Question

Which one of the following animals is a protostome?
A. Acorn worm
B. Spiny skinned animal
C. Alphidium
D. Echidna

Answer 1

Hint: To make the study easier the organisms are classified on a different basis, one such way is to characterize organisms on the basis of their body plan. A basic body plan includes whether the organisms have a single opening for ingestion and egestion or a separate opening for both the processes that is ingestion and egestion.

Complete answer: Body Plan: Although having a difference in shape and size, animals have a body that fits in one of the three basic plans.
-Cell aggregate: Cell aggregate type of body plan is present in sponges. They are clusters of cells with a rudimentary division of labour among them. There are no tissues, or organs.
-Blind sac: Blind sac type body plan is present in Platyhelminthes that are flatworms and coelenterates where the alimentary canal has only one opening.
-Tube within tube: Tube-within-a-tube body plan has been achieved in two different ways along two evolutionary lines. One, called the protostomes, and the other called deuterostomes. In protostomes the first opening to develop in the embryonic digestive tube is the mouth; the anus develops later.
Now let us match this with the given option:-
Acorn worm:- This organism belongs to kingdom hemichordata. They are placed in chordates and non-chordates. They have a deuterostome type plan the anus develops first and the mouth is formed later.
Spiny skinned animal:- This is a characteristic feature of phylum Echinodermata. They have a deuterostome type plan the anus develops first and the mouth is formed later.
Alphidium:- It is a protostome organism that is the first opening to develop in the embryonic digestive tube is the mouth; the anus develops later. This type of plan is present in the annelid, mollusk, and arthropod line.
Echidna:- This organism belongs to Echinodermata. They have a deuterostome type plan the anus develops first and the mouth is formed later.
Hence, the correct answer is Option C.

Note: In protostomes the first opening to develop in the embryonic digestive tube is the mouth; the anus develops later. This is seen in the annelid, mollusk, and arthropod line. In the other, the deuterostomes, the anus develops first and the mouth is formed later. This type of plan is present in the echinoderms and chordates (including the vertebrates). So in evolutionary terms we are closer to the echinoderms (starfish) than to insects or mollusks.