop- 

 ment it hatches ; that is to say, it breaks out of the egg cap- 

 sule which has thus far enclosed it and becomes more or less 

 independent. It is, however, still attached to an abdominal ap- 

 pendage of the mother by means of a filament which runs to the 

 embryonic telson. Three successive growth stages, or molts, 

 occur, during each of which the exoskeleton is shed. The 

 animal finally detaches itself from the mother, and thus becomes 

 a free-living Crayfish of small size, but in general appearance like 

 the adult. 



6. Nervous System 



The nervous system of the Crayfish has the same general struc- 

 tural plan as that which has already been noted in the Earthworm. 

 There is a main ventral nerve cord with ganglia running the 

 entire length of the body. This constitutes the central nervous 

 system, and from it, at the ganglia, peripheral branches are given 

 off which run to all parts of the body. There are, however, certain 

 points of difference between the nervous system of the Crayfish 

 and that of the Earthworm. These may be summarized as follows 

 (W. f. 92) : 



(a) In the Crayfish the arrangement of the ganglia in the ven- 

 tral nerve cord does not correspond to the external segmentation 

 throughout the entire length of the body. This is particularly true 

 in the cephalothorax. Altogether there are 13 ganglia present 

 on the ventral nerve cord, and since there are 21 segments in the 

 body it is apparent that either several ganglia are lacking or else 

 there has been a consolidation of certain ones. The facts indi- 

 cate that a fusion has occurred in certain ganglia of the head 

 and thorax. 



(b) In the Crayfish there are a considerable number of sense 

 organs present which are adapted for receiving various kinds of 

 external stimuli such as tactile, olfactory, auditory, and photic. 

 In every type of sense organ, the sensory neurons receive the 

 stimuli and transmit them to the central nervous system. 



The cererral ganglion, lying just back of the eyes, is regarded 

 as the most important ganglion in the body, and constitutes the 

 brain of the animal, just as in the Earthworm. A number of 

 peripheral nerves are given off from this ganglion, which run ante- 

 riorly and laterally, and innervate the antennules, antennae, and 



