Seashell zoology
ASMR Relaxing Mollusc Lecture (zoology with shell and book sounds) (Có Thể 2024)
Seashell,hard exoskeleton of marine mollusks such as snails, bivalves, and chitons that serves to protect and support their bodies. It is composed largely of calcium carbonate secreted by the mantle, a skinlike tissue in the mollusk’s body wall. Seashells are usually made up of several layers of distinct microstructures that have differing mechanical properties. The shell layers are secreted by different parts of the mantle, although incremental growth takes place only at the shell margin. One of the most distinctive microstructures is nacre, or mother-of-pearl, which occurs as an inner layer in the shells of some gastropods and bivalves and in those of the cephalopods Nautilus and Spirula.
Seashells may be univalved (as in snails) or bivalved (as in clams), or they may be composed of a series of plates (as in chitons). They may also be reduced to small internal plates or granules, as in some slugs. In gastropods, bivalves, and shelled cephalopods, the coiled form of the shell approximates an equiangular spiral or variations of it. In some forms, such as the worm shells (family Vermetidae), however, the coiling of the shell is irregular. Shells are frequently ornamented with complex arrangements of spines, folia, ribs, cords, and grooves, which in some species provide protection against predators, give added strength, or assist in burrowing. The aperture of gastropod shells is particularly vulnerable to predators and may be protected by complex folds and teeth. Many species use a calcareous or horny operculum (trapdoor) on the foot to seal off the aperture when the foot is withdrawn into the shell. In the cephalopods Nautilus and Spirula, the planospirally coiled shell consists of multiple chambers connected by a porous tube called the siphuncle. The chambers contain quantities of water and gas that are adjusted by the siphuncle to achieve neutral buoyancy. Many seashells are brightly coloured in complicated designs by a variety of pigments secreted by special cells in the edge of the mantle. In some cases there is an obvious camouflage function, but in most others the significance of the colours is unclear.
Seashells are collected all over the world because of their endless diversity, elegance of form, and bright colours. (See shell collecting.) They also have been used to make jewelry, buttons, inlays, and other decorative items throughout history. In ancient times certain varieties, such as tooth shells and cowrie shells, were even used as money. (See also bivalve; gastropod; nautilus.)
Sông Amur, sông Đông Á. Đây là con sông dài nhất của Viễn Đông Nga và nó chỉ đứng sau sông Dương Tử và Hoàng Hồ (sông Hoàng Hà) trong số những con sông dài nhất của Trung Quốc. Các đầu nguồn của nó mọc lên ở Nga (Siberia), Mông Cổ và Khu tự trị Nội Mông của Trung Quốc ở vùng núi
Hà Lan - Hà Lan - Con người: Niềm tin phổ biến cho rằng Hà Lan là hỗn hợp của người Frisia, người Saxon và người Franks. Trên thực tế, nghiên cứu đã đưa ra sự tranh cãi hợp lý rằng cư dân tự trị của khu vực này là một hỗn hợp của các nhóm dân cư tiền Đức và Đức, trong quá trình thời gian đã hội tụ ở khu vực đồng bằng chính của Tây Âu. Đã xuất hiện từ các nhóm này trong thế kỷ thứ 7 và thứ 8, một số chính trị lớn dựa trên sự thống nhất về văn hóa và sắc tộc