Phylum Arthropod a 487 



Characteristics of the Phylum. — While the arthropods as a group 

 posses certain unique features, most of their fundamental characteristics 

 have been seen in other phyla. They are bilaterally symmetrical, possess 

 three germ layers, are at the organ grade of construction, and have a 

 well-developed head region with sense organs. In common with the 

 molluscs and annelids, many have determinate cleavage and a schizocoel 

 type of coelom. As in the molluscs, this schizocoel is much reduced 

 and in most cases consists only of cavities in the pericardium, excretory 

 organs, and gonads. Unlike the molluscs, but like the annelids, there 

 are paired appendages and well-developed metamerism. The paired 

 appendages of the arthropods bear only slight resemblance to the setae 

 of the earthworm, for these appendages are jointed, modified into 

 different regions, and move by means of an elaborate musculature. 

 The jointed appendages and the chitinous exoskeleton or cuticle are the 

 truly unique features of the members of this phylum. 



Chemically, chitin is a nitrogenous polysaccharide. It is insoluble 

 in alcohol, water, dilute acids, and dilute or concentrated alkalies. While 

 this material is not unique with the arthropods, its organization into a 

 protective exoskeleton is found in no other animals. In most arthropods 

 regions of this chitinized exoskeleton are harder, that is, they are 

 sclerotised. In such areas there is a smaller amount of chitin, but a large 

 amount of the protein cuticulin. Between the sclerotized plates are 

 softer areas which contain as much as 80 per cent of chitin and are 

 termed membranous. The exoskeleton is often also impregnated with 

 colored materials such as carotin and melanin. 



Bodily movements are made possible in spite of the relatively in- 

 flexible skeleton due to the fact that the sclerotized portions are arranged 

 in plates or segments with membranous portions between them. This 

 chitinous exoskeleton or cuticle is secreted by the epidermis which lies 

 directly below it, and is formed of a thin outer epicuticle and a thicker 

 cndocuticle. The epidermis is made up largely of glandular epithelial 

 cells, some of which secrete special products such as wax, poisons, or 

 special scent materials. These are discharged to the surface by means 

 of ducts which pass through the chitinous cuticle. Other cells give rise 

 to flexible hairs and setae which also pass through pores opening to the 

 outside. 



Inasmuch as the exoskeleton is inflexible, it imposes certain re- 

 strictions upon the size which the arthropod can attain. For addi- 

 tional growth to be possible, this skeleton must be periodically shed 

 in a process known as ecdysis or molting. At the start of molting, the 



