MUD-EATING ANIMALS 50 



certain radii of the upper side of the umhrcUa. iiy the 

 action of ciha, this food is passed along over the edge to the 

 lower surface of the umbrella and ultimately to the mouth. 



A good many marine worms live on mud, and so do the 

 sea-lilies, Crinoids, and the sea-cucumbers, IIolotiiurians. 

 It is a peculiarity of all these animals that the wall of their 

 ahmentary tract, packed as it is with fragments of sand and 

 other samples of the sea bottom, many of them with very 

 sharp corners, is of extraordinary thinness, transparent and 

 tightly stretched, so that one might expect that at any moment 

 the intestine would rupture. But it does not seem to do so. 

 This sand or mud is rich in organic matter, both living and 

 dead, and this it is that supplies the nutriment to the mud- 

 eaters. Marine worms mostly belong to the group Polyciiaeta, 

 i.e. with many bristles. They often eat each other but they 

 cannot digest the bristles, which work their way through the 

 tissues to the outside, much as a needle often does when it 

 has inadvertently been swallowed by a child. 



A rough rule of feeding is that the bigger animals consume 

 the smaller animals and plants. Many unicellular animals 

 ingest and digest bacteria and unicellular algae. Certain of 

 the most infectious bacteria can serve as food for protozoa 

 such as the Amoeba, and apparently cause no disease in the 

 consumer's body. 



Amongst the more simple multi-cellular organisms pretty 

 well any smaller animal or plant may serve as food, though 

 many of the minute protozoa seek to protect themselves from 

 being eaten by forming skeletons of lime or flint or by flinty 

 spicules. As we get higher up in the scale we find animals are 

 more apt to pick and choose what they eat. Earthworms 

 consume great quantities of earth, and the surface soil is 

 teeming with an abundant fauna and flora which affords 

 them ample nourishment. The mineral fragments which 

 cannot be digested are passed out on to the surface of the 

 land as worm-castings. In an average soil there are some 

 150,000 earthworms to the acre, and the amount of earth 

 extruded from earthworm burrows is considerable. Darwin 

 weighed the amount of earth thrown u}) on two sej)aratc 

 square yards in a single year and found it to be C-75 lbs. 



