Chapter 3 - Jawless Vertebrates
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What is a fish?
- A fish is an aquatic vertebrate that respires using gills, is ectothermic, has fins, and
has a skin that is usually with scales. The taxonomic term for fish is Pisces,
which, as you should know, is a paraphyletic group.
PISCES: the fishes:
they may be paraphyletic, but they are still fish. This Florida Museum of
web site gives a basic introduction to the major groups of fishes (with good
photographs showing characteristic features).
- Web Site with more information
Here is a terrific web site on FISHES
from the Biomedia section of the Biological Sciences Website;
Glasgow University Zoology Museum.
"Why fishes instead of fish"
by Clark Hubbs
- "Fishes or Fish?
Although people generally use the word "fish" for both singular and plural, most
usually make a distinction between them. From the scientist's point of view, then,
the difference between "fish" and "fishes"?
Both words can refer to more than one fish. The word "fishes" refers to more
KIND of fish: If you have a tub with three bluegills in it, say: "I have
several fish in this tub."
If you have a bucket with a bluegill, a largemouth bass, and a paddlefish in it, say:
"I have several fishes in this tub."
And remember, a linguist will tell you that all languages constantly change over time and
space, which means that what was "right" yesterday in Texas may not be
tomorrow in Maine! "
- (Hubbs, Fishes of Texas,
to Fishes -- Comparative Anatomy, Auburn University
The Earliest Evidence of Vertebrates
SUBPHYLUM CRANIATA (Fig. 3-3, 3.4)
Learn more about craniates from the UCMP or from the Tree of
- distinct head and brain (cranium)
- neural crest cells in association with
- epidermal placodes that form specialized [paired] sense organs-pronounced cephalization
- one or more semicircular canals
- gills (for respiration) in pharyngeal slits
- duplication of the Hox gene complex
- Endoskeleton of living tissue, primitively cartilage,
later endochondral bone
- Unique excretory (paired kidneys) and circulatory systems
- the craniates include the hagfish and the vertebrates.
- In traditional classifications, the hagfish,
lampreys, and extinct armored jawless fish
are grouped together in the paraphyletic Class Agnatha.
- It is based on the
following primitive craniate characteristics
- jaws absent (there is a mouth, but it lacks internal cartilaginous or
- paired limbs absent (a single pair may be present in fossil forms)
or two semicircular canals
- vertebrae unossified, persistent notochord
- living forms lack bone, external bony skeleton may be present in
- cartilaginous endoskeletons are present
- larval forms resemble lancelets
- Living forms are elongate, scaleless, slimy
parasites and scavengers
more on agnathans from Biomedia, Glasgow University Zoology Museum
- the most primitive group of craniates, includes the living and fossil hagfish.
- middle Pennsylvanian, Recent
- 6 genera, 60 species
- exclusively marine
- elongate (eel-like)
- many mucous glands present for antipredator defense [an enticing picture
of hagfish slime
from Safari Splash]
- feed on polychaete worms, shrimp, and dead or dying fish
- attach to fish, form a knot in
the tail and pass it forward to rip off flesh. Image © BIODIDAC. used with
- usually enter coelomic cavity and feed on
- most "eel skin" is bycatch of scavenging hagfishes
- Table 3.1 lists features of hagfish that are more primitive than the condition in
lampreys and gnathostomes and includes the following:
- anterior nasal opening; pineal not exposed
- single semicircular canal
- poorly developed eyes (lensless)
- no vertebrae
- unsupported fin rays
- partially developed cartilaginous braincase and gill supports (derived from neural
- Almost all you need to know about hagfish from Oceanlink.
Check out the slime
- Learn more about hagfish from the UCMP or from the
- the lowly hag from Safari
- the economic importance of hagfish skins from Sea
- 2 or 3 pairs of semicircular canals
- mesonephric kidneys
- dermal bone
- neural arches
- electroreceptors and lateral-line sensory canals (with neuromast cells)
- brain with optic tectum, cerebellum, and oculomotor nerves
- fin rays associated with musculature
- Learn more about vertebrates from the UCMP or from the Tree
- Vertebrates first
appear in the early Cambrian. Two genera, Myllokunmingia
and Haikouichthys are known
from China. They lack bone, but have skeletons of
cartilage. Haikouichthys looks like a lamprey, Myllokunmingia
looks like a hagfish.
- extinct Paleozoic (Cambrian to Devonian) jawless fish
with an external skeleton of bone . (Figs. 3-1, 3-2,
- The ostracoderms include the Thelodonti,
and the Osteostraci.
The Osteostraci are the sister taxa of the jawed vertebrates (gnathotomes).
- gnathostomes (jawed vertebrates).
Order PETROMYZONTIFORMES (upper Mississippian, middle Pennsylvanian, Recent)
- lampreys (Fig. 3-6)
- have a skeleton of cartilage, previously interpreted as descended from
jawless fish with an external bony armor, which lampreys have secondarily lost.
- currently researchers suggest the absence of a dermal skeleton in lampreys
and absence of other features is primitive.
- dorsal nasal opening
- pineal eye
- two semicircular canals
- cartilaginous braincase
- branchial skeleton, vertebrae (neural arches), and fin rays
- seven gill pouches open directly to exterior
- osmotic regulators
- ammocetes larva
- Ventral view of oral sucker and mouth of the lamprey. Note the horny rasping teeth.
Image © BIODIDAC. used with
- Many lampreys are parasites of bony fish. Image © BIODIDAC. used with
- Information on sea lamprey in the Great Lakes from the USGS.
and River Lamprey from selected fishes of New York State
- A Lamprey-Based Undulatory Robot
from John Ayers. Learn how lampreys swim.
- Europeans have long considered lampreys a delicacy. The lamprey has, so
far, failed to whet North American appetites. Click here
for more information and recipes.
- Learn more about lampreys from the UCMP or from the Tree
The Origin of Bone and Other Mineralized Tissue
The Origin of Fins
- see pages 68-69 (figs. 3-11, 6-12)
- pitch - tilt up and down (around z axis)
- resisted by paired fins: pectoral and/or pelvic;
- side to side
- roll (around x axis - long axis) and yaw (around y
- resisted by dorsal, anal, and caudal fins
A Problem Posed by Gills
- see pages 69-71
- Jon Mallatt's scenario of the evolution of vertebrate gills and jaws (Fig.
The Transition from Jawless to Jawed Vertebrate
- Evolution of the vertebrate jaw from the anterior visceral arches (Fig.
- Generalized jawed vertebrate (Fig. 3-10)
- Derived features of gnathostomes (Table 3.2)
- Mallatt's hypothesis for the evolution of vertebrate gills and jaws (Fig.