BASIC ORGANIZATION OF THE METAZOA 



81 



Fig. 67. — Connective tissue, 

 consisting of cells, matrix, and 

 fibers. 



(the lobster, for example) have irregular spaces among their organs, filled 

 with body fluids, there is sometimes difficulty in deciding whether a 

 cavity is a coelom or not. In general, the coelom must be lined by a 

 definite layer of cells, the peritoneum, which is lacking around the spaces 

 in the lobster, and the principal reproductive organs {gonads) are sus- 

 pended from its walls. 



Tissues. — In practically all metazoa in which the several kinds 

 of somatic cells are very numerous, those of any one kind are grouped 

 together, not necessarily all in one place but 

 usually in a number of places. In Hydra 

 (Fig. 59), as we have seen, all endoderm 

 cells together form a continuous layer con- 

 stituting the inner part of the body wall. 

 The epithelial cells are similarly placed 

 together in the ectoderm. The secreting 

 cells of the foot are together in a small 

 group. The other somatic cells of Hydra 

 are not conspicuously grouped, since the 

 subepithelial cells and the cnidoblasts derived from them do not form a 

 continuous layer. 



In most metazoa the somatic cells of any given sort are more con- 

 spicuously assembled in layers or masses than in Hydra. Such groups 

 or masses of like cells are called tissues. A tissue may be defined as a 

 number of cells of the same kind forming a continuous mass. Ordinarily 

 they perform some function in common, but it is not necessary to know 

 their function to consider them a tissue. Tissues may be classified on 

 the basis of both structure and function. In the vertebrate animals 

 these classes are sustentative, epithelial, contractile, nervous, vascular, and 

 reproductive. 



Sustentative Tissues. — The sustentative tissues are primarily those 

 which support. The typical sustentative tissue is ordinary connective 

 tissue which binds the skin to the flesh beneath or holds the muscles 

 of the thigh together in a mass or helps suspend the intestine from 

 the body wall. It contains scattered cells (Fig. 67), but the serviceable 

 part is made of things secreted by the cells. These things are a gelati- 

 nous matrix, or ground substance, and large numbers of tough fibers 

 imbedded in the matrix. It is the latter that give connective tissue its 

 strength. 



Certain connective tissues of very great strength are given special 

 names. The ligaments binding bones together at the joints, and the 

 tendons joining muscles to the bones which they move, are examples. 

 The essential fea,tures of connective tissue — cells, matrix, fibers — are 

 present in both, but the fibers far outbalance the other parts. 



