Transcript Phylum : Aschelminthes - GCG-42

Dr. Sanjay Kumar Jha
Department of Zoology
P.G.G.C.G. - 42, Chandigarh
GENERAL CHARACTERS
This phylum includes the bagworms. They possesses the following characters:1) Body Form: their body is worm-like, flat or cylindrical, with or without
division into regions and without any segmentation.
2) Symmetry: have bilateral symmetry.
3) Germ layers: the bagworms are triploblastic, i.e., they develop form 3 germ
layers, viz., ectoderm, endoderm and mesoderm.
4) Level of Organization: have organ-system level of body organization.
5) Head & Appendages: anterior end does not have a definite head, though it
has mouth and sense organs and contains brain; no appendages present;
locomotion by cilia in some or muscular in others that have thick cuticle.
6) Body Wall: may consists of firm, resistant cuticle, epidermis and muscle
layer. The cuticle is moulted (changed) during growth period or throughout
life. Epidermis is Syncytial or cellular and possesses low or regulated
permeability as compared to multicellular epithelia. The musculature
contains longitudinal fibres only in forms having thick cuticle that works in
tandam with each other.
General Characters (Contd.)
(7) Body Cavity: A fluid-filled cavity may be present between the body wall and the
digestive tract (Gr. askos – bag; helmins - worm). It is not lined by mesoderm or an
epithelium, hence, called as false or pseudocoelom (eg. Rotifers and large
nematodes). A pseudocoelom is a persistent blastocoel and is in reality a haemocoel
without heart. Some smaller aschelminthes have no body cavity and are Acoelomate
protostomes.
(8) Digestive Tract: The digestive tract is complete, have nonmuscular wall with anterior
and posterior parts are lined by ectodermal epithelium forming stomodaeum and
proctodaeum respectively having a specialized pharynx.
(9) Respiration: occurs by diffusion through the body surface.
(10) Circulatory System: is undeveloped because most forms are very small to small in
size.
(11) Nervous System: The nervous system comprises of a circumpharyngeal ring that
sends nerves forward as well as backward.
(12) Excretory System: the excretory system consists of gland cells, or of canals, or of
both. Some forms have protonephridia.
(13) Reproduction: sexual, as the sexes are separate, fertilization is internal.
(14) Development: Cleavage is determinate, life history simple or complex.
(15) Number of Cells: nuclei is often constant (Eutely), mitosis stops after embryonic
development and growth occurs only by enlargement of cells.
(16) Natural Habitat: occurs everywhere (fresh water, sea water and soil), many parasitic.
Unique Features
There has certain unique feature characteristic to this phylum:a) Syncitial epidermis (single cell of many nuclei).
b) Body wall musculature of longitudinal fibres only in
many forms.
c) Body cavity without a lining of mesodermal epithelium,
hence called as Pseudocoel, in many species.
d) Fixed number of cells or nuclei (Eutely).
e) Non-muscular enteron or gut.
ADVANCEMENTS
The Aschelminthes shows advancements over Platyhelminthes
in the following respects:i. Complete digestive tract which allows food to move
on without mixing successive meals and faeces to pass
out of the anus.
ii. Fluid-filled cavity between the digestive tract and the
body wall.
iii. Sexes are separate with distinguished sexual
dimorphism.
iv. Lack of asexual reproduction.