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يوجد 38 صفحة - انت الآن في الصفحة رقم 1
Jawad Jawad
Learning English
منذ 2 أيام   نشر في  ٢٠٢٦/٠٨/٠٧ م
The nervous system of land planarians has the longitudinal nerve cords reduced to one ventral pair that is located much deeper in the body than in other triclads. These ventral cords are usually connected by many comissures, so that they fuse into a single ventral nerve plate.

Additionally, land planarians have a highly developed ventral nerve plexus just below the epidermis that is probably associated to the presence of a creeping sole.
22

Jawad Jawad
Learning English
منذ 3 أيام   نشر في  ٢٠٢٦/٠٨/٠٦ م
The epidermis of land planarians is composed by a simple epithelium of cubic or columnar cells. The cells are ciliated only on a ventral region, called creeping sole, which the animal uses to glide over surfaces.

Numerous secretory cells open throughout the epidermis, the most characteristic ones being the rhabditogen cells, which produce a secretion in the form of small rod-like structures, the rhabdites. Rhabditogen cells are very numerous on the dorsal epidermis.
28

Jawad Jawad
Learning English
منذ 4 أيام   نشر في  ٢٠٢٦/٠٨/٠٥ م
The most distinguishing feature that characterizes land planarians is the presence of a creeping sole, a highly ciliated region on the ventral epidermis that helps them to creep over the substrate.

The creeping sole may be wide and flat, occupying most of the ventral surface, or narrow and pronounced, being easily distinguished from the rest of the ventral surface.
34

Jawad Jawad
Learning English
منذ 6 أيام   نشر في  ٢٠٢٦/٠٨/٠٤ م
At first they may be confused with slugs or leeches, but they lack the anterior tentacles of slugs and the segmentation of leeches. Their size vary greatly, from a few millimeters in length to about one metre.
20

Jawad Jawad
Learning English
منذ 6 أيام   نشر في  ٢٠٢٦/٠٨/٠٣ م
Land planarians are distinguished from their marine and freshwater relatives by their terrestrial habits, as well as by morphological distinctions. Some species have dull colors, including shades of brown and grey, that make them inconspicuous in their environment, but most species are marked by very colorful patterns.
24

Jawad Jawad
Learning English
منذ 1 أسابيع   نشر في  ٢٠٢٦/٠٨/٠٢ م
At the other extreme, one species in this family, Platydemus manokwari has become an invasive species in both disturbed and wild habitats in the Pacific Islands, and has damaged the endemic land snail fauna. This species has been found in Europe (France) in 2013 for the first time, and in 2015 in New Caledonia, Wallis and Futuna Islands, Singapore, Solomon Islands, Puerto Rico (first record in the Caribbean), and Florida, USA.
25

Jawad Jawad
Learning English
منذ 1 أسابيع   نشر في  ٢٠٢٦/٠٧/٣٠ م
Because of their strict ecological requirements, some species have been proposed as indicators of the conservation state of their habitats.

They are generally animals with low vagility (dispersal ability) and with very specific habitat requirements, so they can be also used to accurately determine the distribution of biogeographic realms.

Today the fauna of these animals is being studied to select conservation priorities in the Atlantic rainforest in Brazil.
18

Jawad Jawad
Learning English
منذ 2 أسابيع   نشر في  ٢٠٢٦/٠٧/٢٩ م
Geoplanidae is a family of flatworms known commonly as land planarians or land flatworms.
These flatworms are mainly predators of other invertebrates, which they hunt, attack and capture using physical force and the adhesive and digestive properties of their mucus.
They lack water-retaining mechanisms and are therefore very sensitive to humidity variations of their environment.
22

Jawad Jawad
Learning English
منذ 2 أسابيع   نشر في  ٢٠٢٦/٠٧/٢٨ م
Various conservation initiatives are being attempted around the world. The Chinese giant salamander, at 1.8 m (6 ft) the largest amphibian in the world, is critically endangered, as it is collected for food and for use in traditional Chinese medicine. An environmental education program is being undertaken to encourage sustainable management of wild populations in the Qinling Mountains and captive breeding programs have been set up.
22

Jawad Jawad
Learning English
منذ 2 أسابيع   نشر في  ٢٠٢٦/٠٧/٢٧ م
Other larvae, especially in permanent pools and warmer climates, may not undergo metamorphosis until fully adult in size. Other populations in colder climates may not metamorphose at all, and become mature for mating while in their larval forms. Neoteny allows the species to survive even when the terrestrial environment is too harsh for the adults to thrive on land.
26

Jawad Jawad
Learning English
منذ 2 أسابيع   نشر في  ٢٠٢٦/٠٧/٢٥ م
The changes that take place at metamorphosis are under the control of thyroid hormones and in obligate neotenes such as the axolotl (Ambystoma mexicanum), the tissues are seemingly unresponsive to the hormones.

In other species, the changes may not be triggered because of underactivity of the hypothalamus-pituitary-thyroid mechanism which may occur when conditions in the terrestrial environment are too inhospitable.
21

Jawad Jawad
Learning English
منذ 2 أسابيع   نشر في  ٢٠٢٦/٠٧/٢٤ م
By the end of the larval stage, the tadpoles already have limbs and metamorphosis takes place normally. In salamanders, this occurs over a short period of time and involves the closing of the gill slits and the loss of structures such as gills and tail fins that are not required as adults.

