xxv PHYSIOLOGICAL DIFFERENTIATION : ORGANS 291 



There are two matters of general importance in connec- 

 tion with the structure of Polygordius to which the student's 

 attention must be drawn in concluding the present lesson. 



Notice in the first place how in this type, far more than in 

 any of those previously considered, we have certain definite 

 parts of the body set apart as organs for the performance of 

 particular functions. There is a mouth for the reception of 

 food, an enteric canal for its digestion, and an anus for the 

 extrusion of faeces : a coelomic fluid for the transport of the 

 products of digestion to the more distant parts of the body : 

 a system of blood-vessels for the transport of oxygen to and 

 of carbon dioxide from all parts : an epidermis as organ of 

 touch and of respiration : nephridia for getting rid of water 

 and nitrogenous waste : and a definite nervous system for 

 regulating the movements of the various parts and forming 

 a means of communication between the organism and the 

 external world. It is clear that differentiation of structure 

 and division of physiological labour play a far more obvious 

 and important part than in any of the organisms hitherto 

 studied. 



Notice in the second place the vastly greater complexity 

 of microscopic structure than in any of our former types. 

 The adult organism can no longer be resolved into more or 

 less obvious cells. In the deric, enteric, and coelomic 

 epithelia we meet with nothing new, but the muscle-plates 

 are not cells, the nephridia show no cell-structure, neither do 

 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 coelomic fluid are to be looked upon as 

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

 extent that in the foregoing types, we can study the 



u 3 



