288 POLYGORDIUS 



the nerve-fibres nor the punctate substance of the brain. 

 The body is thus divisible into tissues or fabrics each clearly 

 distinguishable from the rest. We have epithelial tissue, 

 cuticular tissue, muscular tissue, and nervous tissue : and 

 the blood and ccelomic fluid are to be looked upon as 

 liquid tissues. One result of this is that, to a far greater 

 extent than in the foregoing types, we can study the 

 morphology of Polygordius under two distinct heads : 

 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. 272) 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. 273). Thus certain of the tissues of 

 Polygordius exhibit continuity of the protoplasm, a fact of 

 considerable interest in connection with the question of the 

 origin of multicellular animals discussed in the previous 

 lesson (p. 266). 



