488 The Animal Kingdom 



epidermal cells secrete a new epicuticle under the endocuticle ; then 

 special glands in the epidermis secrete a molting fluid through a duct 

 opening above the new epicuticle. This fluid contains an enzyme cap- 

 able of digesting some proteins and chitin, but not cuticulin. Since the 

 epicuticle is made up largely of cuticulin, it is not affected by the molting 

 fluid, but the endocuticle is rapidly dissolved away. Possibly the old 

 dissolved material is used in the formation of the new exoskeleton. As 

 soon as the new cuticle is formed, the arthropod breaks its way through 

 the old one by expanding portions of the body. The animal then wriggles 

 out of its old "skin" and emerges with a soft new one which rapidly 

 hardens and darkens. 



The fundamental metamerism of the arthropods is variously modi- 

 fied among the members of the different classes. In some it is very 

 clearly visible, while in others fusion of segments obscures the basic 

 plan. Among the centipedes, for example, the segments are clearly 

 seen, while among others it is apparent only in such things as the nerve 

 ganglia, the musculature, and embryology. In most classes, body re- 

 gions are clearly visible; thus the insect has a clearly defined head, 

 thorax, and abdomen, while the crayfish has a cephalothorax (fused head 

 and thorax) and an abdomen. 



The members of a phylum as diverse as the arthropods have so 

 many modifications of their various organ systems that generalizations 

 are difficult as to the details. These are best considered in relation to 

 each class. Briefly, however, they all have a complete digestive tract 

 with various digestive glands and definite regions. The circulatory 

 system is of the open type; nearly all have a central pumping organ 

 of some type, a few blood vessels, and an extensive series of blood 

 sinuses forming a hemocoel. In the majority of species, the respiratory 

 pigment is hemocyanin, a bluish green protein containing copper and 

 sulfur ; a few forms have hemoglobin. Even more variable is the manner 

 in which respiratory exchange is made possible. Some species have 

 gills, others tracheal tubes, still others book lungs, while some simply 

 use the body surface. Excretion is accomplished by special glands in 

 some and by Malpighian tubules in others. These latter are long tubes 

 in the body spaces which pick up uric acid wastes from the body fluids 

 and discharge them into the intestine. Among most species, the nervous 

 system is quite similar to that of the annelids inasmuch as it consists 

 of dorsal ganglia, circumesophageal connectives, subesophageal ganglia, 

 and a ventral nerve cord with ganglia. In general, there are many 

 different types of sense organs, most of which are highly developed. The 



