ARTHROPODA 205 



Many aquatic forms are microscopic in size. Except in certain 

 fixed forms, like the barnacles and parasitic forms like Sacculina, 

 we find bilateral symmetry. 



The Arthropoda walk, leap, burrow, swim, and fly, with loco- 

 motor organs and accessory structures correspondingly developed. 



Geographically they are most cosmopolitan. In the Island of 

 Cyprus, certain centipedes are found on the burning plains and in 

 the snows of the mountains. A fresh water crustacean, Lepidurus 

 glacialisy is found only in the Arctic regions. The brine shrimp 

 Artemia salina is found in brackish water and in pans of exceedingly 

 salty water evaporating in the sun. In certain hot springs, crus- 

 taceans and insect larvae live at a temperature of 60° C. 



Integument and Musculature. — The skin of the Arthropoda con- 

 tains epidermal cells (pigmented membrane of the lobster) which 

 secrete a chitinous exo-skeleton frequently containing lime salts, 

 but found to be extremely thin at the joints. Chitin is anhydride 

 of dextrosaynine^ a sugar derivative. Growth is in most cases ef- 

 fected by a process of ecdysis or moulting^ in which the old shell 

 cracks off and the new skin rapidly hardens. The period just after 

 moulting is a most dangerous one for most Arthropoda, for their 

 brethren are cannibalistic. Under the epidermis we find connective 

 tissues and the dermis with nerves and blood vessels. 



Circular muscles found in the Annelida are absent in the Arthro- 

 poda, the longitudinal muscles causing movement of the segments. 

 The appendages are well supplied with muscles. 



Digestive System. — In parasitic forms the mouth may be ex- 

 tremely rudimentary. The wide difference in mouth parts of in- 

 sects gives a basis for their classification. The digestive tube varies 

 much with the type of food consumed. Crustacea like the lobster 

 have a short gullet leading to a large cavity, the stomach, which has 

 a gastric mill for gizzard-like grinding. 



In insects with mandibles (grasshoppers, crickets, and locusts) 

 we find a well-developed gizzard. In the insects there is great 

 variation in the form and length of the digestive tube. In carniv- 

 orous species the crop, gizzard and large intestine are sometimes 

 absent. In bees the crop is the " honey-bag." There is no true 

 liver in the insects, but the functions of a hepato-pancreas are per- 

 formed by glands in the stomach. 



In spiders the pharynx and gullet are extremely small and the 



