86 THE PLANT CELL WALL 



being "buried" within the cytoplasm hence serving as an internal 

 skeleton. The acantharian spicular system differs from the common 

 pattern in consisting of strontium sulfate instead of silica. 



Metazoa 



The cells of multicellular animals are not characteristically 

 "walled off" on all surfaces from neighboring cells by their secre- 

 tions. We often find extracellular materials which have been elab- 

 orated by the exposed surfaces of cells in the external layer of the 

 metazoan body. Such materials commonly serve as exoskeletons. 

 Among coelenterates, the corals (class Anthozoa) are often iden- 

 tified with skeletal products. In many forms, the skeleton is an 

 ectodermal product consisting either of calcium carbonate or horny 

 proteinaceous substances. 



The exoskeleton reaches high point among the arthropods. 

 The cuticle is a complex and highly structured system of macro- 

 molecules elaborated by the epidermal cells. 



The major building materials of the cuticle are fibrous proteins 

 chitin, phenols, and, in some cases, calcium carbonate. Arthropods 

 characteristically produce the a-chitin which has been described 

 in the fungi. 



Other forms— annelids and mollusks— produce (3-chitin, which 

 is a crystallographic modification on of the a-form. 



In insect metamorphosis, cuticular development begins with 

 the secretion of a lipoprotein layer, cuticulin, by the epidermal 

 cells. Following this, lamellae consisting of approximately equal 

 parts of chitin and the protein arthropodin are laid down. The 

 old cuticle is digested by proteinases and chitinase and reabsorbed. 

 Shortly before molting, phenols are released and incorporated 

 in the new cuticle and a wax layer is deposited on its surface. As 

 the new instar emerges, a final lipoprotein layer is deposited on 

 the surface while the phenols are oxidized to quinones which cross- 

 -link cuticular proteins, forming the final dark, hard outer layers. 



The chitin and protein are internally associated in the cuticle. 

 From X-ray analysis, it may be concluded that chitin and protein 

 lie in planes consisting of parallel rows of oriented protein and 

 chitin chains. The protein is of the extended, [3-keratin type, hence 



