Phylum Cnidaria (a stinging thread) Also known as Phylum Coelenterata (-hollow gut) This phylum contains about 10,000 species worldwide. They are mostly marine with a few freshwater species.
Cnidarians are regarded as originating close to the basal stock of the metazoan line. They have a 700 million back history based on fossil records.
Defining characteristics are: 1. nematocysts (cnidae); complex intracellular organelles that secreted by the animal 2. Planula larva in the life cycle. Larva is bilaterally symmetrical
Cnidarians include; 1. corals
2. sea anemones
3. Fresh water hydroids (Hydra),
4. Jellyfish
Physalia 5. Portuguese man-of-war Portuguese war ship
Cnidarians have a basic radial symmetry. They possess only 2 layers of living tissue; - ectoderm ; the epidermis* - endoderm; the gastrodermis* *Muscle fibers in epithelia They have a Gelatinous layer called mesoglea (non living) in between epidermis and gastrodermis. Amoebocytes (living cells) in the mesoglea involve in food digestion, nutrient transport, storage and defense.
The body is sac-like with an internal gastro vascular cavity called coelenteron and an opening called the mouth. (Open-ended tubular gut; no anus) The mouth surrounds a ring of tentacles. No head Body axis is oral- aboral
Cnidarians are characterized by having stinging cells called cnidoblast (nematoblasts). These cells contain cnidae (nematocysts), a fluid-filled proteinaceous capsule with an opening in one end. Inside capsule is a long, spirally coiled hollow thread/tube. Cnidocil is a cluster of modified cilia in the cnidoblast through which chemical and tactile stimulation perceive to act on cnida
Upon stimulation, the hollow thread/tube shoots out explosively from the capsule, and eject from the lid turning inside out as it goes. It contains toxins which are used in food capture and defense.
The entire process requires only about 3 milli seconds
Cnidarians are primarily carnivorous; but some soft coral feed on phytoplanktons Some obtain nutrients through photosynthesizing symbiotic algae
They show tissue level of organization. Muscle fibres occur at the base of the epidermal and gastrodermal cells. They do not have circulatory, respiratory and excretory systems.
A nerve net consisting of multipolar neurones are located below the epidermis near the mesoglea. Some of them have sense organs such as statocysts and oceli. (Simplest animals to possess true nerve cells and sense organs)
The body form of a cnidarian is constructed following two basic body plans, the polyp form and the medusa form. (Polymorphism) Both forms have tentacles arranged around the mouth.
Polyp/Polypoid form Usually sessile Vase/cylindrical shape Mesoglea thin Mouth at the upper surface Tentacles occur around the mouth Solitary or colonial Sometimes they build hard or soft corals Eg. Corals, Hydras, Sea anemones Medusa/Medusoid form Usually mobile Bell/umbrella shape Mesoglea thick Mouth at the lower surface Tentacles occur along the edge of the umbrella Always solitary Never build skeletons Eg. Jelly fishes
In some species only the polyp form is seen in the life cycle while in some others only the medusa form is seen. Still, in the life cycle of some other species alternation of generation is seen between the medusa and the polyp forms.
Only polyp - Hydra
Only medusa Physalia Aurelia (Jellyfish)
Typical cnidarian life cycle
Cnidarians are divided into four taxonomic classes. Class Hydrozoa includes- hydras portuguese-man-of war Obelia Class Schyphozoa includes-jellyfishes Class Anthozoa - includes- sea Anemones Class Cubozoa corals
Class Schyphozoa Cup animals Include few hundreds (about 200) species of cnidarians All are marine. Eg. Aurelia
Scyphozoans are characterized by the predominant medusoid stage (scyphomedusae) It looks an inverted cup. The polyp stage exists only as a short-lived larval stage. (Scyphistoma larva) Scyphistoma larva
Scyphozoan medusae are big; 2 cm - 2 m in diameter (relative to the hydrozoan medusae)
Most scyphozoans swim actively by contraction of muscle fibres and mechanical properties of mesoglea. Contraction of muscle fibre results the reduction of the volume of space under the bell and excess water expel out. This effects animal to propel opposite direction.
Mesoglea is thick that supports the body structure like a skeleton. It is firm gelatin. Hence, scyphozoans are commonly known as Jellyfish.
Mouth locates at the end of a muscular cylinder-known as manubrium.
They lack a velum(a shelf of tissue projecting inward from the margin of the bell in hydromedusae - Hydrozoan) The medusae bell is circular in cross section and the tentacles are distributed around the circumference. Sensory organs distributed around the edge of the bell, called the rhopalia, contain the ocelli (simple eyes) and statocysts used for determining spatial orientation.
Scyphomedusae always have 4 oral arms surrounding the mouth.
Gastrovascular cavity of scyphozoans is developed as fluid filled gastrovascular canals. Gastrovascular canals connect to the mouth through the manubrium.
