1. Metazoan diseases

2. Phylum Platyhelminthes
commonly known as the flatworms
dorsoventrally flattened, bilaterally symmetrical and acoelomate
They usually lack an anus and specialized skeletal, circulatory and respiratory systems.
The majority of Platyhelminthes are monoecious, i.e. both male and female reproductive systems occur in the same individual.

3. Class Monogenea
ectoparasitic animals with no intermediate hosts involved in the life cycle.
They are about 3 cm in length.
possess a posterior organ of attachment known as the haptor, armed with hooks and or clamps or suckers.
they usually have some form of attachment organ at the anterior end.

4. Subclass : Monopisthocotylea
The haptor is a single unit with one or two pairs of large hooks (anchors) and 12 to 16 lateral hooklets.
The anterior attachment organ is often glandular.
The mouth is not surrounded by the sucker. Eyespots are frequently present anteriorly.
Comprises of two families Gyrodacrylidae and Dactylogyridae.

5. Subclass :Polyopisthocotylea
In this group the haptor is well developed and bears clamps and suckers.
Anchors may be present.
An oral sucker may be present anteriorly.

6. Dactylogyrus and Gyrodactylus
Symptoms:
cloudiness of skin, grey white colour, focal haemorrhages on skin.
The infection is ectoparasitic and can cause considerable damage to the host when present in high numbers.
Opisthaptor-a multiple hooked attachment organ.
Treatment
3 to 5 minute saltwater bath (30-35 ppt).
Glacial acetic acid or hydrogen peroxide dips will also kill these parasites.
Praziquantel baths have also proved to be effective in killing some monogenean worms

7. Class Digenea endoparasitic animals
Both adult and larval metacercarial stages are found in fish
The majority of digeneans have two suckers on their body surface, one at the anterior end and one ventrally located
Hermaphrodite in nature
Families - Sanguinicolidae, Bucephalidae and Allocreadiidae.

8. Class Cestoda A scolex (attachment organ) present at the anterior end.
The adult worms are usually white in colour and may be very elongated.
parasitic in the intestine of the host
The Cestoda includes the orders Proteocephalidea, Pseudophyllidea, Tetraphyllidea, trypanorhychidea and Caryophyllidea.

9. Phylum :Aschelminthes
Bilaterally symmetrical pseudocoelomate animals.
Class Nematoda
elongate worms with a cylindrical body tapering at both ends.
The mouth is located anteriorly.
The gut is clearly divided into an oesophagus and an intestine.
The sexes are separate.
The Nematoda includes the orders Trichuridea, Ascariddea, Spiruridea, Filariidea and Diocrophymidea .

10. Phylum Acanthocephala
Mostly elongate cylindrical worms armed with an anterior retractile proboscis carrying hooks.
Gut absent, sexes are separate.
At least one intermediate host is required in the life cycle.

11. Phylum Mollusca
The larvae of freshwater bivalve molluscs are often found attached to the gills and outer surfaces of fish.
The larvae have this bivalve shells often with little hooks on their inner edge.

12. Phylum Arthropoda:-
Bilaterally symmetrical animals with segmented bodies bearing jointed appendages.
The body in covered with a rigid or semi-rigid chitinoid exoskeleton.
Subclass Branchiura
Body flattened dorsoventrally.
Two pairs of antennae and a preoral proboscis are present.
The second maxillae usually form prehensile suckers, eg. Argulus.

13. Argulus important ectoparasite infecting fish.
Commonly called the "Fish Louse", these are flattened creatures with a very distinctive shape and appearance.
They move about the skin of a fish very effectively and camouflage themselves well on the host.
They suck body fluids from the fish via a sharp stylet that actually injects a small amount of toxin into the fish.
Argulus also possesses a life cycle with pelagic larval stages so the entire aquarium system may have to be treated with organophosphates to control the disease

14. Subclass Copepoda. copepods found on fish are mostly ectoparasitic.
They display a great range of body form, ranging from those types such as
Ergasilus -unfused abdominal segments and
Lernea in which the body segments are fused together and many of the appendages are missing or highly modified.
The Copepoda includes the orders Cyclopidea, Caligidea and Lernaeopodidea.

15. Lernea
infects large scaled freshwater tropical and temperate species of fish
possesses a life cycle that includes microscopic pelagic larval stages
easily visible to the naked eye and may be more than 2 cm in length.
Infected fish scrapes against objects, whitish-green threads hang out of the fish skin with an inflamed area at point of attachment.
treatment
Organophosphates and glacial acetic acid dips
10 to 30 minute bath in 10 mg/l of potassium permanganate.

16. Phylum Annelida Class:- Hirudinea
Segmentaed, coelomate worms with a muscular body.
Class:- Hirudinea
The leeches have a segmented body with anterior and posterior suckers.
There may be eyespots anteriorly.
The body may be round or somewhat dorsoventrally flattened.
Leeches are ectoparasitic on fish.
Families Piscicolidae and Glossiphorniidae.

17. Phylum Chordata Order Petromyzontia Family petromyzontidae
The Lampreys are eel-like, mostly freshwater or anadromous, jawless fishes.
They have a round suctrorial disc-like mouth with horny teeth.
The single nostril is dorsally situated.
The fins have no rays.
They are sanguinivorous in nature