The body cavity of sponges is large, it is open to the outside world, and it enables the sponge to consume food Dawkins Sponges do not have any internal organs or a nervous system. Their skeleton is made of tiny, needle-like splinters, or a mesh of protein called spongin 2 Most of the Demospongiae are made of spongin.
Some of the sponges are made of both spicules and spongin. Bath sponges are made of calcereous or siliceous spicules, and they are therefore soft to the touch. Most of the bath sponges are Demospongiae 2.
Porifera means pore-bearing. Sponges are covered with tiny pores on the outside called ostia 2. Ostia lead to an internal system of canals which leads to one or more larger holes called oscula, which are the openings to the out side.
Ostia are surrounded by donut shaped cells called porocytes. The chambers within the canals are lined up with cells called choanocytes, or collar cells. The collar cells consist a sticky funnel shaped collar and a hair-like whip called a flagellum. The collar cells beat their flagella back and forth to force the water through the sponge.
The water brings in nutrients and oxygen and takes out the carbon dioxide. The two purposes of providing food and removing wastes are served due to this function. The sticky collars also pick up tiny particles of food and bacteria brought in by the water 2. Unlike the cells of most other animals, the cells of sponges do not form into various organs like kidney, liver, or nerves. Sponge cells are totipotent Dawkins The totipotent cells could give rise to reproductive cells or other types of cells of the organism.
In theory, this enables a sponge to live forever Dawkins Sponges can produce sexually as well as asexually. Most sponges are hermaphroditic, which means each adult can act as either female or male.
Asexual reproduction happens by budding and fragmentation 4. With this type of reproduction pieces or fragments of the sponge's body are broken off the by the water currents, and they are carried by the currents to a different location. Prevailing theories suggest that sponges are early animals which produced no subsequent evolutionary line. It is the thinness of the sponge body and the fact that its cells are exposed to circulating water—which supplies food and oxygen, and removes waste—that make organs unnecessary.
Sponges may have been the first multicellular animals. Multicellularity which means that cells adhere to one another, communicate, are mutually dependent for survival, and specialize to perform different tasks is the key to producing more complex organisms.
From round to encrusting to branching, the colors and textures of sponges add to the complexity of the reef. Credit: G. Sponges are found in a wide variety of colors, shapes, and sizes and are often mistaken for plants. The approximately 8, living sponge species are scientifically classified in the phylum Porifera , which is comprised of four distinct classes: the Demospongiae the most diverse, containing 90 percent of all living sponges , Hexactinellida the rare glass sponges , Calcarea calcareous sponges , and Homoscleromorpha the rarest and simplest class, only recently recognized, with approximately species.
Sponges are primarily marine, but around species live in fresh water. Sponges have cellular-level organization, meaning that that their cells are specialized so that different cells perform different functions, but similar cells are not organized into tissues and bodies are a sort of loose aggregation of different kinds of cells.
This is the simplest kind of cellular organization found among parazoans. Other characteristics of sponges include a system of pores also called ostia and canals, through which water passes. Water movement is driven by the beating of flagellae, which are located on specialized cells called choanocytes collar cells. Sponges are either radially symmetrical or asymmetrical.
They are supported by a skeleton made up of the protein collagen and spicules, which may be calcareous or siliceous, depending on the group of sponges examined. Skeletal elements, choanocytes, and other cells are imbedded in a gelatinous matrix called mesohyl or mesoglea. Sponges capture food detritus particles, plankton, bacteria that is brought close by water currents created by the choanocytes.
Food items are taken into individual cells by phagocytosis, and digestion occurs within individual cells. Reproduction by sponges is by both sexual and asexual means. Asexual reproduction is by means of external buds. Some species also form internal buds, called gemmules, which can survive extremely unfavorable conditions that cause the rest of the sponge to die. Sexual reproduction takes place in the mesohyl. Male gametes are released into the water by a sponge and taken into the pore systems of its neighbors in the same way as food items.
Spermatozoa are "captured" by collar cells, which then lose their collars and transform into specialized, amoeba-like cells that carry the spermatozoa to the eggs. Some sponges are monoecious; others are dioecious.
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