专业英语作业

Except for the sponges,animal cells are organized intotissues,which are groups of cells combined into a structural and functonal unit.In most animals the tissues are organized into organs,which are complex structures that are made up by the composition of two or more kinds of tissues.
This developmental stage occurs in all animals.In most animals the blastula folds inward at one point to form a hollow sac with an opening at one end called the plastopore.An embryo with a blastopore is called a gastrula.The subsequent growth and movement of the cells of the gastrula produce the digestive system.The details of early embryonic development differ widely from one phylum of animals to another,as you will see.These details often provide important clues to the evolutionary relationships among the phyla.
Mostcrustaceans have two pairs of antennae,three pairs of chewing appendage,and various numbers of pairsof legs.Crustaceans differ from the insects---but resemble the centipedes and millipedes---in that they have legs on their abdomen as well as on their thorax.They are the only arphropods with two pairs of antennae.
3.1 Chelicerates
The fossil record of chelicerates goer back as far as that of any multicellular animals,about 630 million years.a major group of extinct arthropods,the trilobites,was also abundant then,and the living horsehoe crabs seem to be directly descended from them.by far the largest of the three classes of chelicerates is the largely terrestrial class arachnida,with some 57000named species including the spiders,ticks,mites,scorpions,and daddy longlegs.
3.2Mandቤተ መጻሕፍቲ ባይዱbulates:Crustaceans
The crustaceans（subphylum Crustacea,with a single class)are a large, diverse group of primarily aquatic organisms, including some 35000 species of crabs, shrimps, lobsters, crayfish, barancles, water fleas, pillbugs,and related groups. Often incredibly abundant in marine and freshwater habitats and playing a role of critical importance in virtually all aquatic ecosystems,crustaceans have been called “the insects of the water”.
The spicules and spongin may occur together,or only one of the may be present.These elements not only strengthen
the body of the sponge,but they may also deter predators.425 million years ago. There are about 35,000 named species of spiders ( order Araneae ). The order Acarai, the mites,is the largest in terms of number of species and the most diverse of the arachnids.
2.The Sponges-Animala without Tissues
Sponges are the simplest of animals.The cells of a sponge are not orgnized into tissues.Sponges lack organs,and most sponges completely lack symmetry.The bodies of sponges consist of little more than masses of cells embedded in a gelatinous matrix.There is relatively little coordination among the cells-a sponge can pass through a fine silk mesh,with individual clumps of cells separating,and then reaggregate on the other side.
Most animals reproduce sexually.Their gametes-eggs and sperm-do not divide by mitosis.With few exceptions animals are diploid;their gametes are the only haploid cells in their life cycles.The complex form of a given animal develops from a zygote formed from the union of male and female gametes.In a characteristic process of embryonic development,the zygote first undergoes a series of mitotic divisions and becomes a hollow ball of cells called the blastula.
3.diversity of the arthropods
Many arthropods have jaws,or mandibles,formed by the modification of one of the pairs of anterior appendages（but not the one nearest the anterior end).these arthropods are called mandibulates ； they include the crustaceans,insects,centipedes,millipodes,and a few other small groups.the remaining arthropods,which include the spide-rs,mites,sc-orpions,and a few other groups,
scorpions are probably the most ancient group of terrestrial arthropods. They are known from as early as the silurian period,some 425 million years ago. There are about 35,000 named species of spiders ( order Araneae ). The order Acarai, the mites,is the largest in terms of number of species and the most diverse of the arachnids.
lack mandibles. These animals are called the chelicerates. Their mouthparts,called chelicerae,elolved from the appendages nearest the anterior end of the animals.they often take the form of pincers or fangs.the crustaceans seem to have evolved mandibles separately and are not thought to be directly related to the insects and other mandibulates.
1.Some General Features of Animals
Animals are extraordinarily diverse in form.They range in size from a few that are smaller than many protists to others like the truly enormous whales and giant squid.The cells in animals are exceedingly diverse in form and function and are of fundamental importance in makng up their complex bodies,which function so well under such a wide variety of circumstance.
The body of a young sponge is shaped like a sac or vase.The body wall is covered on the outside by a layer of flattened cells called the epithelial wall.Facing into the internal cavity are specialized,flagellated cells called choanocytes,or collar cells.Between the choanocytes and epithelial wall is a gelatinous,protein-rich matrix in which occur various types of amoeboid cells,minute needles of calcium carbonate or silica called spicules, and fibers of a tough protein called spongin.