ANIMALS WITH JOINTED FEET-THE ARTHROPODS 201 



composed of a protein substance, chitin, plan to that of annelids, is considerably 

 which is impregnated with more or less more centralized. There are fewer ganglia, 

 lime, depending on the species. Some and more independence of all parts of the 

 arthropods, like the lobster, have a high body. This increased integration has re- 

 percentage of lime in the exoskeleton, suited in an animal that is swifter, more 

 whereas the insects have smaller amounts, agile, and better able to cope with its en- 

 The greater the lime content, the harder the vironment. 



skeleton. For the land dweller, exposed to The coelom is much reduced in size, 



rapid desiccation, a waterproof outer cover- being replaced to a large extent by a sys- 



ing becomes essential. Since the exoskeleton tem of blood spaces called the haemocoel. 



of these air breathers is practically water- There are certain modifications in other sys- 



proof, one may assume that this condition tems also, but these will be discussed in the 



made it possible for the animal to invade various groups, as they occur, 

 land. The arthropods include such a wide va- 



One rather serious disadvantage to this riety of forms that it seems as though they 



external plate of armor is concerned with had explored nearly all the possibilities, 



growth. As the animal increases in size, this There are more species in this group of 



suit becomes tighter and tighter, until animals than in all others put together, in 



finally the animal must rid itself of the old fact, several times as many. Furthermore, 



suit and secrete a new one. During the time the number of individuals exceeds all other 



of transition, however, the arthropod finds Metazoa. The arthropods literally encom- 



itself in a very precarious situation. Having pass the earth; they invade the soil, the 



shed the old suit, it must wait for its body water, the air, the frigid polar zones, and the 



size to enlarge and for the new skeleton to torrid equatorial latitudes. Since they feed 



harden before its muscle can again be elfec- on the same foods as man, they have be- 



tive. This procedure may take several hours come his most serious competitor, indeed, 



and during this time the body is very soft it has been said that the main struggle for 



and nearly immovable. The animal is, survival today is between the arthropods, 



therefore, unprotected at such times and particularly the insects, and man. This 



takes special precaution to conceal itself might be true if man does not destroy him- 



from its enemies. The process of shedding self first by his own cunning. Insects are 



the skeleton and growing a new one is carriers of some of the most serious diseases 



called molting. which affect man and his domestic animals, 



Another important difference between and although some species are beneficial, 



the annelids and the arthropods is the pres- it is doubtful that the benefit to man of 



ence in the latter of feet with joints. This some outweighs the damage done by the 



conspicuous character gives the phylum its others, 

 name. The feet and legs of animals with a 

 hard exoskeleton need joints for the pur- 

 pose of movement. In the arthropods many 

 appendages have lost theii' original loco- 

 motor function and have become radically The phylum Arthropoda is divided into 



modified into organs of defense, offense, several classes (Fig. 11-1), of which only 



and even sense organs. The exoskeleton and four warrant discussion in an introductory 



the jointed appendages thus have distinct text. The first is a very small group, the 



advantages and have contributed much to Onychophora, composed of about 70 spe- 



the success of this group. cies, all of which bear the name of Peri- 



The nervous system, while similar in patus. These small animals are as interest- 



THE ONYCHOPHORA. 

 PERIPATUS 



