An Introduction to Zoology
Chapter 11: The Bilateria: Acoelomate Lophotrochozoans:
In the two previous chapters, we discussed the phyla Cnidaria
, the diploblastic
animals that are collectively known as the Radiata
. All other animals are triploblastic
and bilaterally symmetrical
, and so are collectively known as the Bilateria
All of the Bilateria have elongated, bilaterally-symmetrical bodies, at least during the embryo stage. Some of the Bilateria are radially-symmetrical as adults, it’s true, but all of them develop from bilaterally-symmetrical embryos. Most of the Bilateria also show some degree of cephalization
, meaning that their sense organs are concentrated at the anterior
end to form a head
Almost all of the Bilateria have their sense organs and neural tissues
concentrated at the anterior end of the body. This concentration of sensory
and neural tissues at the anterior end of the body is known as cephalization.
The remaining articles in this series will focus on the various phyla within the clade
Bilateria. First, though, we’ll briefly discuss bilaterian classification and diversity.