Sea firs (hydroids), hydras and siphonophores (Class Hydrozoa) The class Hydrozoa comprises over 2,700 species and is subdivided into seven orders: Athecata, Thecata, Limnomedusae, Trachymedusae, Narco medusae, Siphonophora and Hydrocorallinae. Both polyp and medusa are generally present, but the latter may be reduced and then does not always leave the parent colony. Many members of … Read more
Reproduction and development in Cnidarians Cnidarians are usually dioecious, that is, the sexes are separate, but may be hermaphrodite. Gametes are usually shed into the seawater, where fertilisation occurs. In general, medusae produce gametes, but in instances where only the polyp occurs, (e.g. in anthozoans) gonads are borne on the projecting internal septa or (e.g. … Read more
Cnidaria structure Musculo-epithelial cells in Cnidarians The major cell types of the Cnidaria are the musculo epithelial cells, nerve cells, and nematoblasts which produce nematocysts. The ‘body’ is composed mainly of musculo-epithelial cells. These cells, found only in the cnidarians, are a means of producing an epithelium combined with a contractile system. Each cell consists … Read more
The phylum Cnidaria is predominantly marine and contains many common shore animals such as sea anemones and sea firs (hydroids), as well as deeper water forms such as the Portuguese Man o’ War. Freshwater forms are less common, but one group, the hydras, is probably the best-known member of the phylum. Classes of Cnidaria There … Read more
Class Gelatinosa In contrast to the previous class of sponges, the gelatinos sponges contain a jelly, as the name would suggest. This jelly fills the interstices of the body tissue, and the flow of water is thus restricted to a system of inhalant and exhalant canals. Order Calcarea These are sponges of the gelatinose type, … Read more
Class Nuda These are sponges that lack intercellular jelly. In other words, the star-shaped cells and the amoeba cytes form a network through which water percolates, and there are no inhalant or exhalant canals. Glass sponges (Order Hexactinellida) Sponges of this order are characterized by siliceous spicules based on a hexaradiate or six-rayed plan. Suborder … Read more
Calcareous sponges are largely confined to the littoral and the shallow seas, although a few have been dredged at depths down to 600 ft. or beyond, to a maximum depth of 2,700 ft. Hexactinellida is typically deep-sea, the majority of species living at depths greater than 600 ft., but around the Indo-Pacific islands they seem … Read more
With few exceptions, the body of a sponge is supported by a skeleton made up either of spongin fibers, usually forming a network, as in the bath sponges (Spongia), or of spicules (little spikes) of silica or carbonate of lime. Although we speak of the spicules as constituting a skeleton there are some remarkable differences … Read more
Reproduction in sponges [Phylum Parazoa] This is by both sexual and a variety of non-sexual methods. All sponges appear to bear both ova and spermatozoa, although there has recently been a suggestion that this is not invariable, and fertilization may be internal or external. With internal fertilization the embryos develop inside the sponge, which is … Read more
Body structure [Phylum Parazoa] Probably the best way to describe the body of a sponge is to define its various parts: The skin or ectosome is the outer layer or layers of cells; usually this is a single or double-layer but in a few species it takes the form of a thick cortex made up … Read more