Food particles captured by nematocysts on tentacles and oral arm-----ingested at the mouth--- through manubrium-- -- to stomach -----distributed among four gastric pouches.
Wall of the gastrovascular pouch cansists short tentacles with nematocysts that secrete digestive enzymes. Partially digested food particles are phagocytized and digestion completes intracellularly. Circulation of fluid in the gut take place by movement of cilia lining the gastrovascular canals.
The life cycle of a typical scyphozoan (Aurelia) The sexes are separated. The life cycle of Aurelia includes alternating generations of medusae and polyps
Notes The lifecycle of a typical scyphozoan (Aurelia) In medusae, gametes are released from the gastrodermal gonads into the gastric pouches. Ova remain in the coelenteron but spermatozoa exit via the mouth into the sea. They enter the mouth of a female and make their way to the gonad where they fertilize the eggs.
Notes Cleavage begins in the gastric pouches. Embryos are released from the mouth and brooded on the oral arms. A hollow, ciliated blastula develops and invaginates to become a gastrula. The gastrula, known as the planula larva, is capable of independent existence and leaves the mother to swim in the plankton. The planula locates a suitable hard substratum, attaches, and grows into a scyphistoma larva a solitary polyp form. The scyphistoma can feed and may survive more than one reproductive season.
Notes The sessile minute scyphistoma, reproduce asexually by budding or by formation of cysts (podocysts). Under suitable environmental conditions, scyphistomae undergo transverse segmentation and segment metamorphosis (strobilation). The segmented parts of the strobilating polyp (strobila) develop into medusae that eventually break loose and become free swimming young jellyfishes (ephyrae). The ephyra grows rapidly into an adult medusa, completing the life cycle.
Class Hydrozoa (Water animals) 2,700 species of cnidarians include in Class Hydrozoa Include marine and freshwater species Eg. Hydras, Portuguese-man-of war,
Most hydrozoans represent a polyp stage than a medusa stage in the life cycle. Eg. Hydra - do not have a medusa
But, some spend part of their lives as medusa and part as polyp stage. Eg. Obelia gonozoid gastrozoid
Hydrozoan medusae are also known as jellyfishes but, much smaller (few cms or even less) Gastrodermal tissues of hydrozoans do not have nematocysts. They are restricted to epidermis. Mesoglea do not contain cells.
A shelf of tissues called velum extend inward from the edge of the swimming bell towards manubrium This help to eject water in a grater pressure from under the bell through a narrow opening when contraction of musculature
~6,600 species. Marine. Class Anthozoa (Flower animals) corals, sea anemones The anthozoans are a key feature of all marine environments, most notably in the form of coral reefs. They are sessile and either solitary or colonial. Lack a medusa stage entirely (Only polyp stage that look like flowers).
The oral disc is encircled by numerous tentacles (in multiples of 6 or 8) placed at top of a cylindrical column. The body cavity (coelenteron) is divided by mesenteries (septa) also in multiples of 6 or 8. Anthozoans have a muscular pharynx leading from the mouth to the gut.
Coral polyp secrete a prototheca by its epidermal layer; first as a basal plate and then develop as radial folds and a rim as a thecal wall in between mesenteries.
There are two subclasses within the Anthozoa: Hexacorallia Octocorallia
Hexacorallia (Zoantharia) The hexacorals have; - simple tentacles around the mouth in multiples of six - and six pairs of internal mesenteries. Eg. - common sea anemones, Fungia;(They are solitary) and - most are reef forming true stony corals (hard corals) and thorny corals; (They are colonial)
Fungia Sea anemony Coral
Octacorallia (Alcyonaria) Octocorals have eight branching (pinate) tentacles and eight mesenteries (septa). Colonial & soft corals Eg. Gorgonia (Sea fan), sea pens, sea pancies (Renilla). Many show polymorphism.
Class Cubozoa - box jellyfish ~20 species. Marine.
A cubozoan medusa.
The cubozoans are similar to the Scyphozoa but the bell is square in cross section, with a velum-like structure called the velarium. The velarium restricts the size of the opening through which water is expelled when the bell contracts, thus increasing thrust and making them stronger swimmers than the Scyphozoa. There are four clusters of tentacles, one at each corner of the bell.
The rhopalia of the Cubozoa differ from those of the Scyphozoa in possessing very complex eyes with lenses, corneas and retinas. The lens is capable of producing very sharp images, as good as human eyes but the focal length is longer than the distance between the lens and the retina making box jellyfish strangely far-sighted.
Development is also different in the cubozoans. Each scyphistoma forms a single medusa via complete metamorphosis. Cubozoa includes the highly toxic box jellyfish, found in tropical regions and often in swarms which can drift into bays, disrupting human activities.