acentrous

from The Century Dictionary.

• Without a center; specifically, noting a condition of the vertebral column found in some batrachians and fishes, in which bony neural arches are associated with a persistent notochord which shows no trace of segmentation: said also of cells in which a centrosome or centrosphere cannot be detected.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective ichthyology lacking vertebral centra