On the basis of number of axes and rays: Spicules may occur in several forms like the simple rod form or in the form of forks, anchors, shovels, stars, plumes etc. The spicule forms depend on the presence of number of axes and rays. Accordingly, they can be divided into the following forms:.
Monaxon: These kinds of spicules are formed by the growth along one axis. They may be straight needle-like or rod like or may be curved. Their ends may be pointed or hooked or knobbed. Monaxons can be both calcareous and siliceous types. These monaxon spicules are further divided into two kinds,.
Monactinal- the growth of the spicule takes place only in one direction. Diactinal- The growth of the spicule takes place in both the directions. Tetraxon: These spicules have four rays each pointing in different direction.
Usually one of the four rays is elongated giving the appearance of a crown of 3 rays. Such spicules are called as triaenes.
Triaxon: These spicules have three axes that cross one another at right angles to produce six rays. Thus it is also called hexactinal spicule. These triaxon spicules are characteristic of glass sponges of the class Hexactanellida. Polyaxon: These are the spicules with several equal rays radiating from a central point.
They may be grouped to give star-like appearance. Polyaxon spicules are found along with microscleres. The calcareous spicules are secreted by special type of cells called as sclerocytes. These sclerocytes are derived from binucleated mesenchymal scleroblasts. A monaxon spicule or each ray of the triradiate spicule is secreted by a group of two sclerocytes. Among these two sclerocytes one acts as thickener cell and the other acts as the founder cell. The initiation of the formation of the spicule starts with the deposition of a particle of calcium carbonate between the two nuclei of the binucleated mesenchymal cells.
This particle grows drawing apart the two nuclei and then two sclerocytes are formed. Now the thickener cell lays down additional layer of calcium carbonate adding to the thickness of the spicule. When the spicule is fully formed, both the cells i. The scleroblast secreting a calcareous spicule is called as calcoblast, while the scleroblast secreting a siliceous spicule is called silicoblast.
All cell types within the sponge obtain oxygen from water through diffusion. Likewise, carbon dioxide is released into seawater by diffusion. In addition, nitrogenous waste produced as a by-product of protein metabolism is excreted via diffusion by individual cells into the water as it passes through the sponge. Water flow in a sponge : In a sponge, water enters through the body pores and exits in the direction of the osculum direction of blue arrow. This diffusion of water through the body supports major functions in the sponge.
Sponges reproduce by sexual, as well as, asexual methods. The typical means of asexual reproduction is either fragmentation where a piece of the sponge breaks off, settles on a new substrate, and develops into a new individual or budding a genetically-identical outgrowth from the parent eventually detaches or remains attached to form a colony. An atypical type of asexual reproduction is found only in freshwater sponges, occurring through the formation of gemmules.
Gemmules are environmentally-resistant structures produced by adult sponges wherein the typical sponge morphology is inverted. In gemmules, an inner layer of amoebocytes is surrounded by a layer of collagen spongin that may be reinforced by spicules.
The collagen that is normally found in the mesohyl becomes the outer protective layer. In freshwater sponges, gemmules may survive hostile environmental conditions such as changes in temperature. They serve to recolonize the habitat once environmental conditions stabilize. Gemmules are capable of attaching to a substratum and generating a new sponge. Since gemmules can withstand harsh environments, are resistant to desiccation, and remain dormant for long periods, they are an excellent means of colonization for a sessile organism.
Sexual reproduction in sponges occurs when gametes are generated. Sponges are monoecious hermaphroditic , which means that one individual can produce both gametes eggs and sperm simultaneously.
In some sponges, production of gametes may occur throughout the year, whereas other sponges may show sexual cycles depending upon water temperature. Sponges may also become sequentially hermaphroditic, producing oocytes first and spermatozoa later. Oocytes arise by the differentiation of amoebocytes and are retained within the spongocoel, whereas spermatozoa result from the differentiation of choanocytes and are ejected via the osculum.
Ejection of spermatozoa may be a timed and coordinated event, as seen in certain species. Spermatozoa carried along by water currents can fertilize the oocytes borne in the mesohyl of other sponges. Early larval development occurs within the sponge; free-swimming larvae are then released via the osculum.
Sponges are generally sessile as adults and spend their lives attached to a fixed substratum. They do not show movement over large distances as do free-swimming marine invertebrates. However, sponge cells are capable of creeping along substrata via organizational plasticity. Under experimental conditions, researchers have shown that sponge cells spread on a physical support demonstrate a leading edge for directed movement.
It has been speculated that this localized creeping movement may help sponges adjust to microenvironments near the point of attachment.
It must be noted, however, that this pattern of movement has been documented in laboratories, but it remains to be observed in natural sponge habitats.
Privacy Policy. Skip to main content. Search for:. Phylum Porifera. Phylum Porifera Sponges lack true tissues, have no body symmetry, and are sessile; types are classified based on presence and composition of spicules. Learning Objectives Explain the position of the phylum Porifera in the phylogenetic tree of invertebrates. Key Takeaways Key Points As larvae, sponges are able to swim, but as adults, they are sessile, spending their life attached to a substrate.
Although the majority of sponges live in marine habitats, one family, the Spongillidae, is found in fresh water. Calcarea, Hexactinellida, Demospongiae, and Homoscleromorpha make up the four classes of sponges; each type is classified based on the presence or composition of its spicules or spongin. Most sponges reproduce sexually; however, some can reproduce through budding and the regeneration of fragments. The majority of sponges are filter-feeders, but a few species are carnivorous due to the nutrient -poor environment in which they are found.
Key Terms parazoan : include only one phylum known as the sponges endosymbiont : an organism that lives within the body or cells of another organism spongin : a horny, sulfur-containing protein related to keratin that forms the skeletal structure of certain classes of sponges spicule : a sharp, needle-like piece holdfast : a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate.
Advanced search. Skip to main content Thank you for visiting nature. Download PDF. Subjects Zoology. This process allows sponges to adopt a huge variety of shapes and sizes, the authors say. Rights and permissions Reprints and Permissions. About this article. Cite this article How the sponge got its skeleton. Copy to clipboard.
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