Tag: zoology

Non-symbiotic bacteria in the soil and symbiotic bacteria that live in the roots of plants are capable of nitrogen fixation. Bacteria in the genera Clostridium and Azotobacter are non-symbiotic nitrogen-fixing bacteria, while those in the genus Rhizobium are symbiotic bacteria. Pseudomonas species fixes nitrogen under microaerobic conditions in the free-living state and colonizes rice endophytically. Nitrogen fixation in phototrophic bacteria has been found in Rhodospirillum rubrum. 

The body is distinguished into three unequal regions, proboscis, collar and trunk in hemichordates, like Balanoglossus. Hemichordates are most primitive marine chordates, their body is soft and devoid of any skeleton. 

Acorn worms is a hemichordate. They are closely related to echinoderms. They are solitary worm-shaped organisms. They can be traced in burrows and are deposit feeders, but some species are pharyngeal filter feeders. Many are well known for their production and accumulation of various halogenated phenols and pyrroles. 

Acorn worms are unsegmented worms belonging to the phylum Hemichordata. Balanoglossus is an ocean-dwelling acorn worm genus. It is considered to be an evolutionary link between invertebrates and vertebrates.

Balanoglossus is a hemichordate acorn worm. It is the connecting link between invertebrates and vertebrates. It is basically a bottom-dwelling marine worm. It is tubicolous, that is, lives inside U-shaped burrows or tubes in the sand.  In Balanoglossus, the sexes are separate and prominent sexual dimorphism does not exist and fertilization is external, that is, it takes place outside the body of the organism.

Nephridium is an excretory organ which functions similarly to a kidney in vertebrates. These are present in paired form in the invertebrates. It is also in amphioxus. It expels wastes from the body cavity to the exterior.

Excretory organs of Amphioxus is the pronephros. The pronephros is the anterior end of this excretory organ, which is developed before the rest to meet the larval needs.

Amphioxus also known as lancelet is a fish-like marine chordate. Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells that are closely packed to form a toughened rod. The lancelet notochord, unlike the vertebrate spine, extends into the head and therefore they are placed in subphylum Cephalochordata. Hence, option C is correct.

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