268 PRACTICAL LESSONS IN SCIENCE. 



The nervous system consists of a brain, above the oesophagus, 

 the ventral chain of ganglia and the cesophagal ring connecting 

 the brain and the anterior ganglia of the ventral chain. The 

 sense nerves arise from the brain, while the ventral ganglia 

 send nerves to the muscles, organs of locomotion and skin. These 

 animals usually have eyes, organs of hearing, organs of smell 

 and tactile organs. 



The digestive organs are well developed, with salivary glands, 

 hepatic appendages and excretory organs usually present. The 

 circulatory and respiratory organs exhibit the widest range of 

 form. Circulation is sometimes effected by a pulsating organ, 

 sometimes by the movements of the body, and again by regular 

 movements of certain organs, as the intestines. Respiration is 

 frequently effected by means of the surface of the body, sometimes 

 by branched appendages of thelimbs,and in other Cases by means 

 of internal tubes or tracheae. The reproduction is usually sexual, 

 but modified forms sometimes occur. The most of this group are 

 oviparous, some are viviparous, and in most a somewhat com- 

 plex embryonic development is followed by a complicated meta- 

 morphosis. The Division Arthropods includes four classes, the 

 Crustacea, as the crayfish and lobster, the Arachnida, as the 

 spiders, Myriapoda, as the centipede, and the Insecta. 



The Crustacea are for the most part aquatic, breathing by 

 means of gills; some, however, breathe air and are terrestrial. 

 They vary greatly in size and form, but a study of the common 

 crayfish will give us a good idea of the whole class. Crayfish 

 may be found hidden among sticks and stones of shallow water, 

 and may be caught with a light dip net, which may be made of 

 stout wire and mosquito netting. Preserve a number in alcohol 

 for dissection and keep several in fresh water in a pan or glass 

 jar for observation. 



Notice how the crayfish walks, how and which way he jumps 

 when frightened. Looking down upon the crayfish, notice the 

 cephalathorax, with its smooth horny covering called the cara- 

 pace, and the projection of the carapace forward between the 

 eyes, the rostrum. Count the segments in the abdomen, and, 



