In addition to the absence of jaws, Agnatha are characterized by absence of paired fins, the
presence of a notochord both in larvae and adults, and seven or more paired gill pouches.
The branchial arches (a series of arches that support the gills of aquatic amphibians and
fishes) lie close to the body surface.
Agnatha have a light sensitive pineal eye (an eye-like structure that develops in some cold-
bloodedvertebrates)anddonothaveanidentifiablestomach. Theyreproduceusingexternal
fertilization. They are ectothermic, have a cartilaginous skeleton, and a heart with two
chambers.
Many agnathans from the fossil record were armored with heavy bony-spiky plates. The
first armored agnathans - the Ostracoderms – were precursors to the bony fish and hence to
the tetrapods, including humans.
What advantages would the advent of jaws have for fish? Such an adaptation would allow
fish to eat a much wider variety of food, including plants and other organisms. In the next
twosections youwill be introduced to twogroups of fish with jaws: those with a cartilaginous
skeleton and those with a bony skeleton.
Cartilaginous Fishes
The cartilaginous fishes, orChondrichthyes, are jawed fish with paired fins, paired nos-
trils, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. The
approximate 1,000 species are subdivided into two subclasses: Elasmobranchii (sharks, rays
and skates) and Holocephali (chimaera, sometimes called ghost sharks). Fish from this group
range in size from the dwarf lanternshark, at 16 cm (6.3 in), to the whale shark, up to sizes
of 13.6 m (45 ft) (Figure13.8).
Figure 13.8: One of two male whale sharks at the Georgia Aquarium. Whale sharks are the
largest cartilaginous fish. ( 5 )
Animals from this group generally have ratio of brain weight to body size that is close to
that of mammals, and about ten times that of bony fishes. One of the explanations for their
relatively large brains is that the density of nerve cells is much lower than in the brains of
bony fishes, making the brain less energy demanding and allowing it to be bigger.
Since they do not have bone marrow (as they have no bones), red blood cells are produced
in the spleen, in special tissue around the gonads, and in an organ called Leydig’s Organ,
only found in cartilaginous fishes. The tough skin of this group is covered with dermal
teeth, orplacoidscales, although they are mostly lost in adult Holocephali, making it feel
like sandpaper. It is assumed that their oral teeth evolved from these dermal teeth, which
migrated into the mouth.
The sharks, rays and skates are further broken into two superorders, one containing the rays