NEET Biology Animal Kingdom Notes - Phylum-Nemathelminthes

Phylum-Nemathelminthes

 Phylum Nematoda (= Nemathelminthes).

(i) Introduction : Bilateral and protostomial ?organ grade? eumetazoans in which the space between body wall and alimentary canal is a false body cavity, or pseudocoel, derived from embryonic blastocoel. The term ?Nematoda? literally means ?threadworms? or ?roundworms? (Gr., nema = thread + eidos = form). About 12,000 species known.

(ii) 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.

            (iii) Salient features

(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 metres 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) The false body cavity, or pseudocoel is spacious, with a fluid but no free cells

            (6) Straight alimentary tract with terminal mouth and anus

(7) Circulatory system and respiratory organs absent. A simple excretory system, comparatively simpler or complicated sensory organs, and a well?developed nervous system present

            (8) Reproductive system well?developed. Usually unisexual with sexual dimorphism.

(9) 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.

(iv) Classification of Nematoda : On basis of the presence of absence of some specialized sense organs and caudal glands, and characteristics of excretory system, nematodes are classified into two classes:

            (a) Class (1) ? Phasmidia or Secernentea :

            (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, etc.

          (b) Class (2) ? Aphasmidia or Adenophorea :

            (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 ? Tichinella, Capillaria, etc.