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 Hydra viridis
Hydra is included in the phylum Cnidaria, along with sea anemones, jellyfish and coral polyps, and is one of the few freshwater members of this group. One feature all members of the Cnidaria have in common is the possession of thread cells or cnidia or nematocysts -- tiny stinging cells located in the body and tentacles which discharge a paralyzing poison into prey organisms, enabling their capture and ingestion. The nematocysts of Hydra are similar to those of the marine stinging jellyfish which periodically invade beaches, causing injury and sometimes death to humans.
The Hydra's body is a hollow tube consisting of two layers of cells separated by an unstructured gelatinous layer. The outer, clear layer of cells is called the epidermis or ectoderm, and generates the nematocysts. The inner layer is called the endoderm or
gastrodermis, and produces the enzymes which digest the Hydra's food. The separating layer is called the
mesoglea. It is a gel of various secretions and proteins, containing loose cells not organised into any kind of tissue.
The Hydra's tentacles are hollow, tubular extensions of these three layers.
Many members of the Cnidaria establish symbiotic relationships with zoochlorellae which impart a green colour to the polyp. Coral polyps are a well known example of this in the marine environment, and in fresh water, the best known example is Hydra
viridis.
Hydra reproduces asexually most of the time by a process of budding, young polyps becoming detatched from the parent when they are fully developed. Seasonal episodes of sexual reproduction also occur, mature polyps developing gonads on the external body wall. Fertilized eggs give rise to tiny planula larvae which swim away, attatch themselves and develop into polyps which continue to reproduce by budding.
Hyra viridis is usually bright green. The colour is due to large numbers of green algal cells, or
zoochlorellae, dispersed throughout the entire endoderm, the innermost of the two cell-layers forming the Hydra's tubular body.
The zoochlorellae, as they do in the other creatures which harbour them, use sunlight to produce sugars and generate oxygen, both of which benefit the hydra.
The metabolic activities of the hydra generate carbon dioxide which is required by the algae. Because they have their own internal garden, green hydras can survive for long periods between prey captures.
Article by Micrographia
http://www.micrographia.com
 Ostracoda
Like the copepods, the ostracods are very numerous in both freshwater and marine environments. There are 2000 living species. The larger marine species are also known as mussel shrimps or seed shrimps, but the freshwater ostracods are usually smaller than a
millimetre.
There are 10,000 or so fossil species dating from the Late Cambrian period (about 500 million years ago) to recent times. Their great abundance and widespread distribution have made them useful index fossils for dating marine sediments, notably in oil exploration.
In freshwater ponds they are usually found scuttling around among the submerged plants and debris at the shallow edges, and less commonly in the open waters. They swim smoothly with appendages extended from between the two halves of their carapace. When disturbed, they withdraw their limbs and clamp the halves of their tiny shells tightly together.
They are perhaps less attractive creatures than the other small crustaceans due to the opaque and sometimes strongly patterned shell which makes it difficult to see their internal structure. Young specimens are the most rewarding for microscopical examination, as their shells are generally more transparent than those of the adults.
Ostracods are very similar in appearance, making it less than easy for the non-expert to distinguish one species from another -- or even one genus from another.
The ceaseless activity of ostracods can make them something of a nuisance to the microscopist when, under the microscope, they disturb the observation of some other creature. They are bulls in the chinashop of the
sub-millimetre world.
Article by Micrographia
http://www.micrographia.com
 Mollusca
gastropoda
Snails. The largest and most diverse group of mollusks. The phylum
Mollusca in a ancient group that is second in size only to the arthropods.
There are over 50,000 living species and about as many fossil species
known. Mollusks are unsegmented, coelomate protostomes with a ventral
muscular foot, a shell-secreting mantle, and a radula as the feeding
organ. They have a trochophore larval stage similar to the annelids and a
later veliger larval stage. The circulatory system is open and excretion
is by metanephridia. Gastropods make up 70% of all mollusks and they are
well represented in marine, freshwater and terrestrial habitats.
Article by E.S.G. Montana State University
http://www.esg.montana.edu/
 Bryozoa
The bryozoa are sessile colonial aquatic animals. The individuals are called zooids, and they reproduce by a process of budding. Of the 5000 or so known species, only about 50 are found in fresh water.
Although the marine bryozoa are most common in the shallow waters of the continental shelf, they have been found at ocean depths of 8000m. The colonies they form can be large gelatinous or calcareous patches involving many thousands of individuals, or long branching chains resembling plants (the
"polyzoa" and "moss animalcules" of the early microscopists).
Though the individual zooids superficially resemble hydroid polyps such as those of the freshwater Hydra, they are considerably more complex. The freshwater bryozoan Lophopus (shown in MicroMondo's
videoclip) shows the struc-tural features common to all brozoans most conspicuously the numerous long, ciliated tentacles and their supporting structures, collectively called the
lophophore, which generates the feeding current and oxygenates the animal.
They provide one of the microscope's most compelling spectacles, especially in darkfield when the movements of suspended particles shows the feeding currents clearly.
Article by Micrographia
http://www.micrographia.com
 Oligochaete
Annelid worms are mostly macroscopic and free-living. Their bodies are cylindrical or flattened, and are divided into a number of segments which may be as few as a dozen or so, or more than a hundred. Most segments bear bundles of
chaetae, or bristles - numerous in many of the marine forms, or few in the case of the earthworm and the microscopic freshwater forms.
