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  Vertebrate chordates-I, Biology tutorial

Introduction:

Vertebral column may be cartilaginous in nature as is case in some fishes or bony as in most vertebrates. Unlike protochordates, vertebrates are the ones we come across more or less on the daily basis. Like animals include cats, horses, pigeons, chickens, dogs, etc. It should be emphasized that humans are chordates.

Characteristics of Subphylum Vertebrata:

Chordate subphylum, Vertebrata, is characterized by given features:

  • Notochord is absent in adult; it is substituted by spine of cartilaginous or bony column - vertebrae/backbone.
  • Brain encased by cranium that protects and supports it with advanced nervous and sensory structures
  • Most have 2 pairs of appendages: one pair of pectoral and one pair of pelvic appendages.
  • Bony and/or cartilage endoskeleton for structural support and or locomotion.
  • Males and females are separate and distinct.
  • Variety of feeding strategies: herbivores, carnivores, omnivores, filter feeders, parasites.
  • Well-developed ventral heart with 2-4 chambers.
  • Closed circulatory system with haemoglobin as respiratory pigment in blood.
  • Specialized epidermal structures in form of feathers, scales, spines, hair, fur.
  • Body is bilaterally symmetrical and of 3 parts - head, trunk and post-anal tail.

Vertebrates which evolved from fish pass through similar embryonic stages. The flexible notochord develops in back and blocks of tissue known as somites form along each side of it. Somites will become major structures, like muscle, connective tissue, vertebrae, and, later, larger glands of body.

Features of Superclass Agnatha:

Features of Members of superclass are given below:

  • Lack jaws therefore name agnatha (without jaw)
  • Vertebral spine is cartilaginous in nature
  • Mouth is usually round/ circular
  • No scales or exoskeleton.

Characteristics of the Class Cyclostomata (lampreys and hagfishes)

The class cyclostomata derived its name by having round or circular mouth. Class is characterized by given features:

  • Eel-like in shape and Lack exoskeleton/scales
  • lack jaws but contain rows of horny teeth which move in circular motion and give mouth circular shape - therefore name cyclostomata. In absence of jaw, mouth can't close and is always open such that water continuously cycles through it.
  • Prey/parasites on fishes
  • Notochord perseveres in adults
  • Marine habitat with size of 10-90cmin length.

Cyclostomes are extremely unique among vertebrates due to of their semi-parasitic nature. Lampreys, with exception of some small freshwater forms, join themselves to other fishes by their suctorial mouth and then rub off flesh through horny teeth carried by highly-developed tongue. Hagfishes are able to boring their way right in body of prey, consuming all soft parts and leaving skin behind as ordinary empty shell, held by the bones.

Class Cyclostomata comprises of 2 orders: Petromyzontia (or Hyperoartii) and Myxinoidea (or Hyperotreti).

Order: Petromyzontia (or Hyperoartii) E.g. lampreys - Petromyzon marinus.

The Petromyzontes are characterised by the following features:

  • Soft body devoid of scales
  • Pineal (cone-like) eyes
  • Lack bone
  • Endoskeleton made of cartilage
  • Seven gill pouches open directly to exterior
  • Circular sucking mouth used in parasitising other fishes

The hagfishes are characterized by the following features:

Circular mouth fitted with rasping tongue surrounded by short tentacles, gill pouches attached to common external opening on either side, nasal opening at tip of snout rather than on top of head as in lampreys, completely marine, elongate (eel-like) body.

Features of Class Ostracodermi (Extinct):

This class is extinct but had features like: small fish-like animals (only few centimeters long), bottom dwellers, rudimentary fins and bony armor, poor swimmers, no lower jaw, filter feeders or deposit feeders, no teeth, and are marine.

Gnathostomata:  

Features of Superclass Gnathostomata (jaw- bearing animals):

  • The vertically biting tool known as jaws basically composed of two endoskeletal elements, palatoquadrate and Meckelian cartilage, and number of dermal elements known as teeth, at times joined to large dermal bones
  • Paired appendages paired pectoral and pelvic fins supported by the internal skeleton that supports more effective locomotion
  • Interventrals and basiventrals in backbone.
  • Gill arches, that support/hold gills, lie internally to gills and branchial blood vessels, unlike gill arches of all jawless craniates, that are external to gills and blood vessels
  • The horizontal semicircular canal in inner ear
  • Gnathostomes comprise rays, sharks, chimaeras, ray-finned fishes, lobe- finned fishes and land vertebrates comprising humans.

Characteristics of Class Placodermi (Extinct):

The class Placodermi is considered as first set of fish with jaws but is now extinct. Members of the class had the given characteristics:

  • Well-developed fins and armour plating
  • Dermal armour comprising of head armour and thoracic armour. In thoracic armour, primary dermal plates form the complete ring around the body and always comprise at least one median dorsal plate
  • Dwell in both freshwater and marine environment
  • Large size (up to 10 m)
  • Lower jaw and teeth present
  • Characteristics of the Class Chondrichthyes (Cartilaginous fish)

Cartilaginous fishes contain the given characteristics:

  • Internal skeleton is made up of cartilage (rather than bone)
  • Body scales are placoid (tooth-like) with bony base
  • Jaws suspended by 2 gill arches
  • Swim bladder or lung absent; contain oil-filled liver to give buoyancy
  • Claspers (modified pelvic fins) present in males for internal fertilization
  • Notochord present in young and slowly replaced by the backbone of cartilage in adult
  • Gills not covered by operculum

Class Chondrichthyes is separated in two extant subclasses:

Subclass: Elasmobranchii (sharks, rays and skates)

Subclass: Holocephali (chimaera, at times known as ghost sharks). Members of class Chondrichthyes contain backbone which is composed of cartilage. They are not fishes you come across frequently because of their marine.

Features of Class Osteichthyes (Bony fish)

Members of this class have the given features:

  • Bony endoskeleton
  • Body covered by cycloid scales (thin and round bony scales)
  • Paired pectoral and pelvic fins supported by bony rays
  • Bilaterally symmetrical tail fin
  • Visceral cleft as separate gill openings covered by a bony flap - the operculum.

The Osteichthyes are characterized by endochondral ("spongy") bone in endoskeleton, dermal fin rays composed by lepidotrichiae (altered, tile-shaped scales), and 3 pairs of tooth-bearing dermal bones lining jaws (dentary, premaxillary and maxillary). Osteichthyes comprise 2 main subclasses, Actinopterygii and Sarcopterygii.

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