The phylum Echinodermata is unique in itself with its fascinating colours, structures and shapes. They are a group of marine invertebrates, with a famous and widely recognised fellow, the “Star Fish”. These marine organisms are characterised by their unique features. One of the distinctive features is the presence of spines all over the skin, just like in Porcupine. For this reason, Jacob Klein coined the term Echinodermata, which means spiny skin in Greek (Echinos – Spiny; Dermos – Skin).
The creatures of the Echinodermata phylum are exclusively marine and bottom dwellers in seas and oceans. Living on the sea floor can be utterly challenging as there are limited resources. Also, bottom dwellers face the inability to travel long distances. Gradually, they adapted themselves to make their living possible in such inhabitable conditions also. And, in the present scenario, they are widely dominating the sea floor.
As per the recent data, this phylum constitutes around 7000 different species. It is a very diverse phylum which includes the species like:
- Star Fish/ Sea Star
- Sun Star
- Brittle Star
- Basket Star
- Sea Biscuits
- Sand Dollars
- Sea Urchins
- Cake Urchins
- Sea Cucumber
Content: Water Vascular System
- What is Water Vascular System?
- Water Vascular System Components
What is Water Vascular System?
The water vascular system is another unique trait of Echinoderms. We also refer to it as the ‘Ambulacral system’. This is basically a hydraulic system of fluid-filled canals that spreads along the entire body surface. This system is derived from coelom. It is enterocoelomic in origin.
There are several different functions of this system, including:
- Gaseous exchange (Respiration)
- Water transport (Circulation)
Along with these activities, it also mediates movement and locomotion.
- Water enters the madreporite on the dorsal side of the Echinoderm.
- From there, it passes into the stone canal, which moves water into the ring canal.
- The water channelling in this system begins from the madreporite, present on the dorsal side of the body.
- It is a button-like structure through which the water enters the Echinoderm’s body.
- From there, the water moves into the stone canal, which further carries the water to the ring canal.
- The ring canal connects to the radial canals (one radial canal in each arm).
- The radial canal turns into lateral canals (ampullae). The lateral canals end in tube feet.
Water Vascular System Components
The components of the water vascular system are:
- Stone Canal
- Ring canal
- Radial Canal
- Tiedemann’s canal
- Pollian Vesicles
- Lateral Tube
- Tube Feet
The madreporite is a button-like structure on the Echinoderm’s aboral (dorsal) region. It is a hard, sieve-like, circular, rounded, calcareous plate connecting the internal system to the aquatic environment.
Its surface has numerous narrow, radiating, straight or wavy grooves (furrows). The bottom of each groove contains many minute pores, which is why it looks like a sieve plate.
Every pore leads to a fine, short, tubular pore canal that combines into a collecting canal. There are around 200 pores and pore canals, respectively. Later each collecting canal opens into a small bag-like structure called an ampulla beneath madreporite. The ampulla leads the water into the stone canal.
After passing through the madreporite, the water enters an S-shaped stone canal. It was named a stone canal because of its strong walls. The sequence of calcified rings strengthens these walls.
On the inner surface, there are cilia for drawing the seawater in the canal from the outside. One end of the canal opens outside via madreporite while the other opens into the ring canal. A ridge with spirally coiled lamellae occupies the lumen region of the stone canal. From here, the water is pushed into the ring canal.
The ring canal is a wide pentagon-shaped ring vessel beside the mouth.
On the inner side of the ring canal, there are ten small yellowish rounded glandular structures named Tiedemann’s bodies. They give an irregular shape to the canal. In Asterias, only 9 of these bodies are present, and the space of the 10th body remains occupied by the stone canal.
The main function of these bodies is to produce coelomic corpuscles and phagocytes to dispose of the bacteria present in the ring canal.
Pollian vesicles are bulbous structures that branch off of the ring canal, serving as reservoirs to maintain excess water stores. These can be one, two or four little pear-shaped structures that remain inter radially arranged.
Basically, they are thin-walled bladders having long narrow necks. Mainly they serve the role of reservoir or storehouse to manage the fluid circulating the water vascular system. Also, they regulate the water pressure of the entire system.
From the outer surface of the ring canal, five radial canals originate, one entering each arm. The radial canal travels throughout the length of the arm up to the tip. Further, it ends in the lumen of the terminal tentacle. This radial canal moves up to the oral side of the ambulacral muscles.
Numerous pairs of short, narrow, transverse branches originate from either side of the radial canal, known as the Lateral canal or Podial canal. Each lateral canal leads to the tube feet, which remain attached at the base.
Every canal has a valve to prevent water from flowing backwards into the radial canal.
The tube feet are hollow, thin-walled, elastic cylinder-like structures. When you examine the underside of an echinoderm, you will see hundreds of tiny tube feet. These feet extend between the endoskeletal plates. They remain arranged into several rows on every arm.
The tube feet can be divided into three parts:
- Ampulla, which is the upper sac
- Middle tubular podium
- Sucker (Terminal disc)
The ampulla is a water-filled sac that contains circular as well as longitudinal muscles. In contrast, the podium is a tube-shaped structure with only longitudinal muscles. The tube feet are capable of expanding and contracting just like the hydraulic system of the bulldozer.
Functions of Water Vascular System
The water vascular system has three main functions:
- Food capture
The echinoderms exhibit creeping movements for their locomotion. This movement occurs with the help of tube feet. For this movement, the water vascular system built the hydraulic pressure mechanism.
- Firstly, one or two arms slightly rise from the substratum in the direction of movement.
- Then the ampulla of the tube feet contracts and the valve of the lateral canal closes.
- The water flows into the podium, which elevates the hydraulic pressure.
- As a result, the tube feet elongate forward and firmly adhere to the substratum with the help of suckers.
- After getting attached, the tube feet make a vertical posture, thereby pulling the entire body forward.
- Now, the podia contracts causing the water to move from the podia to the ampulla. Consequently, the tube feet shorten.
- Later, the suckers relax, and the tube feet raise again to repeat the process.
The tube feet also help in capturing the food. Mainly the suckers help in opening the hard mollusc shells.
The tube feet enable an echinoderm to attach to the ground or the surface of rocks and corals.
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