Summary of the Characters of Insects, etc, 7 



and by the absence of that hard external skeleton seen in Arthropods 

 and composed of chitin. 



C. The ]\I0LLUSCA or Slugs and Snails have normally a covering 

 or shell composed of calcareous salts, and they have never jointed 

 limbs or processes. The " Shell " may, however, as in Slugs, be 

 rudimentary and internal. 



A. The ARTHEOPODA are di\'ided primarily into the following 

 classes : — 



Crustacea (Crabs, Lobsters, Woodlice, etc.), mainly aquatic 

 animals, but Woodlice are terrestrial. The head bears two 

 pairs of jointed feelers or antennte, and there are organs of 

 locomotion on most of the segments of the body. The young 

 in the Woodlice closely resemble the parents. They breathe 

 by means of gills. 



Myeiapoda (Millepedes and Centipedes). Terrestrial. Head 

 separate from the rest of the body, which is composed of a 

 number of rings each of which bears one pair (Centipedes) 

 or two pairs (Millepedes) of legs. The young resemble the 

 parents. In millepedes the first three segments have each 

 only one pair of legs. The young are hexapod, but the legs 

 are never all on the first three rings. 



Ar.ACHNOiDEA (Spiders, Mites, etc.). These have either eight 

 legs attached to the front part of the body (thorax) or two 

 pairs (Gall ilites). The young or larvai are usually six- 

 legged (hexapod), and thus differ from the adults. The body 

 is, however, never divided into three marked regions. 



This class is subdivided as follows : — • 



ft. Araneida (Spiders). 



Legs eight. Breathe by means of air sacs and trachea. 



Head and mid body or thorax united, forming a so-called 



cephalo-thorax. Eyes simple. Abdomen not ringed. All 



beneficial. 

 1). Acarina (Mites and Eed Spiders). 



These have thorax and abdomen more or less in one, and 



breathe cutaneously and by trachea. T]ie young are six- 



lecged. Ticks are also included here. 



Hexapoda (True Insects). 



Easily told by having the head, thorax and abdomen distinct 

 from one another, and by having six legs. 



