ZOOLOGY OF CHORDATES: STUDY GUIDE - Zakharenko M.O. - 2015

CHAPTER 1. PHYLUM CHORDATA. LOWER CHORDATES

The phylum Chordata encompasses animals that vary greatly in appearance, size, lifestyle, and habitat. Geographically, they are distributed worldwide. The phylum comprises 40,000 species inhabiting terrestrial environments, oceans, seas, and freshwater bodies. Despite this significant diversity, all Chordates share A number of common features that justify grouping them into a single phylum (Fig. 1).

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Fig. 1. Diagrams of the body Structure of a vertebrate animal:

A - sagittal section; B - transverse section in the trunk region; 1 - gill slits; 2 - Brain; 3 - HEAD Kidney (pronephros);

4 - Lungs (or swim bladder); 5 - Pancreas; 6 - Müllerian duct (oviduct); 7 - gonad; 8 - trunk kidney (mesonephros);

9 - Wolffian duct; 10 - pelvic kidney (metanephros); 11 - secondary Ureter; 12 - Spinal Cord; 13 - notochord; 14 - dorsal (neural) arch of the vertebra; 15 - ventral arch of the vertebra; 16 - rib; 17 - coelom; 18 - intestine; 19 - mesentery; 20 - dorsal aorta; 21 - vertebral centrum; 22 - cloaca;

23 - Urinary Bladder; 24 - Spleen; 25 - Stomach; 26 - Bile duct; 27 - Gallbladder; 28 - Liver; 29 - Heart; 30 - Thyroid Gland; 31 - oropharyngeal cavity

The Characteristic Features of chordates include the presence, either in the adult stage or during embryonic development, of the following structures:

1. A well-developed Axial Skeleton represented by the notochord or Vertebral Column. The notochord is of endodermal origin and is characteristic of lower chordates and lower vertebrates. In higher vertebrates, it develops as an embryonic organ and is replaced in adulthood by the spine (cartilaginous or bony), which is of mesodermal origin.

2. The Central Nervous system is represented by a neural tube of ectodermal origin with an internal cavity, the neurocoel. It is located on the DORSAL SIDE OF the body above the notochord.

3. The anterior part of the gut tube (Pharynx) is perforated by gill slits. They function as respiratory Organs in lower chordates and lower vertebrates, as well as in embryos of higher vertebrates.

4. The Circulatory system occupies a ventral position.

In addition to these shared characteristics, chordates possess features that are also common to other animal phyla:

- a secondary Mouth opening, or deuterostomy (characteristic of Echinoderms, pogonophorans, and chaetognaths);

- bilateral Symmetry (manifested in many invertebrates);

- a secondary body cavity, or coelom (found in echinoderms, jawed vertebrates, Mollusks, Arthropods, and Annelids).

- a segmented, metameric arrangement of the main Organ Systems. This feature is less pronounced in chordates than in arthropods and many groups of worms, and it is lost in hemichordates and tunicates. External segmentation is absent.

Chordates are distributed globally and inhabit all ecological realms. The Classification of chordates has long been structured as follows.

The phylum Chordata is divided into 4 subphyla (or three, according to other classifications):

Phylum Chordata - Chordata

Subphylum 1. Acrania - Acrania

Class Cephalochordata - Cephalochordata

Subphylum 2. Tunicates (Urochordates) - Tunicata, seu Urochordata

Class Ascidiae - Ascidiae Class Salpae - Salpae Class Appendiculariae - Appendiculariae

Subphylum 3. Vertebrates, or Craniates - Vertebrata, seu Craniata

Division Agnatha —

Class Cyclostomes - Cyclostomata

Division Gnathostomes - Gnathostomata

Anamniotes - Anamnia

Superclass Fishes - Pisces Class Cartilaginous Fishes - Chondrichthyes

Class Bony Fishes - Osteichthyes

Superclass Tetrapods - Tetrapoda (Quadrupeda)

Class Amphibians - Amphibia

Amniotes - Amniota Class Reptiles - Reptilia

Class Birds - Aves Class Mammals - Mammalia (Theria)

Modern classification looks somewhat different. It is represented by two phyla: Chordata and Hemichordata. The first phylum includes cephalochordates and tunicates, while the second includes all others. Ultimately, it has the following structure.

Phylum Chordates (Chordata)

Subphylum Acrania (Acrania)

Subphylum Tunicates (Tunicata), or Urochordates (Urochordata)

Phylum Hemichordates (Hemichordata)

Class Enteropneusta (Enteropneusta)

Class Pterobranchia (Pterobranchia)

Subphylum Vertebrates (Vertebrata), or Craniates (Craniata)

Infraphylum Jawless vertebrates (Agnatha)

Class Cyclostomes (Cyclostomata)

Infraphylum Jawed vertebrates (Gnathostomata)

Superclass Fishes (Pisces)

Class Cartilaginous fishes (Chondrichthyes)

Subclass Chimaeras, or Wholeheads (Holocephaii)

Class Bony Fishes (Osteichthyes)

Subclass Lobe-finned Fishes (Sarcopterygii)

Subclass Ray-finned Fishes (Actinopterygii)

Class Amphibians (Amphibia)

Class Reptiles (Reptilia)

Subclass Turtles and Tortoises (Testudines, seu Chelonia)

Subclass Lepidosaurs (Lepidosauria)

Subclass Archosaurs (Archosauria)

Class Birds (Aves)

Class Mammals (Mammalia)

Given that most textbooks and study guides are based on traditional Taxonomy, the material is presented in accordance with it as well.