408 ARTHUR '\\ MASTERMAN. 



very generally in the lower types of Metazoa, especially 

 sedentary types in whicli the cilia, no longer motor, are 

 entirely trophic in function. The first important differen- 

 tiation in cilio-trophic alimentation is the separation and 

 retention of food particles and the removal of the water- 

 current. The active areas which are trophic have a charac- 

 teristic form of epithelium which may be termed tropho- 

 phoral; and the areas along which the return water-current 

 is removed have another type of epithelium which may be 

 termed hydrophoral. Trophophoral epithelium is made up 

 of densely crowded elongated cells, covered with active cilia, 

 and very commonly having glandular cells added to them. 

 Hydrophoral epithelium has flat or cubical cells, usually non- 

 ciliated or with few cilia, and often pigmented or even 

 actively excretory. 



For the separation of the food particles from the water- 

 current at least two important general methods are resorted 

 to. The first is the principle of filtration, in which water is 

 allowed to pass through certain apertures and food retained, 

 or vice versa; and the second is by an entanglement of 

 the food particles in mucus strands which allow the water- 

 currents to pass in other directions, whilst they themselves 

 are conveyed into the alimentary canal. 



The first system is probably the most primitive, and in 

 almost every case the second is superadded to it. I have 

 attempted elsewhere (10b) to show that a great number of 

 the characteristic orgaus of the Chordata, including noto- 

 chord, hypochorda, pharyngeal clefts, thyroid gland, and 

 hypophysis, may be traced to an origin which they subserved 

 in either one or other of these methods of food ingestion, 

 and the matter will be further dealt with later. 



In Phoronis both methods are exemplified very perfectly. 

 In the case of water-filtration it is evident that the greater 

 the surface of contact which is effected between the tropho- 

 phoral areas and the hydrophoral areas, the greater will be 

 the efficiency of the filtration attained. The simplest rela- 

 tionships obtain in the larvas of Echinoderms in which the 



