4 Invertebmta. 



bundles called muscles. The first kind occur in the 

 lowest animals and in naked protoplasmic particles ; 

 the second in infusorial animals (fig. 14); the third in 

 all but the simplest animals. Connected with the 

 third set of organs of relation we find a system of fine 

 white threads called nerves, whose endings occur in 

 these organs, and whose starting-points are central 

 clusters of nerve-cells, called ganglia. These threads 

 convey the variously received stimuli from the sense 

 organs to the ganglia, and carry the command for 

 motion from the ganglia to the muscles. 



The function of Nutrition is discharged by four 

 sets of organs : ist, those of feeding, consisting of a 

 cavity or stomach, for the reception of the food, with 

 glands appended thereto, which secrete fluids to assist 

 in assimilation ; 2nd, organs of circulation, which carry 



FIG. i. 



Euplotes Charon, a ciliated infusorium showing the stages of division. 



the assimilated matter, or blood, through the body for 

 the nutrition of the tissues ; 3rd, organs of respiration, 

 by which oxygen is taken in ; and 4th, organs of 

 excretion whereby the waste products are eliminated. 



