xxiv ANATOMY AND HISTOLOGY 289 



anatomy, dealing with the general structure of the parts, 

 and histology, dealing with their minute or microscopic 

 structure. 



One point of importance must be specially referred to in 

 connection with certain of the tissues. It has been pointed 

 out (p. 273) that the epidermis has rather the character of 

 a sheet of protoplasm with regularly-arranged nuclei than of 

 a layer of cells, and that the muscle-plates are covered with 

 a layer of protoplasm with which the ultimate nerve-fibres 

 are continuous (p. 274). Thus certain of the tissues of 

 Polygordius are multinucleate but non-cellular. They are 

 comparable in minute structure to an Opalina or to the 

 plasmodium of a Mycotozoon, and must therefore be dis- 

 tinguished from such definitely cellular tissues as the enteric 

 epithelium. , 



