HIGHER RIGID SUPPORTING TISSUES 



6 7 



THE SUPPORTING CONNECTIVE TISSUES: SPECIALIZED RIGID 



FORMS 



We shall consider as specialized forms of rigid connective tissues, all 

 those cases in which a large number of cells work collectively to produce 

 a single supporting structure or homogeneous supporting material for 

 the body. Such materials are usually produced inside of the body by 

 mesodermal tissues and form an endoskeleton. As examples of such, 

 we shall study the tissues that form the skeletal structure of the common 

 sponge, others that build up the cartilages of the squid and the verte- 

 brate animals, and those that form the bone of the mammals. Many 

 integumental structures are, in effect, skeletal. We therefore refer the 

 student to Chapter XX, Part A, to read, in this connection, the descrip- 

 tions of the shell of an arthropod and the scale of a teleost fish. The 

 "pen" or shell of the cephalopod mollusk, while an ectodermal product, 

 functions purely as an internal organ 

 of support and will be treated of in 

 this chapter. 



The Cellulose Skeleton of Euspon- 

 gia (Fig. 68). In this sponge the 

 skeleton consists of long, curved, and 

 branching rods of a material that re- 

 sembles cellulose and is variously 

 branched and arranged to loosely 

 support and uphold the soft and deli- 

 cate tissues of the body. 



The particular cells that are respon- 

 sible for the production of this fiber 

 are part of the middle layer of con- 

 nective tissue, and are indistinguish- 

 able from their fellows until a fiber is 

 to be made in their neighborhood. 



Then they accumulate in the path 

 of the future fiber and its branches, 

 and secrete and pour out the material 

 of which it is formed. ^ While the fiber FlG 68 ._ Part of a newly forming skeletal 



is growing, they remain attached to it, fiber of Euspongia. (From SCHNEIDER 



forming at this time an epithelium-like after F> E - ScHULTZE -> 

 layer of cells around it. They differ in shape and other features from 

 other mesoglcea cells during this period of fiber production, but when 

 it is finished they retire and again assume the form of the other con- 



nectivdt-issue cells. 

 uf 