Included in this phylum are the leeches, the marine bristle worms, and the earthworms.
The annelids are usually divided into three classes: Oligochaete worms, Polychaete worms, and the
Hirudinae, or leeches. The mostly marine Polychaete worms are not covered here.
This class includes a wide variety of worms, primarily freshwater and terrestrial in habitat, with giant earthworms having pigmented bodies and few chaetae at one end of the scale, and tiny colourless aquatic worms with pronounced chaetae at the other. The smaller aquatic forms are the ones shown in these galleries.
They are usually found in the mud and detritus of the pond bottom, and in the tangles of filamentous algae floating on the surface and anchored at the edges of the pond.
They are scavengers -- anything digestible is absorbed from the food intake during its passage through the intestine. In turn, the faecal pellets which fall to the floor of the pond, consisting of undigested remains loosely bound with mucous secretions from the gut of the worm, form a suitable substrate for the support of other life such as unicellular algae, bacteria, and the ciliates, rotifers and other worms which are normally found in their company.
 Cyclops
Copepods are extremely abundant animals in both freshwater and marine habitats.
The free-living copepods fall into three orders:
- Cyclopoidea.
Includes the well known Cyclops, of which there are over a hundred species. Moderately long secondary antennae, and the females carry twin egg-sacs.
- Calanoidea.
Characterized by long secondary antennae, and in the females, a single egg-sac. Includes
Diaptomus.
- Harpacticoidea.
Much smaller than Cyclops and Diaptomus, and usually found foraging on submerged plants. Their small antennae do not enable them to swim.
The various species of Cyclops are very numerous in freshwater locations during the summer months, and are usually among the much reduced fauna in these same locations during the winter. The main task facing the microscopist is that of catching a specimen for examination under the microscope, as they dart very quickly away when any attempt is made to catch them. Specimens collected from the open waters of a pond or lake often have more transparent cuticles than those at the water's edge, and provide a better view of internal structures.
Article by Micrographia
http://www.micrographia.com
 Stentor caeruleus
Stentor are one of the largest protozoa found in water, and is therefore one of the easiest for the microscopist to study. This article describes where you can find the Stentor and how different types of illumination under the microscope can be used to show the characteristic features of a protozoan.
Protozoa are single-celled organisms and are one of the simplest forms of life. In the modern classification of organisms, the protozoa belong to the kingdom Protista along with the algae and lower fungi. The Protozoa are unique in that the single cell performs all the functions of feeding, digestion, excretion, respiration and reproduction.
Although most protozoans are microscopic, the Stentor is the exception to this rule and can be up to 2mm long! It is therefore larger than many of the multi-cellular organisms found in freshwater such as rotifers and water-fleas, and has been known to eat the smaller members of these groups.
Stentor are usually present in most freshwater ponds. Take a few pieces of the stem of any floating plant eg
Potamogeton, which has spear-shaped leaves. Place the stems in some water in a shallow dish and allow a few minutes for the water to settle. Examine the stems with a 10x hand lens - any Stentor present on the stem will be visible as long trumpet-shaped organisms. Stentor may also be swimming freely, when they become more oval in shape with a narrowing at their posterior.
Autumn is a good time of year to find Stentor because leaves falling into a pond increase the bacteria population feeding on the decaying vegetation. This leads in turn to an increase in the population of protozoa such as Stentor which feed on the bacteria. Colonies of Stentor often occur, in which the lower parts of the animals are covered with a mucilaginous substance which they secrete.
Stentor is remarkable for its regenerative powers; a small fragment one-hundredth the volume of an adult can grow back to a complete organism. This capability has made Stentor a favourite subject amongst biologists for studies of regeneration in
protozoans.
Stentor can be very colourful - there are green (Stentor caeruleus), blue and amethyst coloured species. The green colour is a result of the microscopic green algae which Stentor ingests. The algae live in symbiosis with the
Stentor, ie the algae and Stentor mutually benefit from the close association. The algae uses photosynthesis to convert Stentor's waste products to useful nutrients.
Article by Microscopy-UK
www.microscopy-uk.net/micropolitan/index.html
Hydracna
Water mites can usually be found around the edges of ponds and lakes, either swimming in the vicinity of submerged plants or foraging on their leaves and stems. They are often brightly
coloured, most often some shade of red or green, although blue, yellow and tan colours are also seen. The adults are rarely larger than about 2mm or smaller than 0.5mm. The largest known varieties can reach 8mm.
Having eight legs and a pair of palps, they can appear at first to resemble tiny spiders, but closer inspection reveals that what would be cephalothorax and abdomen in a spider is in the water mites fused into a single body part having no sign of segmentation.
They are almost exclusively freshwater in their habitat, and whilst a few are parasitic upon such creatures as the freshwater mussel, most are free-living and carnivorous. These seize upon worms, small crustaceans and small insects encountered in their
foragings, and use their penetrating mandibles (homologous to the chelicera of the spiders) to pierce the cuticle of the prey and suck out its juices.
There are also less active and less conspicuously coloured mites which spend their time crawing sluggishly amongst the plant debris of the pond floor (see below), and may be present in large numbers.
Article by Micrographia
http://www.micrographia.com
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