In this article we will discuss about the classification of subphylum acrania.
1. Lack distinct head, cranium, brain, eyes, auditory organs and jaws.
2. The nerve cord is dorsal and hollow.
3. Notochord persistent and extends along the whole length of the body.
4. Gill slits numerous and open in the atrium.
5. Blood vascular system and nervous system resemble those of vertebrates.
6. Body wall consists of over fifty myotomes.
7. The larval form is asymmetrical and larval period is long.
Subphylum Acrania has only one class:
1. Small, almost transparent about 5.8 cm long, lance late organisms.
2. The anterior two-thirds of the body is roughly triangular in transverse section, and posterior third is nearly oval in section.
3. The dorsal fin is a median longitudinal fold along the whole of the dorsal border and continued round the posterior end of the body extending forwards as the ventral fin.
4. Below the pointed anterior extremity is a large median aperture, the oral hood, surrounded by a frill-like membrane, the edge of which is beset with numerous tentacles or cirri. The oral hood encloses a cup-shaped vestibule which communicates with the mouth.
5. Finger-like lobes called “wheel organs” are present on the wall of the oral hood.
6. Along the roof of the vestibule runs a ciliated groove — the groove of Hatschek.
7. The notochord is persistent and un-segmented.
8. The nerve cord is dorsal and tubular.
9. Gill slits are numerous and open in the atrium.
10. The body wall consists of about sixty V-shaped’ myotomes.
11. The larval form is asymmetrical and the larval period is long.
Examples: Branchiostoma, Asymmetron.