STRUCTURE AND FUNCTION IN INVERTEBRATES 



It bears a striking resemblance to the cartilaginous ''chondrocranium" of the 

 vertebrate series. 



The arthropod exoskeleton appears to be a different modification of the 

 basic cuticle, which has been thickened and stiffened by additional deposi- 

 tion of organic materials, such as scleroproteins and chitin. In some of the 

 larger aquatic crustaceans, calcium salts have also been added to the skeleton. 

 The most significant skeletal advance among arthropods is the breaking up of 

 the exoskeleton into stiff, inflexible portions termed sclerites, interconnected by 

 thin, flexible joints. Many parts of the arthropod exoskeleton have become 

 folded inward to serve more eflficiently as points of muscle attachment. Al- 

 though these may appear as endoskeletal structures, their origin and nature 

 are clearly like those of the outer exoskeleton. The effect of the development 

 of the rigid and unyielding exoskeleton on growth in the arthropods has been 

 noted (p. 484); mollusks and other enclosed forms grow by the addition of 

 material, continually enlarging the skeleton, but arthropods must shed the 

 exoskeleton and secrete a new and larger one to provide for increase in size. 

 Periodic molting of the cuticular skeleton is also characteristic of the growing 

 stages of Nematoda (p. 352). 



The skeletal structures of echinoderms are components of the body wall and 

 thus resemble exoskeletons. These structures are always produced by tissues 

 of mesodermal origin, however, as are the bones of vertebrates, and are 

 covered externally by epidermis. This epidermal covering may be eroded 

 away, as on the tips of spines, but this does not alter the fundamental 

 relationships. Sea cucumbers commonly have flexible, apparently unpro- 

 tected body walls, but closer inspection reveals the presence of microscopic 

 spicules and plates which are clearly homologous with the skeletons of other 

 echinoderms. 



Reproductive Systems and Their Functions 



All the organs and systems discussed so far function primarily in the main- 

 tenance of the individual animal — in its nutrition, in the regulation and 

 integration of its various physiological processes, in the support and pro- 

 tection of its organs, and so on. Unlike these, the reproductive system is 

 of no value in the day-to-day activities of the individual; the survival of the 

 animal does not depend on the proper functioning of its reproductive organs. 

 This system is devoted entirely to the maintenance of the species and its pro- 

 jection into the future. In the long view, the activities of the individual are 

 all, directly or indirectly, related to the reproductive process. Unless the 

 individual is well nourished and otherwise capable of survival in its environ- 

 ment, it will be unable to produce new generations of its kind. The problem 

 of reproduction involves not only the formation of new individuals but also 

 their production at such a rate, or in such numbers, as to maintain or increase 

 the population of the species, in the face of what might be termed environ- 



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