BIO 342
Comparative Vertebrate Anatomy
Lecture Notes 7 - Digestive System

Digestive tract - ‘tube’ from mouth to vent or anus that functions in:

 Major subdivisions include the oral cavity, pharynx, esophagus, stomach, small & large intestines, and cloaca. Accessory organs include the tongue, teeth, oral glands, pancreas, liver, & gall bladder.

Differences in the anatomy of vertebrate digestive tracts is often correlated with the nature & abundance of food:

The embryonic digestive tract of vertebrates consists of 3 regions:

                  1 - midgut - contains yolk or attached yolk sac

                  2 - foregut - oral cavity, pharynx, esophagus, stomach, & small intestine

                 3 - hindgut - large intestine & cloaca

Mouth & oral cavity. The oral cavity begins at the mouth & ends at the pharynx. Fish have a very short oral cavity, while tetrapods typically have longer oral cavities. The mammalian mouth is specialized to serve as a suckling and masticatory organ (with muscular cheeks).

Teeth are derivations of dermal armor.

Toothless vertebrates are found in every class of vertebrates and include agnathans, sturgeons, some toads, turtles, birds, & baleen whales.

A right whale swims at or near the surface of the water with its mouth open.
Water and food enter through a gap in the front baleen plates, and
food is caught in the matted baleen fringes inside.

Toothed vertebrates:


    1 - have tended toward reduced numbers & distribution

    2 - most vertebrates (through reptiles) have succession of teeth

    3 - most vertebrates (except mammals) replace teeth in ‘waves’ (back to front; every other tooth)

    4 - mammals generally develop 2 sets of teeth: milk (deciduous) teeth & permanent teeth

Morphological variation in teeth:


Tongue mobility:

Using a keen sense of smell, anteaters are able to effectively track down
ant nests on the forest floor. Once a nest is found, the mammal usually rips it open
with its sharp foreclaws to expose its delectable contents. The anteater then proceeds to
catch and eat the ants by repetitively flicking its long sticky tongue in and out of the nest.
The giant anteater's unique tongue can measure as long as two feet (60 cm).

Functions of vertebrate tongues:

Oral glands - secrete a variety of substances including:

Pharynx - part of digestive tract exhibiting pharyngeal pouches (at least in the embryo) that may give rise to slits

     Mammals - an epiglottis is positioned over the glottis so that, when a mammal swallows, the larynx is drawn forward against the epiglottis & the epiglottis blocks the glottis (which prevents food or liquids from entering the trachea)



Stomach = muscular chamber(s) at end of esophagus

Reticulo-rumen (reticulum and rumen)

Reticulum and rumen are often discussed together since each compartment is separated by a low partition. Eighty percent of the capacity of the stomach is related to the reticulo-rumen. The contents of the reticulum and rumen intermix freely. The rumen is the main fermentation vat where billions of microorganisms attack and break down the relatively indigestible feed components of the ruminant's diet. 


After fermentation in the reticulum and rumen, food passes to the omasum. The omasum acts as a filter pump to sort liquid and fine food particles. Coarse fibre particles are not allowed to enter the omasum. Also, the omasum may be the site for absorption of water, minerals and nitrogen.


The abomasum is the true stomach and the only site on the digestive tract that produces gastric juices (HCl and the enzymes, pepsin and rennin). Ingesta only remains here for 1 to 2 hours.

The intestine is located between the stomach & the cloaca or anus & is an important site for digestion & absorption. Vertebrate intestines are differentiated to varying degrees into small & large intestines.

    Fishes - relatively straight & short intestine in cartilaginous fishes & in primitive bony fishes (lungfish & sturgeon). However, the intestine of cartilaginous fishes has a spiral valve.

    Amphibians - intestines differentiated into coiled small intestine and short, straight large intestine

    Reptiles & Birds - coiled small intestines & a relatively short large intestine (that empties into the cloaca)

    Mammals - small intestine long & coiled and differentiated into duodenum, jejunum, & ileum. The large intestine is often relatively long (but not as long as the small intestine). A cecum is often present at the junction of the small & large intestines in herbivores.


Accessory organs - Liver, gall bladder, & pancreas

Ceca - blind diverticula that serve to increase the surface area of the vertebrate digestive tract



Related links:

Variations on a Theme: Specializations of Vertebrate Digestive Systems

Comparative Chordate Digestive Systems

Comparative Anatomy of the Digestive Tract

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