436 



GENERAL CONSIDERATIONS 



tion of the cells formed. Thus there may be found multinucleated cells, 

 such as the striated muscular fibers. There are also living mesh works 

 made up of cells which have only incompletely separated and remain in 

 structural continuity. A tissue of the latter type is called a syncytium 

 (Fig. 291), or, in the case of nervous tissue, a nerve net (Fig. 57). Thefe 

 are protoplasmic bridges which connect the epidermal cells of vertebrates, 

 while in other cases fibrils extend from one cell into an adjacent cell and 

 serve to coordinate the activities of the two. In no metazoan do the 

 individual cells all live as they would if they were alone, but each is 

 affected more or less by the proximity of the others. This is part of the 

 organismal concept. Nevertheless, cells furnish convenient units on 

 which to base our conceptions of morphology. 



470. Organs and Systems. — In Chap. XXI the different systems 

 were enumerated and brief references were made to some of the organs 





MjiL"! :"'■ i':^<%^ 



— >^n 



'%-..#f 





Fig. 291. — Semidiagrammatic representation of a syncytium, as seen in mesenchyme. 



included within each. As each phylum has been taken up in turn, many 

 facts have been given in regard to the development of various systems in 

 it. Here, however, the phylogenetic development of these different 

 systems will be reviewed but only in general terms. It is desirable to 

 begin the discussion of each system with those structures which in lower 

 types, in which the system does not exist, perform the corresponding 

 functions. 



471. Tegumentary System.^ — In the lower protozoans, it has been 

 seen that no cell wall, properly speaking, exists. Many of these forms, 

 however, secrete shells of one kind or another which serve for protection. 

 In the higher protozoans there is a surface layer, or pellicle, which is 

 really a wall secreted by the cell for the purpose of protection. 



The epithelial cells of metazoans always have cell walls. In sponges 

 the body is covered by a single layer of flat pavement cells called a dermal 

 layer. In coelenterates and ctenophores there is no definite epithelium, 

 but the body is covered by an ectoderm. In some cases, as in colonial 

 hydroids, this ectoderm secretes a yerisarc, and in the case of the corals 



