492 The Animal Kingdom 



mental nephridia, imbranched gut, ciliated reproductive ducts, the struc- 

 ture of the eye and the muscular body wall. Unique to the onychophorans 

 are the lack of segmentation, the arrangement of the tracheae, and the 

 double ventral nerve cord without true ganglia. 



Perhaps Peripatiis is a surviving member of a very ancient group 

 of animals, pointing out the fact that the arthropods had a common, 

 though very distant, origin with the annelids. 



THE CLASS CRUSTACEA 



Most widespread of the arthropods are the crustaceans. They live 

 in all portions of the sea, in fresh water, and on land. As they occupy 

 all these varying habitats, a large number of species with a great di- 

 versity of form has evolved. In size, they vary from tiny microscopic 

 floating organisms to the giant sea crabs which spread to 12 feet. Most 

 are free-living, but there are a few parasites. In spite of this diversity 

 there are certain fundamental characters which unite them as a single 

 class : the head bears two pairs of antennae, has paired mandibles, and 

 two pairs of maxillae. Dorsally there is often a carapace over the head 

 and thorax ; the abdominal segmentation is usually distinct, with each 

 segment typically possessing a pair of biramotis (two-branched) ap- 

 pendages which are often highly modified. Respiration is by gills or 

 the body surface and excretion is by special glands. Except for a few 

 species, the sexes are separate. 



The anatomy of the fresh-water crayfish, Cambarus sp., illustrates 

 the characters of this highly varied class. 



The External Anatomy of Cambarus. — The body of the crayfish 

 is formed of nineteen segments which are variously fused and modified 

 in the different regions. The basic structure of each segment is best 

 observed in some of the abdominal ones. In these it can be seen that 

 the protective exoskeleton of chitin consists of several plates. There 

 is a dorsal convex tergnm, two lateral pleura, and a ventral sternum. 

 Between the pleura and the base of the appendages are the small epimera. 



Externally the body consists of two well-defined regions : the an- 

 terior cephalothorax and the posterior abdomen. The cephalothorax is 

 the fused head and thorax and dorsally and laterally it is covered by the 

 hardened carapace. Dorsally the thirteen segments of the head-thorax 

 region are thus concealed, but they are visible from the ventral surface. 

 Anteriorly the carapace projects forward as the rostrum. The portion 

 of the carapace covering the head region is separated from that covering 



