BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.8. Kingdom Animalia (Animals)

2.8.9. Phylum Chordata (Chordates)

The Classification and key CHARACTERISTICS OF THE phylum Chordata are summarized in Table 2.20. The primary defining feature of Chordates is the presence of a longitudinal dorsal rod known as the notochord, which extends between the dorsal nerve cord and the gut (Fig. 2.65). The notochord provides internal structural support for the body and enhances the animal's locomotor capabilities. Chordates presumably evolved from the free-swimming larval forms of their ancient ancestors. A Structure somewhat resembling this primitive condition is still found in several surviving invertebrate chordate groups. However, over the course of evolution, the notochord has been largely replaced in most chordates by bony segments that form the Vertebral Column, or spine. Animals possessing a vertebral column are referred to as vertebrates, while all others are called invertebrates.

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Fig. 2.65. A. General body plan of a chordate. B. Cross-sections of non-chordate and chordate coelomate animals.

Table 2.20. Classification and key characteristics of the phylum Chordata (Chordates)

Phylum Chordata
Key Characteristics A notochord is present at some stage of development. This is a flexible rod composed of tightly packed, vacuolated Cells and enclosed within a tough sheath* Triploblastic coelomate animals Bilateral Symmetry Gill (visceral) slits (pharyngeal openings)* or arches are present A hollow nerve cord* is located dorsally Segmentally arranged Muscle blocks (myotomes) are located along the sides of the body Post-anal tail (the tail originates posterior to the anus)* Appendages span more than one body segment*
Subphylum Vertebrata (Vertebrates)
Key Characteristics In adults, the notochord is replaced by a vertebral column — the spine (a series of vertebrae made of bone or Cartilage)* A well-developed Central Nervous system is present, including a Brain* protected by a cranium Internal Skeleton (endoskeleton) Gill slits are few in number Two pairs of fins or limbs. They connect to the rest of the skeleton via the pelvic and pectoral girdles.
Class Chondrichthyes (Cartilaginous Fishes)	Class Osteichthyes (Bony fishes)	Class Amphibia (Amphibians)	Class Reptilia (Reptiles)	Class Aves (Birds)	Class Mammalia (Mammals)
Skin covered with placoid (tooth-like) scales	Skin covered with cycloid scales (thin, rounded, and bony in composition)	Soft, moist skin capable of gas exchange to supply oxygen to the Lungs. Scales are absent	Skin is dry, covered with horny scales and bony plates	Body covered in feathers; legs bear horny scales	Body covered in Hair (fur); Two Types of glands present: sebaceous and Sweat Glands
Cartilaginous skeleton	Bony skeleton	Bony skeleton	Bony skeleton	Bony skeleton	Bony skeleton
Paired fleshy pectoral and pelvic fins. An asymmetrical caudal fin AIDS in maintaining depth (a swim bladder for buoyancy is lacking)	Paired pectoral and pelvic fins supported by bony rays, providing greater maneuverability. Symmetrical caudal fin	Two pairs of pentadactyl limbs	Usually two pairs of pentadactyl limbs	Two pairs of pentadactyl limbs, with the forelimbs modified into wings	Two pairs of pentadactyl limbs
Typical gill slits present; five on each side	Four pairs of gill slits opening externally via separate apertures in the pharyngeal region, all covered collectively by a bony operculum	Gills present only in aquatic larvae (tadpoles); adults, which are typically terrestrial, possess lungs	Gills are never formed; Respiration is pulmonary	Gills are never formed; respiration is pulmonary	Gills are never formed; respiration is pulmonary
		Life cycle includes metamorphosis from larva to adult
External ear absent	External ear absent	External ear absent	External ear absent	External ear absent	External ear present In addition to the middle and Inner ear
Ova produced, internal Fertilization	Ova produced, external fertilization	Ova produced, external fertilization. Adults must return to Water to reproduce	Yolked, fertilized eggs are laid on land or retained until hatching. Eggs have a leathery shell. Internal fertilization	Similar to reptiles, but eggs possess a calcareous shell. Internal fertilization	Embryos develop within the maternal body. Females possess Mammary Glands that produce milk to nourish their young. The only exceptions are two egg-laying genera: the platypus and echidna. Internal fertilization
				Teeth are absent, replaced by a beak	Different types of teeth specialized for various Functions
					Muscular Diaphragm separating the thoracic and abdominal cavities
Poikilothermic ("cold-blooded")	Poikilothermic	Poikilothermic	Poikilothermic	Homeothermic ("warm-blooded")	Homeothermic
Examples: Scyliorhinus (dogfish) Other sharks and rays	EXAMPLES: Clupea (herring)	EXAMPLES: Rana (frog) Bufo (toad) Also newts and salamanders	EXAMPLES: Natrix (grass snake) Crocodylus (crocodile) Also lizards, alligators, turtles Dinosaurs (extinct)	EXAMPLES: Columba (pigeon) Aquila (eagle)	EXAMPLES: Homo (human) Canis (dog)
* Diagnostic features

Here we will focus exclusively on vertebrate chordates. This is such a significant natural group that it has been granted subphylum status (Vertebrata). The general body plan of chordates is illustrated in Figs. 2.65 A and B, while representative examples of vertebrates are shown in Fig. 2.66.

Table 2.21 lists the evolutionarily important key characteristics of various animal groups.

Fig. 2.66. A. Representative bony fish — herring (Clupea harengus). Paired pectoral and pelvic fins act as depth rudders, controlling upward and downward movement. Unpaired dorsal and anal fins act as keels, allowing the fish to turn and change direction. B. European tree frog (Hyla arborea). C. Lizard. D. Blue tit (Parus caeruleus). E. Hazel dormouse (Muscardinus avellinarius).

Table 2.21. Summary of key evolutionarily significant traits across animal phyla

Phylum	Radial symmetry	Diploblasty	Bilateral symmetry	Triploblasty	Acoelomate	Coelomate	Hemocoel	Metamerism
Cnidaria	•	•	х	х	_	—	—	—
Platyhelminthes	х	х	•	•	•	х	х	х
Nematoda	х	х	•	•	Body cavity		х	х
					develops
					differently
Annelida	х	х	•	•	х	•	х	•
Arthropoda	х	х	•	•	х	•	•	•
Mollusca	х	х	•	•	х	•	•	х
Echinodermata	•	х	х	•	х	•	х	х
Chordata	х	х	•	•	х	•	х	•

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