Tag: diversity in living world

Hemichordates are enterocoelous coelomates with radial cleavage. Enterocoelic coelom (enterocoel) develops as mesodermal pouches, arising from the gut of the embryo. 

Balanoglossus is an ocean-dwelling 'acorn worm', genus of Hemichordata. It is an 'evolutionary link' between invertebrates and vertebrates. It has a notochord in the upper part of the body and has no nerve chord. Hence, option B is correct.

A tornaria is the planktonic larva of some species of Hemichordata. Trochophore is the larva of marine annelids and most groups of molluscs.
A tadpole is a larval stage in the life cycle of an amphibian. Ammocoete is the larval stage of lamprey.

Protochordates are the invertebrate members of Chordata and include urochordates (sea squirts) and cephalochordates (amphioxus, lancelets). Cephalochordates are filter feeders that use cilia to generate a current of water and phytoplankton particles into the mouth. 

Amphioxus are fish-like marine chordates. The notochord is a flexible rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In case of Amphioxus, it is retained in the adults.

Protochordates consists of the organisms that are categorized under subphyla Urochordata and Cephalochordata. Protochordates are also called as Acraniata, due to the lack of head and cranium. These organisms are extremely marine and have small sized bodies. In protochordates, kidneys contain protonephridia, whereas, in euchordates, kidneys contain meso- or metanephridia.

Hemichordata is a phylum of marine deuterostome animals. They show characteristics of both Echinoderm and Chordates. They show bilateral symmetry and are triploblastic. Body is unicellular and is composed of epidermis. Body consists of three regions, proboscis, collar and trunk.

Protochordates are closely related to vertebrates. They differ from the chordate by not having a backbone. They are marine worms that share some, but not all of the characteristics of chordates. All living protochordate are strictly marine and are isosmotic with the sea.

Protochordates are closely related to vertebrates. These lack some of the peculiar characteristics of chordates like the true skull, brain, skeleton. These are invertebrates. This group is composed of the phylum Hemichordata and the subphylum Urochordata and Cephalochordata.
So, the correct answer is option D.

Amphioxus or lancelets comprise approximately of 32 species of fish-like marine chordates. In the lancelet, there is a nerve cord above the notochord, but no cranium or vertebrae or eyes.
So, the correct answer is option A.