Question

Which of the following does not pertain to a protostome?
A. Spiral cleavage
B. Blastopore is associated with the anus.
C. Schizocoelom
D. Annelids, arthropods, and molluscs.

Answer 1

Hint: Protostomia is a clade of phyla-containing animals such as arthropods, annelids, and molluscs. Bilateria, composed mainly of species with bilateral symmetry and three germ layers, is made up of its members along with the deuterostomes and xenacoelomorpha.

Complete Answer:
- In embryonic development, the key variations are observed between deuterostomes and protostomes and are based on the embryological origins of the mouth and anus.
- In most, but not all protostomes, the mouth forms first, then the anus, whereas the reverse is true in deuterostomes. The embryo forms a dent on one side in animals at least as complex as earthworms, the blastopore, which deepens to become the archenteron, the first stage in the growth of the gut.
- The original dent becomes the anus in deuterostomes, whereas the gut eventually tunnels through to make another opening that forms the mouth.
- The protostomes were so called because it was once assumed that the embryological dent shaped the mouth in both situations, while the anus was later shaped, at the opening created by the other end of the gut.
- It is now understood that the blastopore 's fate in protostomes is highly unpredictable. While the evolutionary distinction remains valid between deuterostomes and protostomes, the descriptive accuracy of the 'protostome' name is disputed. In some ways, protostomes and deuterostomes vary.
- Deuterostome embryos undergo radial cleavage during cell division early in development, while several protostomes undergo spiral cleavage.

So the answer is “Option B”.

Note: Animals from all classes have a complete digestive system, but the first opening of the embryonic intestine grows into the mouth in protostomes, and the anus forms secondarily. The anus emerges first in deuterostomes, while the mouth secondarily grows. Most protostomes have schizocoelous growth, where cells simply fill the mesoderm inside the gastrula to develop. The mesoderm forms in deuterostomes through enterocoelic pouching, by endoderm invagination. Yet there is deuterostome-like growth in two of the most basal and ancient protostome phyla, the chaetognatha and priapulida.