ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013
LECTURE 13. GENERAL CHARACTERISTICS OF THE PHYLUM CHORDATA (Chordata)
5. General characteristics of the infraphylum Agnatha(
1. It includes fossil (class Ostracodermi) and modern vertebrates (a single class Cyclostomata, containing two subclasses: Lampreys (Petromyzones) (Fig. 54) and Hagfishes (Myxini)), in which the notochord serves as the primary supporting axis of the body throughout their life; rudimentary vertebrae develop only in some forms.
Fig. 54. Lampreys (after Naumov, Kartashev, 1979):
1 — sea lamprey; 2 — European river lamprey.
2. The neurocranium encloses the brain only from below and from the sides; it is cartilaginous (partial ossification was observed in some fossil forms).
3. Jaws are absent (hence the name of the infraphylum).
4. The mouth is round or triangular, suctorial.
5. A single (unpaired) olfactory sac opens externally via a single nostril.
6. The inner ear capsule contains only two semicircular canals (sometimes one).
7. The caudal fin and one or two dorsal fins are more or less well-developed; paired fins are absent.
8. There are 7–20 gill slits in which gill pouches develop.
Ecological significance of cyclostomes: acting as semi-parasites and semi-predators, they regulate the population of their prey (which are usually diseased).
Significance of cyclostomes to humans: they are used as food (e.g., in Japan); they also cause damage to fisheries by destroying valuable commercial fish.
1. It includes fossil (class Ostracodermi) and modern vertebrates (a single class Cyclostomata, containing two subclasses: Lampreys (Petromyzones) (Fig. 54) and Hagfishes (Myxini)), in which the notochord serves as the primary supporting axis of the body throughout their life; rudimentary vertebrae develop only in some forms.
Fig. 54. Lampreys (after Naumov, Kartashev, 1979):
1 — sea lamprey; 2 — European river lamprey.
2. The neurocranium encloses the brain only from below and from the sides; it is cartilaginous (partial ossification was observed in some fossil forms).
3. Jaws are absent (hence the name of the infraphylum).
4. The mouth is round or triangular, suctorial.
5. A single (unpaired) olfactory sac opens externally via a single nostril.
6. The inner ear capsule contains only two semicircular canals (sometimes one).
7. The caudal fin and one or two dorsal fins are more or less well-developed; paired fins are absent.
8. There are 7–20 gill slits in which gill pouches develop.
Ecological significance of cyclostomes: acting as semi-parasites and semi-predators, they regulate the population of their prey (which are usually diseased).
Significance of cyclostomes to humans: they are used as food (e.g., in Japan); they also cause damage to fisheries by destroying valuable commercial fish.
Last update: 14/08/2026
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