affinities of cyclostomata
Answers
Answer:
Cyclostomata is a group of agnathans that comprises the living jawless fishes: the lampreys and hagfishes.
Explanation:
Cyclostomata includes 2 orders:
Order 1. Petromyzontia (Lampreys)
Order 2. Myxinoidea (Hag fish or slime eels)
Lampreys live parasitically in the sea for about 2.5-3 years and then migrate to freshwater where they breed. Petromyzon has a peculiar breeding habit and breeds only once in its life. Mature males and females migrate from sea to rivers for breeding, known as anadromous migration.
During migration the alimentary canal becomes non-functional and feeding stops. They use the fat stored in the muscles and beneath the skin. They travel hundreds of miles and reach the place where their parents and grandparents bred. In the river a male and a female select a site in clear moderately fast-flowing water and build a nest. The buccal funnel is applied to remove the stones.
The nest consists of a rounded pit with sandy floor. The female fixes to a stone by its buccal funnel near the nest. The male attaches to the female by its buccal funnel near head. Both the female and the male wriggle back and forth and discharge eggs and sperms in the nest. This mating is repeated many times. After each mating the eggs are covered by sand brought in by the tail. After spawning the parents leave the nest and die within a few days.
Fertilization is external and development is indirect. The larva is called ammocoete. Affinities of the Cyclostomes:
According to Hackele, cyclostomes are removed farther from fishes as fishes are from mammals. Cuvier united cyclostomata with cartilaginous fishes. Aggassiz separated them from the true fishes in 1857. Their separate treatment as a group is justifiable for no other reason but they have a rasping tongue. This alone proves that they form a divergent group.
On the other hand the development of anterior myotomes, the skull which does not enclose the vagus, the absence of medulated nerve fibres and the presence of a ciliated groove and a thyroid gland in ammocoete larva, points to the conclusion that the cyclostomes stem has originated from the common craniate stock at a point far below that from which gnathostomes arose. Sedgwick is, however, still in the favour of uniting cyclostomes with fishes.
As a most primitive living form of vertebrate, the class cyclostomata is very important to understand the vertebrate evolution. Besides, they possess some specialized characters as well. Thus cyclostome appears as a curious mixture of degenerate, primitive and specialized features.
The affinities of cyclostomes with Amphioxus and Gnathostomes, along with their specialized and degenerate characters are mentioned below:
Affinities with Amphioxus (Primitive Characters):
The following characters are in which vertebrates resembles found the Amphioxus:
(1) Presence of unconstricted continuous notochord (but with an added sheath).
(2) Segmental musculature but little modified from head to tail.
(3) Relatively large number of gill slits (in some hag fishes).
(4) Absence of gonoducts.
(5) Alimentary tract ciliated, straight and without much regional specializations.
(6) Endostyle in lamprey’s larva.
(7) Unmyelinated nerves.
(8) External fertilization.
At the same times cyclostomes are highly organized than Amphioxus.
They differ from the Amphioxus in following characters:
(1) Presence of several layered epidermis.
(2) Presence of cartilaginous skeletal structure.
(3) Presence of a distinct head with sense organs.
(4) Highly organized brain and vascular system.
(5) Presence of anterior and posterior dorsal fins.
Affinities with Gnathostomes:
The cyclostomes resemble the vertebrates in many respects.
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