At the same time, eyelids develop, the mouth becomes wider, a tongue appears, and teeth are formed. The aqueous larva emerges onto land as a terrestrial adult.
18

Jawad Jawad
Learning English
منذ 2 أسابيع   نشر في  ٢٠٢٦/٠٧/٢٣ م
The tadpoles are carnivorous and the larval stage may last from days to years, depending on species. Sometimes this stage is completely bypassed, and the eggs of most lungless salamanders (Plethodontidae) develop directly into miniature versions of the adult without an intervening larval stage.

15

Jawad Jawad
Learning English
منذ 3 أسابيع   نشر في  ٢٠٢٦/٠٧/٢٢ م
Pond-type larvae may have a pair of rod-like balancers on either side of the head, long gill filaments and broad fins. Stream-type larvae are more slender with short gill filaments—in Rhyacotriton and Onychodactylus, and some species in Batrachuperus, the gills and gill rakers are extremely reduced, narrower fins and no balancers, but instead have hind limbs already developed when they hatch.
15

Jawad Jawad
Learning English
منذ 3 أسابيع   نشر في  ٢٠٢٦/٠٧/٢١ م
In temperate regions, reproduction is usually seasonal and salamanders may migrate to breeding grounds. Males usually arrive first and in some instances set up territories. Typically, a larval stage follows in which the organism is fully aquatic.

The tadpole has three pairs of external gills, no eyelids, a long body, a laterally flattened tail with dorsal and ventral fins and in some species limb-buds or limbs.
14

Jawad Jawad
Learning English
منذ 3 أسابيع   نشر في  ٢٠٢٦/٠٧/٢٠ م
One species, the Anderson's salamander, is one of the few species of living amphibians to occur in brackish or salt water.

Sierra newt amplexus found in stream at Woolman Semester in Nevada County, California
Many salamanders do not use vocalisations, and in most species the sexes look alike, so they use olfactory and tactile cues to identify potential mates.
17

Jawad Jawad
Learning English
منذ 3 أسابيع   نشر في  ٢٠٢٦/٠٧/١٨ م
The northern slimy salamander (Plethodon glutinosus) has a wide range and occupies a habitat similar to that of the southern gray-cheeked salamander (Plethodon metcalfi).

The latter is restricted to the slightly cooler and wetter conditions in north-facing cove forests in the southern Appalachians, and to higher elevations above 900 m (3,000 ft), while the former is more adaptable, and would be perfectly able to inhabit these locations.
18

Jawad Jawad
Learning English
منذ 3 أسابيع   نشر في  ٢٠٢٦/٠٧/١٧ م
There are about 760 living species of salamander. One-third of the known salamander species are found in North America. The highest concentration of these is found in the Appalachian Mountains region, where the Plethodontidae are thought to have originated in mountain streams.

Here, vegetation zones and proximity to water are of greater importance than altitude. Only species that adopted a more terrestrial mode of life have been able to disperse to other localities.
18

Jawad Jawad
Learning English
منذ 3 أسابيع   نشر في  ٢٠٢٦/٠٧/١٦ م
They also lived on the Caribbean Islands during the early Miocene epoch, confirmed by the discovery of Palaeoplethodon hispaniolae, found trapped in amber in the Dominican Republic.

Vertebrae fossils recovered from the Murgon fossil site have been tentatively attributed to that of a salamander, though its true identity is disputed. If the vertebrae truly belong to a salamander, they would represent the only salamanders in Australia.
16

Jawad Jawad
Learning English
منذ 4 أسابيع   نشر في  ٢٠٢٦/٠٧/١٥ م
Salamanders are found only in the Holarctic and Neotropical regions, not reaching south of the Mediterranean Basin, the Himalayas, or in South America the Amazon Basin. They do not extend north of the Arctic tree line, with the northernmost Asian species, Salamandrella keyserlingii, which can survive long-term freezing at −55 °C, occurring in the Siberian larch forests of Sakha.
58

Jawad Jawad
Learning English
منذ 4 أسابيع   نشر في  ٢٠٢٦/٠٧/١٤ م
The oldest known total-group (Caudata) salamander is Triassurus from the Triassic of Kyrgyzstan. Further salamander fossils are known from the Middle Jurassic of England, Scotland, China, and Kazakhstan. The oldest known crown-group salamander (Urodela) remains uncertain but recent analyses suggest it is Valdotriton from the Late Jurassic of Spain.
22

Jawad Jawad
Learning English
منذ 4 أسابيع   نشر في  ٢٠٢٦/٠٧/١٣ م
Some salamander species use tail autotomy to escape predators. The tail drops off and wriggles around for a while after an attack, and the salamander either runs away or stays still enough not to be noticed while the predator is distracted.

The tail regrows with time, and salamanders routinely regenerate other complex tissues, including the lens or retina of the eye. Within only a few weeks of losing a piece of a limb, a salamander perfectly reforms the missing structure.
16

Jawad Jawad
Learning English
منذ 4 أسابيع   نشر في  ٢٠٢٦/٠٧/١١ م
A correlation exists between the toxicity of Californian salamander species and diurnal habits: relatively harmless species like the California slender salamander (Batrachoseps attenuatus) are nocturnal and are eaten by snakes, while the California newt has many large poison glands in its skin, is diurnal, and is avoided by snakes.
17

Jawad Jawad
Learning English
منذ 1 شهور   نشر في  ٢٠٢٦/٠٧/١٠ م
In California, the palatable yellow-eyed salamander (Ensatina eschscholtzii) closely resembles the toxic California newt (Taricha torosa) and the rough-skinned newt (Taricha granulosa), whereas in other parts of its range, it is cryptically colored.

24

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