460 SUMMARY AND CONCLUSION. 



dages, and of the sensory and respiratory organs. Its chief significance therefore 

 lies, not in its occasional and purely incidental usefulness, especially in the higher 

 forms, as a material to construct supplementary organic instruments, or weapons 

 of offense and defense, but in its compelling influence on form, and in its 

 creation of an internal organic environment distinct from that of the surrounding 

 medium. 



Chiten appears at a precisely defined period in arthropod embryos, as the 

 result of some chemical transformation that takes place on the outer surface of 

 ectoderm cells. Its presence is at once recognized by the way in which it pre- 

 vents the penetration of stains and other chemical reagents. There is no reason 

 to doubt that it appeared in the ancestral arthropods with corresponding rapidity, 

 and that it had an immediate and persistent transforming effect on those animals 

 in which it occurred. From the earliest period, therefore, every member of 

 the arthropod stock has been a practically closed mechanism. Its sensory, 

 respiratory, and excretory relations with the exterior were necessarily confined to 

 precisely located, and definitely constructed points, or openings. The location and 

 attachment of muscles, and the movements of one external part on another, were 

 controlled by the location of flexible, hinge-like joints in the armor; and no 

 notable increase in volume or in organic activity could take place without special 

 provisions for the circulation of a blood-like plasma that should serve at the same 

 time as a uniform internal environment, distinct from the external one, and that 

 was suitable both for cell growth and as a means of transporting nutritive and 

 waste substances to their place of consumption or point of discharge. 



The presence, therefore, of a chitenous exoskeleton in the arthropods may 

 be regarded as one of the primary causes of their slow growth in volume and of 

 the early historic appearance of precisely located sensory, motor, excretory, cir- 

 culatory, and respiratory organs; and of their sharply defined, closely knit, and 

 potentially sound organization. 



These conditions are in sharp contrast with those in the naked, soft bodied 

 ccelenterates, and in many worms, molluscs, and acraniates, where the surface 

 of the body is more uniformly exposed to external agents, and the external medium 

 has great freedom of access to the interior. 



The sudden loss of a chitenous exoskeleton, after a longer or shorter period 

 of control, is no less significant than its initial formation. When some com- 

 paratively insignificant change in its chemical composition, or in its mode of 

 growth, led to a radial change in its physical properties, or to its complete dis- 

 appearance, the body was again released from its control and a new set of form- 

 creating factors arose, but on a different level from the old. The change from a 

 chitenous armor to one of cellulose was a powerful factor in the creation of the 

 tunicate type; and the almost total absence of chiten in Balanoglossus, Cephalo- 

 discus, Amphioxus and the polyzoa; the substitution for it of a heavy armor of 

 isolated calcareous plates in the echinoderms; and the predominance in it, of 

 a heavy calcareous deposit in the cirripeds, were all factors of great 



