204 INVERTEBRATE ZOOLOGY 



substances, especially lime, are frequently added to the chitin, 

 thus giving the skeleton additional strength and thereby afford- 

 ing greater protection to the underlying parts. The skeletal 

 plates are more or less telescoped at the more prominent joints, 

 so the body surface presents an uninterrupted armor, 



Ecdysis. — With age, the chitinous covering increases in thick- 

 ness and becomes an effective barrier to further growth of the 

 parts which it encases. Growth is then rendered possible only by 

 periodic shedding of the cuticula in a process known as molting 

 or ecdysis. Immediately following the ecdysis, the body cover- 

 ing is extremely soft and pliable and during this period there is 

 rapid increase in size. Frequency of ecdysis varies greatly in 

 different groups of the arthropods but it is of much more frequent 

 occurrence in early stages of development. The Crustacea 

 continue to shed the skin periodically throughout life. In the 

 insects, attainment of the adult form marks the last ecdysis 

 through which the individual passes, except in the mayflies which 

 molt once after the wings become functional. 



Number of Segments. — In the fundamental structural plan 

 of an arthropod, each segment bears a pair of appendages. 

 Fusion of segments and degeneration of appendages on some 

 segments frequently obscure this plan, but even in such instances 

 evidences of the primitive condition are often still observable 

 in the embryo, for the vestiges of appendages occur here even 

 though they may be entirely wanting in the adult. 



Characteristic Organ Systems. — ^The chief organ of the 

 circulatory system, a dorsal vessel or a heart, lies dorsal to the 

 digestive canal. The circulatory system is of the open type, for 

 the body fluid is not restricted to vessels throughout its course 

 but is frequently spilled into sinuses or lacunae. These sinuses 

 are so prominent that they are frequently mistaken for a coelom. 

 These cavities are a development of a portion of the circulatory 

 system, hence they are designated as a haemocoel, for they are 

 not a true coelom. 



The central nervous system consists of a ladderlike chain of 

 ganglia (Fig. 123) and a brain or supraesophageal ganglion of 

 which all but the brain is ventral to the digestive tract. 



Various modifications for respiration and excretion are found, 

 but these will be discussed under the several classes. 



Development is always sexual but modified types of sexual 

 development are found in the establishment of parthenogenetic 



