General Characteristics of phylum Nematodes/ Nemathelminthes/ Aschelminthes:
- Nemathelminthes or nematodes are mostly aquatic, but some may also be terrestrial.
- The phylum includes both free-living as well as parasitic organisms.
- They have unsegmented, bilaterally symmetrical and triploblastic body.
- A well-defined head is absent in them.
- Their whole body is covered with thick, non-cellular and flexible cuticle.
- Most of their phyla bear adhesive organs.
- Cilia and muscles are the locomotory organs in them.
- Their body cavity may either be pseudocoelomate or some may be acoelomate.
- They bear non-muscular, straight gut with a mouth and an anus.
- They have muscular and highly specialised pharynx.
- Respiratory and blood vascular systems are absent in them.
- Their nervous system consists of circumenteric nerve ring with longitudinal nerves.
- Excretory organs in them are protonephridia and flame cell.
- They are generally dioecious.
- Some of their phyla exhibit determinate cleavage.
- Their fertilisation is internal.
- They doesn’t have any larval stage in their life cycle.
- Sense organs in them are poorly developed which includes phasmids, amphids, bristles, pepeline.
- They are ubiquitous in nature.
- They reproduces sexually by the fusion of gametes.
Classification of phylum Nematodes/ Nemathelminthes/ Aschelminthes:
Based on the presence or absence of phasmids (sensory organ) nematodes are classified into two types along with their examples, as follows:
CLASS – l
- Phasmids are absent in them.
- They doesn’t bear any excretory canal.
- Amphids are usually present behind their lips.
- They are mostly found in free living condition, but some are parasitic too.
- Free living species among them are usually marine inhabitants.
- Examples: Enoplus, Xiphinema and Mermis.
CLASS – ll
- Phasmids are present in them.
- They bear excretory canals.
- Amphids are usually present in their lateral lips.
- They are mostly parasitic in nature (thus they can also be called as parasitic organisms).
- Free living species in them are soil inhabitants.
- Examples: Encylostoma, Euthereria, Ascaris