THE MICROSCOPE IN ZOOLOGY. 179 



olood passes in currents in all directions, without vascular 

 walls, running into the antennae, wings, extremities, etc., 

 and returning as a venous current, forming two lateral 

 currents towards the end of the ahdomen, it is brought 

 by the diastole of the heart through lateral fissures ex- 

 isting in it. 



O 



The respiration is effected by means of tracheae, two 

 or more large vessels running longitudinally, giving off 

 branches in all directions, and opening to the air by short 

 tubes, connected at the sides of the body with orifices 

 called spiracles. Aquatic larvae often have branchiae in 

 the form of plates, leaves, or hairs, through which the 

 tracheae ramify (Plate XVII, Fig. 136). 



The nervous system consists of a series of ganglia ar- 

 ranged in pairs, one for each segment of the body. They 

 are situated between the alimentary canal and the under 

 surface of the body, and are usually connected by longi- 

 tudinal nervous cords. From the ganglia nerves are dis- 

 tributed to all parts. 



The muscular system of insects is quite extensive. Ly- 

 onet dissected arid described more than four thousand in 

 the caterpillar of the goat-moth (Cossus ligniperda). 



XVIII. ARACHNID A. This class of animals includes 

 mites, ticks, spiders, and scorpions. They are destitute 

 of antennae ; the head and thorax are united ; they have 

 simple eyes (ocelli), and eight jointed legs. 



The cheese-mite, the u ticks," the itch-insect (Sarcoptes 

 scabies), and the Demodex folliculorum, which is parasitic 

 in the sebaceous follicles of the skin of the face, are com- 

 mon examples of Acari. They are best mounted in fluid. 



The respiratory apparatus in spiders differs from that 

 of insects, the spiracles opening into respiratory sacs, which 

 contain leaf-like folds for aeration of blood. The spinning 

 apparatus is also interesting. 



The minute anatomy of vertebrated animals affords the 



