47 



CHARACTERS OF INVERTEBRATE ANIMALS 



ment, each separate bit corresponding to a cell. The lining of 

 heart and blood-vessels is of this nature. Stratified scaly epithe- 

 lium resembles the preceding, but is several cells thick. Examples 

 are to be found in the lining of the human mouth, and the epi- 

 dermis of Frog. Simple columnar epithelium consists of prismatic 

 cells packed together in a single layer, and is very characteristic 

 of stomach and intestines. Other examples will be noted else- 

 where. In many cases the surface cells of epithelium are provided 



Fig. 288. Different kinds of Epithelium, enlarged to various scales. All except c are in vertical 

 section. Nuclei represented in black 



A, Simple columnar; B, simple glandular; c, simple scaly; c', simple scaly, surface view; D, stratified 

 scaly; E, simple columnar, with flagella; F, simple columnar, with cilia; G, stratified, with cilia. 



with numerous slender threads of protoplasm, cilia, which by their 

 united action can set up currents. An individual cilium possesses 

 the power of alternately bending and straightening itself. Various 

 examples have already been given of the presence of such struc- 

 tures (pp. 49, 428, 445). 



The ectoderm and endoderm of Hydra are to be regarded 

 as tissues, but do not exhibit the high degree of specialization 

 found in the phyla so far dealt with. They are more of the nature 

 of epithelium than anything else, and this form of tissue is justly 

 regarded as a comparatively primitive one. Beginning with the 

 ectoderm (fig. 287), we find that the largest elements contained 

 in this are what may be termed tailed-cells, since each of them 

 is drawn out at its inner end into a fibre which appears to be of 

 muscular nature. These tails all take a longitudinal direction, and 

 collectively constitute a muscular layer by means of which the 

 body can be shortened, supposing the fibres to contract or shorten 



