THE Triploblastic Acoelomate Body Plan
Members of the phyla Platyhelminthes, Nemertea and Gastrotricha show following advance characters:
- They are first bilaterally symmetrical animals. Therefore, they are more Complex than the cnidarians.
- All these animals are triploblastic.
- They are acoelomate (without a coelom).
- They are classified into three phyla:
(a) Phylum Platyhelminthes: It includes the flatworms. They are free living (e.g.. turbellarians) or parasitic (e.g.. flukes and tapeworms)
(b) Phylum Nemertea: It includes a small group of worms. These worms are elongated, unsegmented and soft bodied. They are mostly marine and free living.
(c) Phylum Gastrotricha: These animals live in the space between bottom sediments.
The evolutionary relationship of Platyhelminthes to other phyla is controversial
There are three views about this evolution:
(a) First View: Evolution from radial diploblastic animals
Triploblastic acoelomate is an intermediate group between the radial, diploblastic plan and the triploblastic coelomate plan. Therefore, the flatworms are an evolutionary side branch from a triploblastic acoelomate ancestor. Thus evolution of flat worms took place from radial ancestors. It formed a larval stage. This larva became sexually mature. Sexual maturity in a larval body form is called paedomorphosis.
(b) Second view: Evolution from bilateral ancestor
Other zoologists believe that triploblastic acoelomate was formed from a bilateral ancestor. Primitive acoelomates and triploblastic were formed earlier than the radiate phyla. Therefore the radial, diploblastic plan was secondarily derived from it.
(c) Third view: Acoelomate derived from coelomate
There is recent discovery of a small group of worms (Lobatocercebridae, Annelida). These worms show both flatworm. and annelid characteristics. It suggests that the acoelomate body plan is secondary characteristic. Thus, the flatworms represent a side branch. It is formed as a result of loss of a body cavity.