Practical Course in Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Chordates
Subphylum Vertebrata, or Craniata

General characteristics

Vertebrates represent the most highly organized and complex group of Chordates, comprising a vast number of species (about 40,000) adapted to a wide range of environments. They are of immense and diverse significance to human economic activity: the overwhelming majority of domestic animals are vertebrates; wild vertebrates provide humans with diverse food sources (fish, birds, mammals) and valuable Materials (such as fur); and vertebrates represent a vital natural reservoir for breeding new economically useful domestic animals.

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Fig. 118. Embryonic membranes of amniotes:

A–G — stages of embryonic development; 1 — ectoderm; 2 — endoderm; 3 — mesoderm; 4 — gut cavity; 5 — extraembryonic coelom; 6 — amnion; 7 — amniotic cavity; 8 — serosa; 9 — allantois; 10 — yolk sac.

Based on reproductive traits, structural Organization, and embryonic development, vertebrates are divided into two natural groups: anamniotes and amniotes (Fig. 118). Anamniotes include cyclostomes, Fishes, and amphibians. Fertilization in these animals is external, the larvae invariably develop in Water, and they lack extraembryonic membranes.

Amniotes include reptiles, birds, and mammals. Fertilization of eggs in these animals is internal and occurs within the female reproductive tract. An embryonic membrane, the amnion, forms around the developing embryo. The Amniotic Fluid is contained between the amnion and the embryo. The posterior region of the embryonic gut forms a thin-walled, sac-like outgrowth known as the allantois.

It performs two Functions: Respiration and the storage of embryonic nitrogenous wastes. Externally, the embryo is further enclosed by a third extraembryonic membrane, the serosa. Classification of extant vertebrates:

Division Agnatha (Jawless vertebrates)

Class Cyclostomata (Cyclostomes)

Division Gnathostomata (Jawed vertebrates)

Superclass Pisces (Fishes)

Class Chondrichthyes (Cartilaginous fishes)

Class Osteichthyes (Bony fishes)

Superclass Tetrapoda (Tetrapods, or Terrestrial vertebrates)

Class Amphibia (Amphibians)

Class Reptilia (Reptiles)

Class Aves (Birds)

Class Mammalia (Mammals)

Cyclostomes and cartilaginous fishes are not covered in this practical course.