THE OEGANS. 49 



itself from tliis point onwards either in tlie form of numerous 

 follicles^ wliicli beset a large portion of tlie canal^ or it forms a large 

 group of glands, whicli open separately or together into the 

 alimentary canal. The differentiation of the liver leads to a gradual 

 separation of that organ from the digestive tube, so that finally it is 

 merely connected to the canal by its ducts (higher Mollusca, Verte- 

 brata). 



Respiratory Organs of the Enteron, 

 § 42. 



The general differentiations of the primitive gut (archentei*on), 

 formed by the endoderm, which have been already mentioned, give 

 rise, in obedience to the principle of division of labour, to organs 

 which serve for the ingestion and digestion of nutrient matters; 

 these do not confer any essentially new functions on the gut. But 

 such a new function does appear when the gut acquires relations to 

 respiration. It is not certain whether this function obtains in the 

 primitive gut, although this is probable, for the endoderm is bathed 

 by the surrounding medium, like the outer layer of the body, while 

 the water which is taken in with the food may serve for respiration. 

 The relation becomes much more definite when we note the regular 

 streaming in of water into the rectum, which obtains in many 

 Vermes and Mollusca, This phsenomenon is an indication of the 

 respiratory function of the gut, but has no bearing upon the forma- 

 tion of distinct respiratory organs, which are differentiated from the 

 digestive tube. 



Such a respiratory organ is brought into existence in the most 

 anterior portion of the gut, by its walls being broken through by 

 lateral pores ; by special relations of these pores to the vascular 

 system it acquires a respiratory significance. This arrangement, 

 which already makes its appearance in the lower divisions, occurs 

 again in the Vertebrata. Processes, known as branchiae, in which 

 the respiratory vascular network is distributed, axise on the walls of 

 the clefts of this cavity. A region of the primitive digestive tube 

 is thus converted into a special portion, a branchial cavity, at the 

 hinder end of which the tube which serves exclusively for nutrition 

 commences. 



Aziother form of respiratory organ is developed from the wall 

 of the gut, in the form of a diverticular outgrowth of the anterior 

 portion of that organ. This appendage of the gut is filled with air, 

 and in fishes has merely a hydrostatic function. As the relations 

 of the circulation become changed it is gradually converted into a 

 respiratory organ, and becomes the lungs; in the higher divisions 

 of the Vertebrata new organs, namely, those for the production of 

 a voice, are developed on the passages leading into it. 



