Question

Which of the following organisms is not triploblastic?
1. Worms
2. Arthropods
3. Molluscs
4. Jellyfish

Answer 1

Triploblastic animals are those whose embryo is developed by three germinal layers, mesoderm, in between the ectoderm and endoderm. These germ layers are found in gastrula and form the whole of the body of the animal. It is present from the phylum platyhelminthes to phylum chordate but it’s not present in starting phylum like porifera, coelenterata and ctenophora.

Complete answer:
In triploblastic organisms, all the three layers present are ectoderm, mesoderm and endoderm. These layers form during the process of gastrulation, in which from each layer the organ will be formed.
Option 1). Worms include aschelminthes, platyhelminthes and annelida. They all have organ system levels of organization. They have all the three layers endoderm, ectoderm and mesoderm and are called triploblastic organisms. So, the opt 1 is not correct.
Option 2). Arthropods are the largest phylum of animalia. Over two-thirds of all named species on earth are arthropods. They have organ system levels of organization, bilaterally symmetrical, triploblastic, segmented and they are coelomate animals. So opt 2 is also not correct.
Option 3). Molluscs are the second largest animal phylum. They are terrestrial or aquatic and have the organ system level of organization. They are bilaterally symmetrical, triploblastic and coelomate animals. So opt 3 is also not correct.
Option 4. Jellyfish come under the phylum called coelenterata and these are diploblastic organisms. They don't have a mesoderm layer, instead they have a gel-like layer called mesoglea. So opt 4 Is correct answer

Note:
Coelom is also a very important aspect of differentiating an organism. Coelom means the presence or absence of a cavity between the body wall and the gut wall is very important in classification. The body cavity, which is lined by mesoderm, is called coelom. On the basis of coelom, organisms are of three types: acoelomates, coelomates and pseudocoelomates.