1. Have the epidermic exoskeleton either absent, or very
2. The spinal column may persist as a notochord with a
membranous sheath, or it may exhibit various degrees of
chondrification or ossification. When the vertebrae are distinct,
their centra have no epiphyses.
3. The skull may be incomplete and membranous, more
or less cartilaginous, or osseous. When membrane bones are
developed in connection with it, there is a large parasphenoid.
The basisphenoid is always small, if it be not absent.
4. The occipital condyle may be absent, or single, or
double. When there are two occipital condyles they belong
to the ex-occipital region, and the basi-occipital region is unossified
or very imperfectly ossified.
5. The mandible may be absent, or be represented only by
cartilage. If membrane bones are developed in connection
with it, there is usually more than one on each side. The
articular element may be ossified or not, and may be connected
with the skull by the intermediation of a quadrate and
a hyomandibular element, or by a single fixed plate of carti
lage representing both these and the pterygo-palatine arch.
A stapes may be present or absent.
6. The alimentary canal may or may not terminate in a
cloaca. When there is no cloaca, the rectum opens in front of
the urinary organs.
7. The blood-corpuscles are always nucleated, and the
heart mav be tubular, bilocular, or trilocular.
8. There are never fewer than two aortic arches in the
9. Respiration takes place by branchiae during part, or the
whole, of life.
10. There is no thoracic diaphragm.
11. The urinary organs are permanent Wolffian bodies.
12. The cerebral hemispheres may be absent, and are
never united by a corpus callosum.
13. The embryo has no amnion, and, at most, a rudimentary
14. There are no mammary glands.