They have an open circulatory system and lack a heart. On the other hand, echinoderms have a well-developed coelom and a complete digestive system. Echinoderms use pheromones to communicate with each other. They detect the chemicals with sensory cells on their body surface. Some echinoderms also have simple eyes ocelli that can sense light. Like annelids, echinoderms have the ability to regenerate a missing body part.
Some echinoderms can reproduce asexually by fission, but most echinoderms reproduce sexually. They generally have separate sexes and external fertilization. Eggs hatch into free-swimming larvae. The larvae undergo metamorphosis to change into the adult form. During metamorphosis, their bilateral symmetry changes to radial symmetry. Living echinoderms are placed in five classes. These five classes show many similarities. Organisms in each class are described in Table below. Believe it or not, this is an animal.
See the mouth and arms? Echinoderms Echinoderms are marine organisms that make up the phylum Echinodermata. Echinoderm Reproduction Some echinoderms can reproduce asexually by fission, but most echinoderms reproduce sexually. Echinoderm Classification Living echinoderms are placed in five classes. Summary Echinoderms are marine invertebrates. They include sea stars, sand dollars, and feather stars. Echinoderms have a spiny endoskeleton.
Without echinoderms, many areas of the ocean would be greatly affected and therefore, echinoderms are an important animal phylum to learn about. It is estimated that there are up to 13, extinct species of echinoderms and that the very first echinoderm was alive in the Lower Cambrian period. This period of time would range from million years ago. The oldest fossil available is called Arkarua.
This species was small, round and disc-like with five grooves extending from the center Echinoderm Fossils. The first echinoderm was thought to be very simple Knott, The organism was motile and bilateral in symmetry.
Bilateral symmetry means the organism can be cut right down the middle and be split into two equal halves. The echinoderm ancestry later developed radial symmetry as it was thought to be more advantageous to the species. The bilateral symmetry can still be seen in the larvae of echinoderms but once they reach adulthood, they develop radial symmetry. The first picture below shows an echinoderm larvae and the bilateral symmetry is clearly shown.
The concept of radial symmetry is clearly illustrated in starfish including the Horned starfish Protoreaster nodosus , shown below. Species of starfish, like the common starfish, have five radially symmetrical projections projecting from a central disk.
These feet have symmetrical outer and inner structures Zubi, Radial Symmetry in an adult Starfish. Out of these it is clear that they form a monophyletic group, however there is doubt as to their phylogenetic relationship within the tree itself. This debate is based on whether Brittle Stars Ophiuroidea and Starfish Asteroidea form a sister clade, i.
Today there are only really two well supported hypotheses those are as follows:. Asterozoan Hypothesis: In this hypothesis it is believed that Brittle Stars and Starfish form a sister clade, and just like in the Cryptosyringid hypothesis Sea Urchins and Sea Cucumbers form another sister clade and Sea Lilies is the most basal group.
This hypothesis is based off of molecular phylogenetic studies which help to show that even though Brittle Stars has a pluteus-type larva which is the larval form of both Sea Urchins and Sea Cucumbers this could just be a result of convergent evolution or that Starfish reverted to an older form of larval form Telford, Cryptosyringid Hypothesis: Similar to the previous hypothesis, Sea Lilies is the most basal group, however in this hypothesis Brittle Stars and Starfish do not form a sister clade.
This hypothesis has support in the development of the organism so that Brittle Stars are sister to Sea Urchins and Sea Cucumbers. This is because they all share a common larval state during early development which could imply that Brittle Stars are more closely related to the sister group containing Sea Urchins and Sea Cucumbers than Starfish Telford, Now that their placement among themselves is better understood, where do Echinoderms in general fit in with other animals and other organisms?
Echinoderms fit in the superphylum deuterostomes of which composes animals who during development the anus forms first unlike the protostomes which have mouth first development. Humans also fall into this superphylum whereas snails and insects develop mouth first.
The above figure represents the phylogenetic tree of the Echinodermata back to the supergroup Unikonts Keeling, The associated divergence dates, or estimated time periods a group split from a common ancestor, are included above in millions of years MYA Hedges, The oldest echinoderms found to date are from the Cambrian period. This period was about million years ago. Some fossils have been found that may be an ancient echinoderm, but there is no definite proof at the moment.
The ancient phyla of echinoderms was divided into classes based on body geometry, type of plating, body symmetry and the absence or presence of appendages. Three basic body plans emerged during the Cambrian echinoderms Scripps Institution of Oceanography, From the middle of the Cambrian period to the mid to late Ordovician period, the class diversification of the echinoderms occurred twice.
According to the fossil record, the diversification decreased at the end of the Cambrian period but this may be due to the lack of artifact preservation. The class level during this period was as high as From the Cambrian period to the Ordovician period, eleven new classes originated. Since this peak of diversification, the amount of class diversity gradually decreased. Eventually the amount of classes decreased to eight.
With the Blastoids, Ophiocistiods and Isorophid edrioasteroids going extinct in the Permian period, there were only five classes that survived the Mesozoic. Echinoderms developed many key evolutionary characteristics that define all species within the phylum, making them one of the most unique animal phyla.
Kjerschow-Agersborg studied the physiological anterior end in Pycnopodia helianthodes [29]. Thorpe studied the orientation of locomotion in echinoderm [30]. O'Donoghue studied the migration behaviors of certain starfish [31]. Rodenhouse studied the morphology and behavior in Pteraster tesselatus [32]. Smith studied the neural system and behavior of starfish [33]. To explore this question, we weighed starfish arms and central disks to determine their center of gravity and symmetric mechanism.
