11.9.2 Developmental Data for the Phylogenetic Tree of Animals

Question 1

Developmental Data for the Phylogenetic Tree of Animals

Answer 1

• Looking at patterns in embryological development provides information about phylogenetic relationships between taxa.
• By looking at their embryological development, it is apparent that the radial symmetry of echinoderms evolved secondarily
from a bilateral condition.
• Radiata are diploblastic, and bilateria are triploblastic.
• The next major branching point in the phylogenetic tree of animals involves the presence or absence of body cavities.

Question 2

note

Answer 2

• Members of the phylum Echinodermata (sea stars and sea urchins) are radially symmetrical but are classified with the bilateria rather than radiata.
• Because genes control development, phylogeny can be determined by looking at patterns in embryological
  development.
• The developmental patterns of echinoderms reveal that the radial symmetry of these organisms evolved secondarily from a bilateral condition—possibly as an adaptation to a more sedentary way of life. Echinoderms are correctly placed in the bilateria grouping.
• Other characteristics of development help to distinguish radiata from bilateria. Looking at the illustration on the left, you can see that as a group, radiata are diploblastic (have two germ layers). Bilateria are triploblastic (have three germ layers.)
• The developmental pattern on starfish reveals that they are triploblastic, not diploblastic.

Question 3

An organism with two layers of germ cells in embryonic development will probably have what type of symmetry?

Answer 3

• radial

Question 4

Which of the following characteristics are found in both radiata and bilateria?

Answer 4

• a dominant diploid stage

Question 5

Radial body symmetry is present in

Answer 5

• cnidarians

Question 6

Bilateral symmetry in the animal kingdom is uniquely associated with

Answer 6

• three germ layers.

Question 7

Animals can be classified by the number of germ layers present in embryonic development. How many germ layers do the animals classified as Radiata usually have during embryonic development?

Answer 7

• two

Question 8

Triploblastic animals usually have what type of symmetry?

Answer 8

• bilateral

Question 9

When classifying animals, why are starfish grouped with the animals that possess bilateral symmetry?

Answer 9

• Starfish developed radial symmetry after they developed triploblastic characteristics.