184 ANIMALS WITH JOINTED LEGS— THE ARTHROPODS 



when some of them are exposed to injury on the outside of the body. 

 However, the endoskeleton has an important advantage in growth ; the 

 skeleton and the body parts can grow together, so the skeleton places no 

 limitation on the size of the animal. The exoskeleton, on the other hand, 

 incloses the remainder of the body and restricts the room that is avail- 

 able for growth. Some of the arthropods partially avoid this disad- 

 vantage by shedding their skeletons completely, in a process called molt- 

 ing, and then growing larger ones for increased size. This method has 

 its drawbacks, however, and even with molting the body size is defi- 

 nitely limited. Since, in general a large animal has an advantage over 

 a small one, this gives a point of advantage to the animals with an endo- 

 skeleton. We may well be thankful that we have this advantage in size, 

 else we would not have a chance in competition with the arthropods. 

 Imagine our consternation if we should find spiders as big as tigers lurk- 

 ing in dark places ready to pounce on us when we came near. Suppose 

 there were wasps as big as buzzards that could zoom down out of the 

 air and pierce us with a dagger-like sting that almost certainly would be 

 fatal. Huge ants traveling in packs like wolves could terrorize an entire 

 countryside. With all our modern weapons and ingenuity we would 

 probably have little chance to survive attacks from such large arthropods 

 with their protective armor and highly specialized weapons. In their 

 present small size they can still give us considerable misery. 



Class — Crustacea 



This class name means hard shell and refers to the hard, crustlike 

 exoskeleton found among members of this group. They are also charac- 

 terized by a large number of paired appendages which typically are 

 branched into two terminal portions. This is called a biramous type of 

 appendage. They use gills for respiration, although a few of them live 

 on the land and absorb their oxygen from damp air circulating over the 

 gills. They have a body composed of many segments, as do all arthro- 

 pods, but these are somewhat combined to form two or sometimes three 

 main body parts. There are about 20.000 species of Crustacea ranging 

 in size from tiny microscopic water fleas to huge crabs that have a leg 

 spread of ten or twelve feet. 



We will study the crayfish, Cambarns, as a typical representative of this 

 class as well as the phylum as a whole. The name crayfish is the com- 

 mon name of this animal, as found in books, but the names crawfish and 

 crawdad are more commonly used in referring to them. They live in 

 fresh water and seem to prefer ponds and sluggish streams where the 

 bottom is somewhat muddy. Many a child has spent happy hours "fish- 



