THE WORM-LIKE ANIMALS 353 



as it slips past segment 14, at least one egg-cell is dis- 

 charged from an oviduct, and as the ring passes segment 

 10, some stored sperm-cells are ejected from the sperm- 

 reservoirs, which were previously filled with cells from 

 another worm. When the ring slips off the worm's an- 

 terior end, its open ends contract and it becomes a closed 

 egg-case. The egg-cell is fertilized by a sperm-cell and 

 then divides into numerous cells, which form the body of 

 an earthworm embryo. There is no larva, as in marine 

 worms (301), but a young earthworm about one inch long 

 is hatched. 



304. Animals with Blood. All backboneless animals 

 higher than and including the segmented worms, have 

 blood or similar liquid which is made to circulate, usually 

 by a heart. The functions of this circulating liquid are the 

 same as in the frog ( 52), for the complexity of the bodies is 

 such that there must be a circulating medium to distribute 

 oxygen and digested food, and to carry excretions from dis- 

 tant cells to the excretory organs. 



The blood of these animals lower than the vertebrates is 

 commonly colorless, resembling lymph of vertebrates, and 

 containing only white cells or corpuscles. Red blood-cells 

 occur only in backboned animals. Some of the backboneless 

 forms have blood with a reddish tint, which is caused by 

 dissolved haemoglobin, the substance which gives the red 

 color to blood-cells of vertebrates. This dissolved haemo- 

 globin makes it possible for a given quantity of blood to 

 carry more oxygen than could be carried in colorless blood ; 

 hence even the reddish tint of the blood is an advantage to 

 the animals which have it. Blood which does not have the 

 coloring matter can probably carry about as much oxygen in 

 solution as water could. 



Examples of animals which have blood without red blood- 

 cells are the larger worm-like animals, the crabs, lobsters, 

 spiders, insects, clams, oysters, snails, cuttle-fishes, star- 



2A 



