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Lower invertebrate bsc 1st semester playlist link 🔗:-
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Notes:-
Aurelia (Jellyfish) part - 5
Nervous system and sense organ of aurelia
Nervous system of Aurelia (Jellyfish)
a. Main nerve net
b. Diffuse nerve net
c. Rhopalial ganglia
Sense organs of Aurelia (Jellyfish)
1. Tentaculocyst or statocyst
2. Ocelli
3. Olfactory pits
Nervous system of Aurelia (Jellyfish)
It consists of the a. main nerve net, b. a diffuse nerve net, and c. 8 rhopalial
ganglia
a. Main nerve net
Each nerve net or plexus consists of nerve cells and fibers.
The main nerve net is more developed and lies in the subumbrellar surface
and extends into tentacles, rhopalia, manubrium, and oral arms.
The presence of the main nerve net on the subumbrellar side is correlated
with the presence of well-developed musculature on that side; the former
co-ordinates muscular movements during locomotion.
Its nerve elements form a sort of nerve ring along the margin of the
umbrella near-circular canal.
The main nerve net is thickened along per- and interradii due to the
concentration of its nerve element along these radii.
Each radial thickening, near the margin of the umbrella, is connected with
the rhopalial ganglion, situated near the rhopalium on that radius.
b. Diffuse nerve net
It lies in the epidermis of the subumbrella as well as the exumbrella.
Its nerve elements consist of smaller cell bodies.
It is also connected with rhopalial ganglia.’It controls the local responses,
like feeding, and can inhibit contractions of the umbrella because the 2
nerve nets are joined through tentaculocysts.
c. Rhopalial ganglia
These are formed by the aggregation of nerve cells.
There are 8 such rhopalial ganglia, one near each sense organ or
rhopalium.
Nerve impulses received by the sense organs are conducted through nerve
nets to the muscle fibers which react accordingly.
Sense organs of Aurelia (Jellyfish)
The sense organs of Aurelia are 8 rhopalia which are situated one in each of
the per-and interradial marginal notches. each rhopalium comprises
tentaculocyst or statocyst, 2 ocelli, and 2 olfactory pits.
1. Tentaculocyst or statocyst
It is a hollow club-shaped structure, situated in the marginal notch between
two marginal lappets.
It is covered on the outer side by the process of bell margin, termed hood,
which also connects the bases of 2 marginal lappets.
Just below the club is a pad of tall ciliated sensory epithelial cells which are
connected with the subumbrellar nerve net lying below the epidermis.
Tentaculocyst is hollow tentacles. Projecting into tentaculocyst is an extension of a
circular canal lined by gastrodermis. Lying in the distal part of the tentaculocyst is a
mass of polygonal statolith cells of gastrodermal origin.
Each statolith cell contains a self-secreted particle, the statolith (Gr., statos,
standing + lithos, stone), composed of calcium sulfate and calcium
phosphate.
Statoliths act as a weight, causing the club of tentaculocyst to bend up and
down at its base, whenever the animal tilts to one side or other, during
swimming.
Tentaculocysts control the equilibrium of the umbrella during swimming. If
the umbrella is tilted, the clubs of tentaculocysts press against their sensory
pad beneath, the sensory cells which become stimulated. Higher the tilt
greater the stimulation. The impulse is conducted through the subumbrellar
nerve net to muscle fibers which react accordingly.
In response, the upper half of the umbrella drives less water than the lower
half at each beat, so that the umbrella automatically rights itself.
2. Ocelli
There are two ocelli, one of ectodermal and the other of
endodermal origin.
The former, known as pigment spot ocellus, consists of a patch of
pigmented and sensory epidermal cells on the outer side of the
club of tentaculocyst.
The latter, known as pigment cup ocellus, consists of a cup-
shaped cavity lined by pigmented and sensory gastrodermal cells
and is situated on the inner side of tentaculocyst, in association
with statoliths.
The sensory cells of both the ocelli are connected with their
respective underlying nerve nets.
Ocelli are photoreceptors.
3. Olfactory pits
These are in the form of depression of thickened epidermis containing
sensory cells.
One such depression lies at the base of the hood. It is termed the outer or
aboral olfactory pit.
The other, known as the inner or adoral olfactory pit, is situated on the inner
side of the tentaculocyst at the base of the pad of ciliated sensory epithelial
cells
These olfactory pits are probably chemoreceptors.
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