Seventh Entry

Fish... Fish... Fish

Like all animals the fish's body is basically a long tube that is twisted up on itself a bit in the middle and has a layer of muscles and ancillary organs around it. This tube has the mouth at one end and the anus or cloaca at the other. Mostly we consider the mouth to be the entrance to the tube and the anus to be the exit, food items come in and faeces go out. Different things happen in different parts of the tube and for the sake of study and understanding we give the various parts names.

Mouth - Pharynx - Oesophagus - Gizzard - Stomach - Intestines - Rectum.

However not all fish have all these parts, some, like many of the Cyprinids and Cyprinidonts, lack a stomach, while a gizzard is only found in a relatively few species.

Mouth

Food is brought into the body via the mouth, and the jaws of modern teleost fish are a mechanical wonder, and the way the many bones work together is quite inspiring. However there is, as always a large variety in fish as a whole and the mouths of a Basking Shark, a Yellowfin Tuna and a Seahorse are quite different in both form and function. Lips are rare in fish, most species have a hard edge to their mouth. Some suction feeders that take in small prey items have small protractible lips that help give the mouth the form of a tube with a circular opening.

The teeth of most fish are the fore-runners of vertebrate teeth with an outer layer of enamel and an inner core of dentine. A fish may have teeth at the front of its mouth and along the jaws and in the pharynx as well as on its tongue.

The buccal cavity (the empty space in an empty mouth) secretes mucus to aid in the swallowing of food, but there are no special organs involved, no salivary glands, and this mucus is a lubricant only, it contains no digestive enzymes such as mammalian saliva does and is lined with squamous epithelium.

Pharynx

Immediately behind the mouth is the pharynx which is the continuation of the tube started at the mouth and in which the are found the gill clefts, through which water flows out of the alimentary canal and into the gills. It is short which leads to the oesophagus.

Oesophagus

It is constructed of two layers of non-striated muscle, one of which is longitudinal and the other circular, strangely in some species of fish the longitudinal muscle is the inner layer while in others the circular muscle is the inner layer. With so many species generalisations only apply to the majority, there are always exceptions.

The whole oesophagus contains many mucus cells, the mucus keeps the food lubricated and helps it to move along the tube.

Gizzard

The gizzard is really a highly muscular modification of the first part of the stomach. Its main purpose is to grind up coarse food items into smaller pieces thus facilitating their later digestion. In those fish which have a gizzard, such as Shad, it is the place where digestion begins because as well as its muscular activity the gizzard also secretes digestive enzymes into the food.

The Liver

Is a large organ that play various roles in the fishes body, it is the site of glycogen storage, it produces a variety of substances, including enzymes that help with the digestion and it is a major chemical factory producing various hormones as well as numerous other important molecules.

Links/Resources:

http://www.infovisual.info/02/033_en.html
http://www.aqualex.org/elearning/fish_feeding/english/digestion/structure.html
http://en.wikipedia.org/wiki/Fish