GENEEAL DESCRIPTION 325 



the whole body. In most arthropods, however, a heart, usually tubular 

 in shape, is present in the dorsal part of the body cavity, the beating of 

 which keeps the blood in circulation. In no arthropods, however, is there 

 a closed vascular system, as the blood, even in the highest, passes from the 

 tissues to the respiratory organs through open spaces of the body cavity. In 

 most arthropods respiratory organs in the form of projections from the 

 legs or the sides of the body are present. In the crustaceans these pro- 

 jections extend into the water and form the gills, while in the air-breathing 

 forms the projections extend into the body cavity and become the so-called 

 lungs of the arachnids or the tubular tracheae which carry respiratory 

 'air directly to the blood. The muscular and nervous systems of arthro- 

 pods are highly developed. The muscles are all striated and are probably 

 the most energetic among animals. 



The main nervous system, like that of annelids, consists of a number 

 of pairs of segmentally arranged ganglia and nerves connecting them. 

 The anterior pair constitutes the brain and is situated in the dorsal 

 portion of the head; from it nerves go to the eyes and the antennae. 

 The remaining parts are ventrally situated, one pair being typically 

 in each somite. In most arthropods, however, fusion has occurred among 

 the ganglia so there are fewer pairs of them than of somites. This fusion 

 has gone so far in some of the highest arthropods that all the ventral 

 ganglia have come to form a single mass. In all arthropods the brain 

 is distinct and is joined with the ventral ganglia by a pair of connecting 

 nerves, one of which passes on each side of the oesophagus. 



With a few exceptions arthropods are unisexual, the hermaphroditic 

 forms being a few parasitic or sessile crustaceans and a few arachnids. 

 The gonads are usually paired, tubular glands which open to the outside 

 by a pair of openings in crustaceans and by a single median abdominal 

 opening in most other arthropods. Parthenogenesis occurs among certain 

 crustaceans and insects, and pasdogenesis occurs as a rare phenomenon 

 among the latter. 



Arthropods are generally characterized by the care they take of their 

 eggs and young. Many of them carry their eggs until they hatch, and 

 often the young animals are also carried. Among insects a family life 

 of remarkable complexity characterizes many species, which often leads 

 to the formation of colonies characterized by a division of labor among 

 their individual members, as in the case of the bees, termites, and many 

 others. 



Distribution. Arthropods constitute the largest phylum of animals, 

 numbering about 400,000 known species, or four-fifths of all known spe- 

 cies of animals. They form also one of the most widely distributed 

 groups, being found in all parts of the sea and land. The crustaceans 



