Download Seastar coloring - davis.k12.ut.us

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EPIDERMIS
59
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Color A through H and their related titles. Note
that although the two surfaces of the sea star are
covered with spines, the individual spines do
not receive a separate color except in the two
magnified points of the arms. After coloring,
read below.
The central disc and rays of the sea star have two surfaces: the aboral (upper) surface, and the oral
(lower) surface on which the mouth is located. The
body is supported by an endoskeleton of calcareous
plates or ossicles with small spines that push up
through the epidermal-lined surface of the body wall.
Nests of dermal gills, projecting from the coelom up
through spaces between the ossicles, function in gas
exchange. At the distal extremity or tip of each ray, a
small pigmented spot and a single tentacle can be
seen. These are sensors receptive to light, touch, and
light pressure.
On the aboral surface of the central disc, two
openings can be seen: the madreporite, which is the
opening to the water vascular system (to be discussed), and the anus. The oral surface exhibits a
longitudinal arnbulacral groove along each ray, bordered on either side by one or two rows of suckerlike
tube feet. The term arnbutacrai is said to be assigned
with reference to walking with tube feet or a fantasized "walk" among the tubefeet tambulo = to walk).
The mouth opens in the center of the central disc on
the oral surface.
and reading below on the water vascular structures and related titles.
Just below the oral surface is a system of peritoneallined tubules that are formed as outpocketings of the
coelom. These tubules and canals make up the water
vascular system. The function of this system is to provide hydraulic pressure to the tube feet, which can
then be extended considerably to adhere the organism to a substrate to aid in feeding and locomotion.
The fluid of this delicate tubular system is very much
like coelomic fluid, and it is possible that the sea
water that comes in through the madreportte (located
on the aboral surface of the central disc) acts as a
pressure relief device. The madreporite opens into a
canal ringed by calcareous ossicles, the stone canal.
The stone canal opens into a ring canal from which
project five radial canals, one within each ray.
Throughout its length, the radial canal, surrounded
by ossicles, gives off lateral canals, each of which
leads to an ampulla (bulb) above and an epidermallined tube foot. Surrounding the ampullae are muscles which help in changing the pressure within
the tube foot. Extension of the tube foot is created by
muscular constriction of the ampulla, which forces
the fluid into the tube foot, extending it. Retraction of
the tube foot is accomplished by muscular contraction and by dilation of the ampulla, which draws in
fluid from the tube foot, creating a suction cup
device. The next plate illustrates the tube feet from
another perspective and in more detail.
It may be helpful to see Plate 60 while coloring
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60
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Color the structures of the body wall and water
vascular system, A through I, and related titles.
The view shown is one of a longitudinal section
through one arm and the central disc. After coloring, read below.
The body wall of the sea star consists of the following
from outside to in: a cuticle (not shown) secreted by
the underlying epidermis; a subepidermal connective
tissue layer supporting a calcareous skeleton of
variously shaped jointed plates to which are attached
a number of muscles situated close to the deepest
layer of the body wall, parietal peritoneum.
The epidermis surrounding and covering the endoskeletal spines contains little pedtcellariae (ped-ihcell-air-ee-ee) or calcareous jaws, which have muscles
connected at their base and give them a pincerlike action. They function in keeping the body surface clean
and free of detritus, settling larvae, and such. The calcareous plates (ossicles) that make up the skeleton
articulate together in such a way that attaching
muscles can move them, permitting the sea star to lift
and move its arms during locomotion and in capturing prey. The arms can be lifted and bent in all directions during such maneuvers. Projecting up through
spaces or breaks in the interlocking ossicles are extensions of the internal cavity or coelom, which are covered only by peritoneum and epidermis. These are
the dermal gills (dermal branchiae), through which
oxygen from the water diffuses into the coelomic
interior and from which it is distributed to all the cells
and tissues. Parietal peritoneum lines the coelom,
and supported from it by double layers of peritoneum
(mesentery) are the organs lined by visceral
peritoneum.
The water vascular system has been discussed in the
preceding plate. The structure and relationships of
the tube feet and ampullae can be appreciated here.
Parietal peritoneum
lines the ampulla and surrounds the tube foot; connective tissue and muscle
fibers surround the peritoneal-lined,
fluid-filled
cavity of each foot; epidermis lines the outer part of
the foot. The end of the foot is modified as a sucker or
terminal disc.
Color the structures of the digestive tract and
the gonads through Q2). Then read below. Color the aboral view of the dissected sea star at upper left. The arm labeled 1 is the deepest view,
and succeeding views 2-5 are progressively
more superficial views, ending at arm 5 showing
the aboral surface.
Some of the smaller prey of the sea star are caught by
the tube feet and moved to the mouth on the oral surface. The mouth opens into the cardiac stomach via a
short esophagus. The cardiac stomach is pulled at
each of the centers of the rays by gastric ligaments
(not shown). The pyloric stomach is located just
aboral to the cardiac stomach and sends out ducts
into each ray. Each duct bifurcates (divides) to form a
pair of pyloric ceca (digestive glands). The center of
the pyloric stomach opens into a short intestine that
has two diverticula or rectal glands. The intestine
extends to the aboral surface and opens to the outside
at the anus. By increase of the intracoelomic pressure, the cardiac stomach can be everted about its
prey (such as a dam). Enzymes are released onto the
prey that digest and liquify it. The prey is then
brought back in to the interior of the body by retraction of the cardiac stomach. Digestion continues to
occur within the pyloric stomach. Absorption of nutrients takes place in the pyloric ceca. The residue of
the digestive process is passed through the intestine
and anus or through the mouth to the outside.
Within each ray of the sea star is a pair of gonads
with a single duct leading to a pore on the aboral surface of the periphery of the central disc. These gonads
or sex glands produce and release eggs in the female
and sperm cells in the male. Fertilization occurs outside the animal in the sea water. Note that the gonads
and the ducts are surrounded by peritoneum.
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