654 - Heredity and Evolution 



radula; an entirely marine group; 400 living 

 and 10,000 fossil species. 



THE ARTHROPODA (LITERALLY, "JOINTED 

 LEGS") 



This is by far the largest phylum in the 

 animal kingdom. The diverse forms total to 

 more than 700,000 species. In fact, the arthro- 

 pods outnumber all other kinds of animals 

 by approximately four to one. 



The phylum Arthropoda includes: all 

 crabs, shrimps, lobsters, barnacles, and other 

 crustaceans; the flies, bugs, beetles, butter- 

 flies, and other insects; spiders, scorpions, 

 ticks, horseshoe crabs, and other arachnids; 

 and all centipedes and millipedes. Also there 

 are some other less familiar forms, such as a 

 small, very primitive genus. Peripatus (Fig. 

 32-30), and a large extinct group, the Trilo- 

 bita (Fig. 32-31). 



The biological success of the arthropods 

 is reflected by their wide range of habitats. 

 They occur almost everywhere — at heights 

 and depths exceeding 20,000 feet; in salt, 

 brackish, and fresh waters; on land and in 

 the air; and on or in other organisms, as 

 ecto- or endoparasites. Moreover, Arthropoda 

 are of tremendous practical importance: as 

 pollinating agents in many plants; as carriers 

 of disease (lice, ticks, flies, mosquitoes); as a 

 food base for fish and birds; as a direct source 

 of food for man (lobsters, shrimps, crabs, 

 etc.); as destroyers of man's food, crops, and 

 clothes; and in many other ways. 



Characteristics of the Phylum. In some 

 respects arthropods resemble other complexly 

 organized animals. They display bilateral 

 symmetry, a tubular enteron, and a coelom. 

 The coelom, however, is reduced. It includes 

 mainly just the cavity enclosed within the re- 

 productive organs. 



Arthropods, like annelids, are distinctly 

 segmented animals. But unlike annelids, the 

 arthropods, except for Peripatus and a few 

 other primitive transitional forms, all display 

 jointed appendages. Typically, one pair of 

 jointed appendages is borne by each of the 



body segments; but among higher forms, 

 especially insects, many of the appendages are 

 missing. 



The entire body of the arthropod is cov- 

 ered by a strong chitinous exoskeleton, which 

 may or may not be hardened by a deposition 

 of calcareous material. Such a complete en- 

 casement of the body does not allow for 

 continuous growth. Consequently arthropods 

 must shed the exoskeleton periodically. This 

 process is called ecdysis, or moulting. After 

 each moult the animal quickly swallows a 

 large amount of water (or air) rapidly in- 

 creasing the size of its body before the newly 

 secreted exoskeleton becomes too hard. 



In contrast to chordate animals, arthropods 

 generally do not possess any endoskeleton. 

 Also the central nervous system consists of a 

 double chain of ganglia, which, except for 

 the first pair (cerebral ganglia), lie ventral to 

 the digestive tract. Typically there is a heart 

 and frequently there are blood vessels leading 

 to the principal organs. But the circulatory 

 system is of the open type. The hemolymph 

 passes through the tissue spaces and comes 

 into direct contact with the cells. Aquatic 

 Arthropoda (chiefly the Crustacea) possess 

 gills, except for a few small species that 

 respire directly through the body surfaces. 

 But terrestrial forms have tracheae (insects, 

 centipedes, and millipedes) or book lungs 

 (spiders and other arachnids). 



Arthropoda, compared with Annelida, tend 

 to display a more distinct differentiation of 

 the principal body parts, namely head, 

 thorax, and abdomen. Several fused seg- 

 ments can usually be recognized in the head 

 and thorax regions, although the segments 

 of the abdomen tend to be separate. Among 

 Crustacea and Arachnida the head and 

 thorax are frequently fused, forming a body 

 part called the cephalothorax. 



The exoskeleton of the arthropod not 

 only serves to cover and protect the soft in- 

 ternal structures, but also provides for the 

 muscular movements of the animal. The suc- 

 cessive rigid parts of the exoskeleton are 

 hinged together by flexible joints and the 



