A) The animal is flat enough to permit effective gas exchange by diffusion.
B) There is not enough room for a circulatory system.
C) The animal lacks muscles, so it could not have a heart to pump blood.
D) Only bilaterally symmetrical animals need a circulatory system.
E) Only vertebrates have circulatory systems.

A) Nematodes
B) Sponges
C) Cnidarians
D) Annelids
E) Flatworms

A) Mammalian erythrocytes lack nuclei.
B) Mammalian erythrocytes respond to changes in water concentration.
C) Mammalian erythrocytes are oval with swollen centers.
D) Other vertebrates do not have erythrocytes.
E) Other vertebrates use a respiratory pigment other than hemoglobin.