Phylum : Aschelminthes/Nemathelminthes (round or thread worms), its classification and characteristics
- multicellular, triploblastic, bilaterally symmetrical with system grade of body organization.
- tube within tube body plan.
- mostly parasitic, few are free living found in sea water, fresh water or damp soil.
- body is elongated, cylindrical, unsegmented without lateral appendages.
- body is covered by cuticle, formed by syncytial epidermis.
- body cavity is pseudocoel. Thus, they are pseudocoelomate.
* coelom (true body cavity)- present between body wall and gut wall formed by splitting of mesoderm.
* pseudocoel– cavity present in body is persistent blastocoel.
- complete alimentary canal with both mouth and anus.
- respiratory and circulatory organs are absent.
- excretion occurs through proto-nephridia or H- shaped renette cells.
- unisexual (dioecious). They have sexual dimorphism.
- nutrition is holozoic or parasitic.
- digestion is extracellular.
- development is mostly indirect.
- mostly small sized. Some are microscopic, a meter or more long.
- sense organs are present in the form of papillae which are of two types. They are amphids (mouth) and phasmids (Anus).
Image source: askiitians
Classification
Roundworms are classified into 5 classes. They are:
Class 1: Nematoda
Commonly known as roundworms, they are aquatic or terrestrial.
- may be free-living or parasitic.
- body wall is with syncytial cuticle and longitudinal muscles in four bands.
- digestive system is complete, with muscular pharynx and glands.
- lack respiratory and circulatory system.
- sexes are generally separate.
- examples: Ascaris, Rhabtites, Oxyrius, Ancylostoma, etc.
Image source: qsstudy
Class 2: Rotifera
- microscopic and aquatic animals.
- eye spots and antennae act as sensory organs.
- commonly called wheel animalcules having a trunk and a tail.
- body wall is thickened into plates or lorica.
- anteriorly, they have a ciliated trochial disc.
- parthenogenesis usually occurs, dioecious.
- examples: Limnias, Rotara (Rotifera) etc.
Class 3: Nematomorpha
- commonly known as hair worms, found in spring (aquatic).
- body is unsegmented and covered with a thick cuticle.
- pseudocoel is present which is filled with parenchyma.
- lack circulatory, respiratory and excretory system.
- digestive system is complete.
- examples: Paragordius, Nectonema etc.
Class 4: Gastrotricha
- microscopic and may be fresh water forms or marine.
- body is covered by cuticle which bears spines.
- body long and ventrally flattened.
- mouth is surrounded by bristles.
- ventral surface bears cilia which provide gliding movement.
- excretory system consists of two protonephridia.
- pharynx is tri-radiate and muscular.
- posterior end of the body is forked.
- dioecious or monoecious.
- examples: Macrodasys, Chaetonotus etc.
Class 5: Kinorhyncha
- marine, microscopic and worm like forms.
- body is superficially segmented.
- dioecious.
- nerve ring and ventral cord present.
- alimentary canal with salivary gland.
- body surface is spherical and consists of spiny cuticle.
- development includes several larval forms.
- examples: Trachydemus, Echinoderes, etc.
References:
i) https://thebiologynotes.com/phylum-aschelminthes/
ii) https://link.springer.com/referenceworkentry/10.1007/0-387-30843-1_27
iii) https://ucmp.berkeley.edu/aschelminthes/aschelminthes.html