98 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



in having jointed appendages and a definite number of 

 segments. 



The skeleton is outside, and consists of articulated 

 segments or rings. The limbs, when present, are like- 

 wise jointed and hollow. The jaws move from side to 

 side. The nervous system consists mainly of a double 

 chain of ganglia, running along the ventral surface of 

 d the body under the 



alimentary canal. 

 The brain is con- 

 nected to the ventral 



FIG. 53. Diagram of the structure of an Arthropod o-Qncrlip hv p rincr 



(after Schmeil) : a, antenna ; c, circulatory system; o""^o^ "" J ** ' *** 



d, alimentary canal; n, nerve cord; g, ganglion; encircling" the "ul- 

 s, skeleton. 



let. The alimentary 



canal and the circulatory apparatus are nearly straight 

 tubes lying lengthwise the one through the center, 

 and the other along the back. The skeleton is com- 

 posed of a horny substance (chitin), or of this substance 

 with carbonate of lime. All the muscles are nearly 

 always striated. 



There are five classes, of which the first almost exclu- 

 sively is water breathing, having gills, and the others 

 principally air breathing, being provided with tracheae. 



CLASS i. Crustacea 



The Crustacea, with few exceptions, are water breath- 

 ing Arthropoda, usually with two pairs of antennae. 17 

 Among them are the largest, strongest, and most vora- 

 cious of the branch, armed with powerful claws and 

 a hard cuirass, bristling with spines. Although con- 

 structed on a common type, crustaceans exhibit a won- 

 derful diversity of external form : contrast, for example, 

 a barnacle and a crab. We will select the lobster as 

 illustrative of the entire group. 



