《海洋生物专业英语》第三部分-protozoa(原生动物)

The three major classes of mollusks ：
Gastropods(abalone, bivalves cephalopods
snail, slug,),
(including squids and octopods are protected by a single, spiraled shell and have a distinct head with eyes at the tips of tentacles.

Most
of the 150,000 know species of mollusks are marine, though some inhabit flesh water, and there are land-dwelling mollusks in the form of snails and slugs.
Flatworms
Flatworms
are the simplest bilateral animals. leaflike or ribbonlike, ranging from about 1 millimeter to about 20 meters length. about 20,000 species living in marine, freshwater, and damp terrestrial habitats. Planarians(涡虫） are examples of freeliving(nonparasitic) flatworms

Living
in partially digested food in the intestines of their hosts, tapeworms simply absorb nutrients across their body surface.
Roundworms
Roundworms(Phylum Nematoda) get their common name from their cylindrical bodies, which are usually tapered at both ends. Roundworms are among the most diverse and widespread of all animals. About 90,000 species of roundworms are known, and perhaps ten times that number actually exists.
Annelids
Worms with body segmentation, which is the division of the body along its length into a series of repeated parts. 15,000 speicies, ranging in length from less than 1 mm to 3-meter giant Australian earthworm. Live in the sea, most freshwater habitats, and damp soil. Three main classed :the earth worms and polychaetes, and the leeches.

Roundworms range in length from about a millimeter to a meter. They are found in most aquatic habitats, in wet soil, and as parasites in the body fluids and tissues of plants and animals. Free-living roundworms in the soil are important decomposers. Other species are major agricultural pests that attack the roots of plants. Humans host at least 50 parasitic roundworm species, including pinworms, hookworms,



The whole body, including the appendages, is covered by an exoskeleton. The two distinct regions of the body are the cephalothorax (consisting of the head and thorax) and the abdomen. The head bears a pair of eyes, each situated on a movable stalk. The body is segmented, but this characteristic is only obvious in the abdomen. Specialized appendages, including pincers, walking legs, swimming appendages, and two pairs of sensory antennae. Even the multiple mouthparts are modified legs, which is why they work from side to side rather than up and down (as our jaws do).
Bivalves
include clams（蛤）, oysters（牡 蛎）, mussel（贻贝）, and scallops（扇贝）. Most bivalves are sedentary, living in sand or mud in marine and freshwater environments. They use their muscular foot for digging and anchoring.

Roundworms exhibit two evolutionary innovations not found in flatworms.
have a complete digestive tract, which is a digestive tube with two openings, a mouth and an anus. This anatomy contrasts with the digestive sacs, or gastrovascular cavities, of cnidarians and flatworms, which use a single opening as both mouth and anus.
Bivalves
cephalopods
the
shell is small and internal (as in squids) or missing altogether (as in octopuses). Cephalopods are marine predators that use beaklike jaws and a radula to crush or rip prey apart. The cephalopod mouth is at the base of the foot, which is drawn out into several long tentacles for catching and holding prey.


A second evolutionary innovation we see for the first time in roundworms is a body cavity, which in this case is a pseudocoelom.
Mollusks
Mollusks(from the Latin molluscus, soft) are softbodied animals, but most are protected by a hard shell. Slugs, squids, and octopuses have reduced shells, most of which are internal, or they have lost their shells completely during their evolution. Many mollusks feed by using a strap-like rasping organ called a radula to scrape up food.

General characteristics
In contrast to the matching segments of annelids, however, arthropod segments and their appendages have become specialized for a great variety of functions. Specialization of segments, or fused groups of segments, also provides for an efficient division of labor among body regions. The appendages of different segments are variously modified for walking, feeding, sensory reception, copulation and defense.