11th Class Biology Animal Kingdom Phylum Aschelminthes (Nemathelminthes) - The Round Worms

(Gk. nema = thread; helmin = worm)

Brief History : Ancient people were familiar with certain large-sized nematode parasites of domestic animals. Minute nematodes were discovered only after the invention of microscope. Linnaeus (1758) included these in “Vermes” Rudophi (1793, 1819) included these under “Nematoidea” Gegenbaur (1859) ultimately proposed “Nemathelminthes” for these.

General Characters

(1) Many endoparasites of various animals and plants; others free–living and widely distributed in all sorts of water and damp soil.

(2) Mostly minute or small; some large (1 mm to 25 cm); some upto several meters long.

(3) Slender, cylindrical, elongated body usually tapering towards both ends, and unsegmented.

(4) Body wall formed of a thick, tough and shiny cuticle, a syncytial hypodermis beneath cuticle, and innermost layer of peculiar, large and longitudinally extended muscle cells arranged in four quadrants.

(5) Triploblastic, bilaterally symmetrical, pseudocoelomate, false coelom derived from embryonic blastocoel, unsegmented.

(6) Straight alimentary tract terminal mouth and anus. These are first animals to have complete gut.

(7) ‘Tube within a tube body’ plan, organ-system grade of body organization.

(8) Circulatory system and respiratory organs absent. A simple excretory system consists of protonephridia, comparatively simpler or complicated sensory organs, and a well–developed nervous system present

(9) Reproductive system well–developed. Usually unisexual with sexual dimorphism.

(10) Many kinds of Nematodes are parasites of useful plants and domestic animals. Some of these are pathogenic to their hosts, causing serious diseases. Even man  is a host for more than 50 species, of which Ascaris lumbricoides and Enterobius vermicularis (pin worm) are very common. Other common human nematodes are Wuchereria which causes Filaria, Trichinella causing trichinosis, and Ancylostoma causing hookworm disease.

Classification of Nemathelminthes : On basis of the presence or absence of some specialized sense organs and caudal glands, and characteristics of excretory system, nematodes are classified into two classes –

Class 1.  Phasmidia or Secernentea or Rhabditea

(1) Mostly parasitic.

(2) Possess a pair of unicellular, pouch-like sense organs, called phasmids, near hind end of body.

(3) Another pair of reduced, pore–like sense organs, called amphids, present near anterior end.

(4) Excretory system with paired lateral canals.

(5) Caudal glands absent.

Examples – Ascaris, Enterobius, Ancylostoma, Wuchereria, Trichuris, Trichinella, Dioctophyma, Rhabditis Necator, Gnathostoma, Dracunculus, Loa, etc.

Class 2. Aphasmidia or Adenophorea or Enoplea

(1) Mostly small, free-living.

(2) No phasmids.

(3) Amphids spiral, cord like or disc like, seldom pore like.

(4) No lateral excretory canals.

(5) Caudal glands present.

Examples : Enoplus, Dorylaimus, Mermis, Halichoanolaimus, Monohystera, Desmoscolex, etc.