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13. Simple life
| 13.18 |
Level III life: Sponges
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| Sponge, any of several thousand species constituting a phylum of simple invertebrate animals. Sponges are mainly marine, with a few freshwater species. They are abundant throughout the world and especially in tropical waters, where they and other invertebrates such as corals are important in the formation of calcareous (chalky) deposits. | ||
| All major sponge groups in existence today had representatives living in the Cambrian period, about 570 million years ago. Considerable debate exists as to the relation of sponges to other invertebrate groups. That they are a multi cellular line derived from single-celled protozoans is generally accepted, but their relationship to the cnidarians (coelenterates-jellyfish and corals) is less clear. One widely held view is that both cnidarians and sponges had a common ancestor. | ||
| 13.18.1 | Key Characteristics | |
| The primitive cellular construction of sponges consists of an outer layer of covering cells and an inner layer of flagellated cells that move water through the animals. Between the two layers are numerous wandering, amoeba-like cells and supporting skeletal structures, often in the form of hard, crystalline spines called spicules. Adult sponges are sessile and attach themselves to rocks or other hard surfaces. A sponge feeds by moving water into lateral incoming pores called ostia and then out through a large opening at its top, called the osculum. Each flagellated cell has a thin collar around a single flagellum. These cells line either large chambers or small ones called flagellated chambers. If the latter are present, they are connected by canals, and the entire sponge can build up pressure and shoot water long distances from the osculum-apparently a mechanism to avoid reuse of oxygen- and food-depleted water. | ||
| Reproduction occurs either sexually or asexually. In the former case, the sponges are usually hermaphroditic but cross-fertilize one another. Eggs and sperm unite to produce a free-swimming larva that settles on a new surface and metamorphoses into a new sponge. Reproduction can also occur by small, internal asexual buds called gemmules, each one able to give rise to a new sponge. The gemmules are able to withstand unfavorable conditions, such as freezing or desiccation (drying out). Sponges have been of great interest to developmental biologists because they are able to reconstitute themselves if their cells are separated into a suspension. | ||
| Scientific classification: Sponges make up the phylum Porifera. Four major groups exist. All the Calcarea are marine, with skeletal spicules composed of calcium carbonate. The Hexactinellida are mostly found in the deep sea; because their skeleton is made of silica in beautiful six-pointed arrangements, they are called glass sponges. The Demospongiae (95 per cent of extant sponge species) include the few freshwater forms. Their skeletal network is made of spongin, and in some species silica spicules are also present. The Sclerospongiae (coralline sponges) are all marine and have a combination of a thin silica and spongin skeleton that surrounds a bulkier, central calcareous skeleton. | ||
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