Tissue-level organization Vs organ-level organization
The tissue level of organization occurs in Hydra. Cells are grouped in
coherent masses for performance of one or more special functions
musculo-epithelial tissue, nervous tissue .
The tentacles may be given the status of organs
, consisting of several types
of tissues (musculo-epithelial cells, sensory cells, interstitial cells,
nematoblasts, endoderm cells) coordinated for performance of one or more
(food capture, conveyance of food to mouth, looping movements).
Division of labours among cells
lead to formation of organs specialised for
more sophisticated functions in tapeworm and earthworm
e.g. ovary and
testis in tapeworm, nephridia in earthworm.
In tapeworm, in adaptation to its parasitic mode of life, there is no alimentary
canal. Digested food of the host is absorbed over whole surface. The nervous
system is reduced
. A very large number of proglottides are produced. The
oldest proglottis is the most posterior
. Every mature proglottis contains a full
set of male and female reproductive organs
Acoelomate Vs coelomate
Movements of body wall occur independently of peristaltic movements of gut
wall and digestive processes.
The coelom provides a large cavity in which organs can be developed and
can perform their functions without interference from other organs. Location of
these organs in coelom necessitates excretory and
reproductive ducts to
convey products to exterior.
Coetomic fluid gives support
, protection and assistance with locomotion
Development of coelom has necessitated a blood-vascular system for
transport of digested food and oxygen.
Proglottis Vs metamerism
exhibits metameric segmentation. Typically every segment
exactly similar. Although the segments are partitioned from one another, they
are coordinated and do not function as independent units
. The main
advantage of segmentation is that it provides an opportunity for specialization
in certain segments
without interference with others.
In contrast to the proglottis of tapeworms, in earthworm there is a definite
number of segments. There is no region capable of permanent proliferation of
segments. The oldest segment is the most anterior.