What are agnathans living relatives?

The group Agnatha consists of the jawless fishes, the most primitive group of extant vertebrates. While most agnathan species are now extinct, fossil evidence indicates that the group was once highly successful and extremely varied. Two lineages of agnathans have survived to the present, the lampreys and the hagfish.

Does Agnatha have a brain?

The basic organization of the brain of the Agnatha, such as the prosencephalon, mesencephalon, and rhombencephalon are common to the gnathostomata, although some variations in development are included in the sensory centers and higher centers of information processing.

What is the gill arch theory?

Scientists pointed out that within the fish embryo, the two body parts develop from two different sets of cells – fins come from mesoderm cells, while gill arches come from neural crest cells. …

Which fins are in pairs?

Fins are composed of two groups, unpaired and paired. The unpaired fins are the dorsal, caudal, and anal; the paired fins are called pectorals and pelvics, or ventrals. Pectoral fins are present in almost every fish.

Where can agnathans be found?

The hagfish is found in cold ocean waters in the Northern and Southern Hemispheres. It is found on muddy sea floors and may live in very large groups of up to 15,000 individuals.

How do Agnatha breathe?

“Water breathing” through gills which are modified pharyngeal slits. (gas exchange between water and blood). Gills can absorb lower concentrations of available oxygen.

Do hagfish have brain?

A brain and spinal cord constitute the central nervous system of hagfishes, the extant sister group of lampreys and gnathostomes among the craniates. A massive fore-brain includes olfactory bulbs, twin telencephalic hemispheres and a diencephalon.

What is fin fold theory?

According to the widely accepted fin-fold theory, the paired limbs are derived from the local persistence of parts of a continuous fold that in ancestral vertebrates passed along each side of the trunk and fused behind the anus into a single fin.

Who discovered fin fold theory?

Two major theories have been dominant within these fields regarding the origin of these appendages. The “fin-fold theory,” or “lateral fin-fold” hypothesis of Thacher,1 Mivart2 and Balfour3 claims that these paired appendages evolved from a bilateral median fin-like structure (Figure 1).