Which one of the following is not true about deuterostomes ?

Question

Accepted Answer

The correct option is 1: Cleavage is usually spiral.

The statement "Cleavage is usually spiral" is **not true** for deuterostomes, making it the correct answer to the question. Deuterostomes, which encompass phyla such as Chordata (vertebrates), Echinodermata (sea stars, sea urchins), and Hemichordata (acorn worms), are defined by an early embryonic cell division pattern known as **radial cleavage**. In radial cleavage, the planes of cell division are either parallel or perpendicular to the vertical axis of the egg, meaning the resulting daughter cells, or blastomeres, stack directly on top of each other, creating a radially symmetrical embryo. Furthermore, deuterostomes typically exhibit **indeterminate cleavage**, where the developmental fate of the early blastomeres is not fixed immediately, allowing each cell to potentially develop into a complete embryo if separated. Spiral cleavage, where division planes are oblique and the cells are offset in a spiral arrangement, is the characteristic feature of the other major branch of bilateral animals, the protostomes.

The incorrect option is 2: The mouth form a new from the surface ectoderm.
This statement is a correct description of deuterostome development and therefore cannot be the answer to what is "not true." The term "Deuterostome" itself translates to "second mouth" from the Greek. During the embryonic process of gastrulation, the first opening that forms, the blastopore, develops into the anus. The mouth then forms later, as a secondary opening, which arises from an invagination of the outer embryonic layer, the surface ectoderm, at the opposite end of the primitive gut. This developmental sequence is a key defining feature that separates deuterostomes from protostomes.

The incorrect option is 3: The mesothelium arises by enterocoely.
This is a correct characteristic of coelom formation in many deuterostomes, and is therefore not the false statement. Enterocoely describes the process where the coelom, or main body cavity, forms from outpocketings of the primitive gut (archenteron). These pouches pinch off to form the mesoderm, which then develops into the lining of the coelom, or mesothelium. This process is contrasted with schizocoely, where the coelom forms from the splitting of solid masses of mesodermal tissue, which is the typical method of coelom formation in protostomes.

The incorrect option is 4: The blastopore typically becomes anus.
This statement is universally true for deuterostomes and is the most fundamental feature of the group. The fate of the blastopore, the initial opening that forms during gastrulation, is to develop into the anus in all deuterostome animals. This is a crucial evolutionary landmark in animal classification. The development of the anus from the blastopore is the inverse of the pattern seen in protostomes, where the blastopore develops into the mouth.