DIFFERENCES BETWEEN DIFFERENT ORGANISMS. 3! 



opens into a permanent digestive cavity or stomach ; but 

 this, in turn, opens directly into the body-cavity or general 

 chamber enclosed by the walls of the body (fig. 5, B). As 

 a result of this, the nutritive fluid prepared from the food, 

 which we may call the blood, gains direct access to the 

 body-cavity, where it is largely diluted with the sea-water, 

 which is also freely admitted to this cavity. The nutritive 

 fluid, thus weakened, is kept in constant circulation by 

 means of innumerable little vibrating hair-like processes or 

 " cilia," with which the lining membrane of the body-cavity 

 is furnished ; and this constitutes the only representative of 

 the circulatory apparatus of the higher animals. As in the 

 Amoeba, there are no distinct respiratory organs, and no 

 special apparatus by which effete matters may be got rid of. 



Fig. 6. Diagrammatic section of a Whelk, a. Mouth, with masticatory apparatus ; 

 b Salivary glands ; c Stomach ; dd Intestine, surrounded by the liver, and ter- 

 minating in the anus (e); g Gill ; k Heart ; f Nervous ganglion. 



In a Mollusc, again, such as the Whelk (fig. 6), nutrition 



