4i8 ANIMAL RESPIRATION 



grooves are thus converted into two ciliated canals, in one of 

 which an inward current of sea-water is maintained, while the 

 other is traversed by an outward current (fig. 546). The in- 

 flowing water carries with it not merely small organisms and 

 particles which serve as food, but also dissolved oxygen which 

 can be used for internal respiration. 



SPONGES (Porifera) AND ANIMALCULES (Protozoa) 



As explained in the preceding section, the body of a simple 

 Sponge is shaped like a cup or vase with walls perforated by 

 numerous holes (see fig. 265). By means of ciliary action sea- 

 water is caused to stream continually through these holes into 

 the central cavity, and thence to the exterior by the opening of 

 the cup. In this way the animal, in spite of the fact that it is 

 fixed, obtains an abundant food-supply together with plenty of 

 oxygen, while at the same time all the products of waste are 

 swept away. In the more complex Sponges, which are to a 

 great extent colonial, the body is traversed by a labyrinth of 

 canals, parts of which are ciliated, but the feeding and breathing 

 arrangements are essentially the same as in the simple forms. 

 Although no doubt the outer surface of a Sponge helps in 

 breathing, it may be said that this function is chiefly performed 

 by the internal surface, the area of which is greatly augmented 

 when the canal-system is complex. 



ANIMALCULES (Protozoa) 



The minute size of these simplest animals renders specialized 

 breathing organs unnecessary, since the outside of the body offers 

 a sufficient surface for exchange of gases between the substance 

 of the animal and the surrounding medium. In such cases as 

 that of the Proteus Animalcule (^Atiio'ba) there Is not even an 

 external membrane to hinder diffusion of carbonic acid gas out- 

 wards, and oxygen inwards, and even where such an investment 

 is present it is exceedingly thin. If as an example of a fixed 

 form, we take the stalked Bell Animalcule {Voriicel/a) it may be 

 noted that the broad end of the body is provided with cilia, the 

 currents set up by which provide both food and oxygen as in so 

 many other cases. 



