STRUCTURE AND FUNCTION IN INVERTEBRATES 



from sheep and cattle, as by-products of slaughter-house operations. 

 This same lack of specificity is characteristic, within limits, of many en- 

 docrine systems among invertebrates. Extracts, or entire glands, from one 

 species of crustacean will exert their usual eflfect when appropriately ad- 

 ministered to other kinds of crustaceans. It has recently been announced that 

 from several thousands of silkworms a minute amount of a purified, crystalline 

 substance has been prepared which represents the "growth and diflferentiation 

 hormone" of this species, produced in the prothoracic glands (pp. 456-457). 

 It is a relatively simple organic compound with the empirical formula 

 C,^H3„0^. It is very powerful in its action and is efTective in initiating 

 metamorphic changes in several insects other than the silkworm. The 

 "juvenile hormone" produced in the corpus allatum has also been identified 

 in extracts of certain insects and will probably soon be isolated. 



In contrast to the typical invertebrate endocrine system as now under- 

 stood, endocrine organs in vertebrates do not as a rule involve neuro- 

 secretory cells. An exception to this statement is provided by the posterior 

 lobe of the pituitary gland in mammals. This organ appears to act chiefly as 

 a reservoir for hormones produced by neurosecretory cells in the part of the 

 brain to which it is attached. It is noteworthy that in both vertebrates and 

 invertebrates modified neurons have become adapted as endocrine organs, 

 thus furnishing a physical as well as a functional relationship between the 

 dual mechanisms of coordination, nervous and chemical. 



Supporting Mechanisms: the Skeleton 



The skeletons of animals furnish support for the body and serve as places 

 of attachment for muscles, which for efTective action must have a firm 

 anchorage. To a v'arving degree, certain kinds of skeletons may also func- 

 tion in enclosing and protecting the soft parts of the animal; and some 

 superficial structures related to skeletons seem principally adapted only 

 to this last function. Skeletons mav usually be classified in two general 

 t\pes. exoskeletons and endoskeletons. Exoskeletons typically cover the 

 surface of the bodv, contain no cells, and are secreted by the underlying 

 epidermis. In contrast, endoskeletons usually lie deeper within the body, 

 enclose cells which produce the skeletal material, and are of mesodermal 

 origin. In some animals exoskeletons become embedded in the superficial 

 tissues of the body, and in others endoskeletons appear to lie at the surface, 

 so that position alone is not a reliable criterion. The majority of skeletons 

 among invertebrates are exoskeletons, but there are many examples of endo- 

 skeletons also. 



Even among Protozoa, supporting and protective structures have been 

 developed that parallel the two types of metazoan skeletons. Many Sar- 

 codina, such as Arcella, radiolarians, and foraminiferans, lay down protective 

 coverings and supporting skeletons of organic or various inorganic materials. 



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