Mouth

Definition: an in-turned cavity, the buccal cavity, that leads to the pharynx

Embryology of the Mouth

- Originates as an in-pocketing of ectoderm called the stomodeum [reminder: in deuterostomes, the anus forms at the blastopore; the embryonic anus is called the proctodeum] (Fig. 13.2c)

- The buccal cavity eventually meets up with the pharynx, which is of endodermal origin. The membrane between the two perforates and the archenteron (primitive gut) becomes continuous with the mouth. (Fig. 13.3a)

Fishes Palate

flat with no openings

Amphibians Palate

flat with paired openings called internal nares or choanae

Reptiles and birds Palate

• arched with a pair of longitudinal folds forming a channel from the choanae to the pharynx (fig. 13.5b)

  • Reptiles have an additional pair of openings leading to the vomeronasal organ (VNO) or Jacobson’s organ.

  • Amphibians and mammals also have a VNO, but it is accessed through the nasal openings.

Crocodilians also have a secondary palate, but this was independently derived from the mammalian secondary palate

Mammals Palate

Mouth Morphology difference in Mammals

In mammals, a trench (oral vestibule) separates the gums from the lips and cheeks. Non-mammals do not have this feature.

Special modifications of the palate

• • Teeth: some fishes, amphibians and reptiles have palatal teeth

  • Transverse ridges: some mammals have ridges of keratinized epithelium for manipulation of food

  • Baleen: elongated transverse ridges forming a series of parallel plates in the mouth of toothless whales

Tongue- Aganatha:

cartilage capped by spines; not homologous with the tongue of other vertebrates

Tongue- Jawed fishes and aquatic amphibians

pad on top of hyoid cartilages with little musculature (no protrusion). This type of tongue is called the Primary tongue.

Tongue- Terrestrial amphibians

• Primary tongue

• Glandular field: produces sticky substances for prey capture

• Hypobranchial muscle

• - Note: Anuran tongues are anchored at the front!

Tongue- Reptiles and Mammals

• Primary tongue

• Glandular field

• Muscle: Mandibular arch

• Muscle: Hyoid arch

• Muscle: Hypobranchial

• Innervated by 4 cranial nerves

Fishes Oral glands

some mucus cells; no salivary glands

Tetrapods Oral glands

most have salivary glands

• Mammals: salivary glands include parotid, submaxillary, and sublingual glands

• Mammals and a few other tetrapods: salivary glands produce the enzyme amylase for digestion of starches

• Snakes and one lizard (gila monster): some salivary glands are modified for venom production

Pharynx Embryology

- The stomodeum (ectoderm) opens into the pharynx

- The pharynx forms primarily from the endoderm of the anterior foregut

- Pharyngeal pouches form along the pharynx and grow outwards to meet in-pockets of skin ectoderm called branchial grooves

- In fishes (and larval amphibians), the partition between the pouches and grooves become perforated to form functional gill slits

- In most tetrapods, the partitions either do not perforate or quickly seal over

Gills of extant agnathans

- Lampreys (Fig. 11.15):

 Each gill is housed in a pouch-like gill chambers, each with external and internal pores

 Water enters and exits the gill chambers through the pores (the mouth is unavailable because it is attached to its prey)

- Hagfishes (F ig. 11.16):

 Water enters through the nasal opening and flows into the pharynx and over the gills

 The water then flows into a common duct and exits through a pore

Gills of sharks

water path 

Water passes in through the mouth, flows over the gills, and exits via the gill slits

 Bottom-dwelling elasmobranchs use the spiracle to bring fresh water into the mouth

 The function of the spiracle in the sharks is not clear (chemosensory?)

shark Gill structure

Gills of sharks

function 

- Blood flows through the gill lamellae in the opposite direction to the flow of the water, setting up an efficient counter current exchange system
-When blood first contacts water, the water is of very low oxygen concentration
-The oxygen content of the blood is still lower, so oxygen diffuses into the blood
-As the blood continues through the lamellae, its oxygen content increases, but so does that of the ever “fresher” water with which it comes into contact
-A diffusion gradient is thus maintained over the entire respiratory surface

Gills of bony fishes

- structure follows the same basic plan as sharks; also has counter-current exchange

- gills are covered by a protective bony operculum, which developed as a fold from the hyoid arch (Note: The chimera, a chondrichthyan, also has an operculum, but it is fleshy)

- The spiracle was primitively present in bony fishes; it is present in a few primitive bony fishes (e.g., the coelocanth, sturgeons) but has been lost by other living forms

- Gills also regulate salt concentration and help in the excretion of nitrogenous wastes, such as ammonia (teleosts)

Fate of the pharyngeal pouches in tetrapods

- First pouch:

-> Auditory cavity of the middle ear

-> Eustachian tubes (leading from the pharynx to the middle ear)

- Second Pouch: palatine tonsils

- Third and fourth pouches: parathyroid gland

- Fifth pouch: part of the thyroid gland in mammals

- Variable pouches contribute to the thymus

- a hydrostatic organ; filling or emptying the air bladder with gas changes the specific gravity of the fish and thus helps it to maintain a proper depth
- In the more primitive actinopterygiins (and the embryos of more "advanced" forms), the bladder is connected to the gut via a  short pneumatic duct.  This is the  physostomous condition.
- The more modern taxa have lost the connection to the gut (the physoclistous condition).
- For physostomous fishes, gasses enter and leave the bladder via the pneumatic duct.
- In physoclistous fishes, air is secreted into the bladder from the blood through a red body, a gas gland that is richly supplied with capillaries (rete mirabile).  Gas may be resorbed from a sunken pocket, the oval, which can be closed off by a sphincter.

Lungs

Amphibians Lungs:

-> Inner surface is either smooth or contains a few ridges of connective tissue that increase the surface area for gas exchange

-> Fill lungs by swallowing air - a force pump mechanism using positive pressure.

-> Rely heavily on cutaneous respiration in addition to breathing through lungs (see 11.6)

Reptiles Lungs

Birds Lungs

Mammals Lungs

-> The lung interior is finely subdivided with blind pouches called alveoli

-> The suction mechanism is made more efficient by the addition of the diaphragm whose contraction helps to expand the thoracic cavity.

The Origin of Lungs: The Facts

- Swim bladders and lungs appear to be homologous, but it is not clear which came first.

- Neither lungs nor air bladders are present in agnathans or elasmobranchs

- One fossil placoderm has lungs, but it is not certain whether lungs of later vertebrates arose from these lungs or were independent derivations.

  Lungs (respiratory organs) appear to have been present in the primitive bony fishes and were important for survival in the warm waters and periodic droughts of the Devonian.

The Origin of Lungs: The Hypothesis

-> Lungs initially were paired, lateral structures that developing from pharyngeal pouches
-> The ancient lungs shifted ventrally to form the lungs of tetrapods and lungfishes

-> In most ray-finned fishes:
  -> The lungs shifted dorsally to merge over the gut (or maybe one lung was lost)
  -> The respiratory function was lost and the  hydrostatic function became important as these fishes moved into deep-water habitats

-> Elasmobranchs completely lost the ancestral lungs and thus lack both lungs and air bladders.

Lungs

Species Structural Differences