Arteries are thicker than veins, and the elastic layer is necessary to support the additional weight of arteries.

Valves present in veins provide a mechanism for withstanding high blood pressure flow going through veins.

The total length of arteries in the body is more than the total length of veins, and the elastic layer helps push the blood over the longer length.

Blood pressure is higher in arteries than veins, and the elastic layer helps maintain the structure of the artery.

Planaria

Hydra

Lumbricus

Ascaris

The longitudinal muscle is contracted laterally to allow it to wiggle forward.

The cilia rhythmically beats the water to allow it to swim forward.

The longitudinal muscle and circular muscle are contracted alternately to allow it to move forward.

The simple muscle is contracted and elongated to allow it to move forward.

Through branching structures called tracheoles.

Through a series of lungs within their body.

By opening their mouths and creating negative pressure that draws air in to move through their tissues.

By opening spiracles that allow oxygen to enter their body cavity directly.

Lophotrochozoa and Ecdysozoa.

Eumetazoa and Protozoa.

Protostomia and Deuterostomia.

Chordata and Hemichordata.

the ability to produce milk.

where early development of the young occurs.

the presence of hair.

teeth are mainly molars.

the presence or absence of a four (4) chambered heart.

the presence or absence of a single closed circulatory system.

teeth are mainly incisors.

the presence or absence of a three (3) chambered heart.

the presence or absence of a double closed circulatory system.

the presence or absence of a pouch.

Absence of sagittal crest, weak projection of face, basal position of foramen magnum, high vaulting of braincase.

Basal position of foramen magnum, high vaulting of braincase, weak projection of face, absence of sagittal crest.

Weak projection of face, basal position of foramen magnum, high vaulting of braincase, absence of sagittal crest.

Basal position of foramen magnum, absence of sagittal crest, weak projection of face, high vaulting of braincase.

the greatest variety of mitochondrial sequences are found among modern Africans.

the fewest number of mitochondrial mutations are found among modern Africans.

all Homo sapiens have the same number of chromosomes.

mitochondrial DNA is inherited only from one’s mother.

Planaria

Hydra

Lumbricus

Ascaris