We then counted the number of times that starfish used each arm and statistically calculated their behavioral symmetric plane. We concluded that starfish are slightly bilateral in behavior, and they are, to some extent, bilateral animals. Asterias amurensis is a very common species of sea star in East Asian coastal areas. We designated the arm opposite to the madreporite as Arm 1, and the others follow clockwise successively in aboral view Fig.
Our numbering system is different than the previous ones, shown in Table 1 [34]. Arm 1 is the arm opposite to the madreporite, and the other arms follow clockwise successively in aboral view. The coordinate system is as shown in the figure. Point O, the origin, is located at the center. Arm 2 lies on the positive x-axis. All weights were measured on analytical balances that were accurate to at least 0. The position of the last pair of side pedicellaria at the base of each arm was designated as the cutting line line L in Fig.
The arms of the dried starfish were cut off and weighed. Because some starfish may regenerate a new arm when it is broken, those with notably different arms were excluded. Due to the large number of starfish used in this experiment, minor differences resulting from inconspicuous regeneration can be regarded as insignificant. Line h, the distance between the center and bottom of the arm, is 1.
Line H, the length of an arm, is 2. Line L is the cutting line. When weighing, the arms were cut off along Line L. The central disk is the light-colored regular pentagon. We assumed the central disk to be homogeneous, so Point O, the center of gravity of the central disk, lay in the center of the regular pentagon. The unit of action is the shaded part.
Point U is the center of gravity in the plane unit, and we assigned the frequency of action of each arm to it. After cutting off all of the arms, the intact central disk was weighed Fig. Because the organs are within the central disk and the organs are soft with irregular shapes, it was difficult to divide the central disk into five equal parts and weigh them precisely. The following experiments were conducted in calm seawater, and the starfish used were all healthy and sound. We lifted the starfish wholly, not just with one single arm.
No specific permits were required for the described field studies. The starfish were turned upside down and left to turn back freely. Generally, the starfish firstly extended its arms upwards, then bent two adjacent arms against the ground for support, stamped the ground with the opposite arm and lifted the other two arms upwards on each side, finally the opposite arm lost its contact with the substrate and the starfish turned over [35] , [36] Fig.
In such cases, we recorded the number of the stamping arm. Occasionally, they only bent one arm against the ground for support, stamped the ground with the opposite two arms and lifted the other two arms upwards on each side. In these cases, we recorded the two stamping arms and assigned each a weight of 0.
Generally, the starfish firstly extended its arms upwards, then bent two adjacent arms against the ground for support, stamped the ground with the opposite arm and lifted the other two arms upwards on each side, finally the opposite arm lost its contact with the substrate and the starfish turned over.
The starfish were placed in water and left to crawl freely. In such cases, we recorded the number of the backward arm. In such cases, we recorded the two arms backward and assigned each a weight of 0.
The arrow indicates the direction of movement. The level of the seawater was lowered enough to bare the central disk of the starfish. A drop of alkali solution was placed in the center of the starfish's back, and we observed its escape. The same arms were recorded as in the crawling experiment. During the statistics of mass, we compare the five arms of the starfish within itself, which means each starfish was designated with the same weight 5 and contributed the same to the sum weight.
V-tests were carried out on the weight and behavior data of each arm of the starfish in order to detect the tendency of skewing on one direction. The course is as follows: Assume the data either weight or behaviors for each arm was d1, d2, d3, d4, d5, and D means the sum of the five data. The center of gravity in a bilateral animal is supposed to lie on the plane of bilateral symmetry.
Because its behavior on both sides is also bilateral, the behavioral center of frequency should also lie on the symmetric plane.
A straight line can be drawn joining the two centers and is considered to be the vertical projection of the plane of bilateral symmetry. We obtained three planes from our three behavioral experiments. We projected the starfish and the three planes of bilateral symmetry to the same plane and got the two-dimensional figure Fig.
All computation was in a plane coordinate system with the projection of the planes of bilateral symmetry. During calculation, we assumed that the central disk was homogeneous. We found that the ratio had little influence on the final calculation of the position of the center of gravity of the intact starfish.
The maximum value 1. Suppose a starfish is a five-pointed star and the distance between the center and bottom of an arm is 1, which is h. Through software simulation, we adjusted the ratio until the five-pointed star figure aligned with the real starfish, ultimately obtaining the distance from the endpoint of arms and the center as 3. Thus the length of an arm H was 2. According to our calculation using data from starfish Table 2 , the center of gravity of an intact starfish lies at the coordinates, 0.
We calculated the center of frequency with frequentness of action in a plane analytic fashion. We then assigned the frequency of action of each arm to the centroid of the unit, shown as the point U in Fig. The center of frequency was then obtained from averaging the centers of the five units.
If the distance between the center and the bottom of an arm is equal to 1 and the distance between the center and the endpoint of an arm is equal to 3. The blue, yellow, green and red planes represent the symmetric planes of turning-over, crawling, fleeing and average, respectively.
Anterior and posterior directions are as shown. The center of frequency in the turning-over experiment was located at the coordinates 0. The center of frequency is not the same with the origin point, v-test, 0.
The symmetric plane is shown in blue in Fig.
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