The Morphology of Tissues. 169 



There are four main classes of tissues among animals j but 

 these are again subdivisible. To use a metaphor borrowed 

 from systematic zoology, there are four genera of tissues, and 

 each contains a number of more or less closely allied species. 



The four genera of tissues are (i) epithelial, (2) muscular, 

 (3) nervous, and (4) connective. 



We cannot here enter into anything like a full account of 

 these several tissues, or even their principal modifications ; 

 these matters are more suitably treated of in detail in a physio- 

 logical handbook, and will be so treated in a later volume of 

 this series. 



But a few general facts and conclusions may be suitably 

 introduced into a handbook of zoology. 



The epithelial tissties may be exemplified by the epidermis 

 of the earthworm figured upon p. 168. Similar tissues are the 

 epidermis of the frog, the cells which line the gut of the same 

 and other animals, the tissues which form the tubules of the 

 kidney, etc. In all these cases the tissue is composed of 

 groups or layers of cells similar to each other, which have 

 preserved the typical cellular form. In the simplest animals, 

 such as Hydra, all the tissues are of this kind. 



The musadai- tissties are characterized by the fact that a 

 large portion of the cells of which they are composed have 

 been metamorphosed into fibres of a contractile substance. 

 All protoplasm is contractile, as is shown by the movements of 

 an amoeba and by the circulation of the protoplasm in a 

 vegetable cell. But in these cases there is no line to be drawn 

 between those portions (of the ectoplasm, at least) which are 

 especially contractile and those which are less so. In the Vorti- 

 cella, on the other hand, there is a definite layer of the ectoplasm 

 — .the myophan layer — in which contractility principally resides. 

 In the ectoderm of the multicellular Hydra, special cells are set 

 apart to perform the part of contractility; but it will be 

 observed that these cells have not lost their typically cellular 

 character. A process, or two processes, have grown out from 

 their under surface which are not, or are hardly, distinguishable 

 from the protoplasm of the rest of the cell ; these are the 

 muscular processes of those cells. In animals belonging to 



