Question

All triploblastic animals are
(a) Acoelomate
(b) Bilaterally symmetrical
(c) Schizocoelomates
(d) Pseudocoelomate

Answer 1

Hint: Triploblastic animals are those who acquire three germ layers during the gastrulation process of embryos. The mesoderm is the middle layer of those and only the triploblastic animals possess this special layer. The true coelom is derived from this layer.

Complete step by step answer:
All the triploblastic animals found on earth are acoelomate. By acoelomate, it is referred to as the animals who have a true coelom that is characterized by a body cavity that is completely lined by a peritoneum attached to organs. These animals include mostly vertebrates and other bilateral animals.

Additional Information: The germ layers formed during gastrulation determine if an animal is diploblastic or triploblastic. Triploblastic animals basically acquired three germ layers during the gastrulation process which are endoderm, mesoderm, and ectoderm. Except for the Porifera, cnidaria, and Ctenophora, all animals are triploblastic animals.
All triploblastic animals are believed to be coelomates as they all have a true coelom lined on both sides by mesoderm. The coelom is basically a fluid- filled body cavity such as an alimentary canal. These animals are again divided into two types
- Schizocoelom: the coelom develops as a split in the mesoderm sheet. Found in annelids, mollusks, and arthropods.
- Enterocoely: In this case, the mesoderm arises from the enteron as hollow outgrowths. Found in echinoderms and chordates. When the cavity gets filled with blood and it is called hemocoel which is found in arthropods and Mollusca.
When an animal can be divided into identical two halves by a single plane, they are called bilaterally symmetrical. But it is only in case of external appearance. For example, humans are bilaterally symmetrical but the human heart and liver are positioned asymmetrically.
Pseudocoelomates also have a body cavity like coelomates but unlike them, these animal's body cavity isn't derived from mesoderm, instead, it is a derivation of the blastocoel. In the case of these animals, the body fluids bath the organs which receive the nutrients and oxygen from this fluid. Generally, animals like rotifers, nematodes show this type of coelom.
So, the correct answer is ‘Acoelomate’.

Note: The true coelom generally arises from the mesoderm layer of gastrulation. Therefore, only the triploblastic animals can show euchromatic characters. Except for some lower phyla, most of the animals possess this character. As for bilateral symmetry, it is not shown by all the triploblastic animals. Only animals like rotifer, nematodes, etc have pseudocoeloms.