READING IN CLASS
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Class calcareous sponge-Calcarea
Members of this class, known as calcareous sponges, are distinctive in having spicules con posed of calcium carbonate. All the spicules are of the same general size and are monaxons or three or four-pronged types; they are usually separate.| Spongin fibers are absent. All three grades of structure asconoid, syconoid, and leuconoid are encountered. Many Calcarea are drab, although briliant yellow, red, and lavender species are known. They are not as large as species of other class most are less than 10 cm in height. Species of сalcareous sponges exist throughout the oceans of the world, but most are restricted to relatively shallow | coastal waters. Genera such as Leucosolenia and Sycon are commonly studied examples of asconoidi and syconoid sponges.
The subclass Sphinctozoa contains a single recently discovered representative (Neocoelia) from shaded recesses on Indo-Pacific reefs. The Sphinetozoa were abundant from the late Paleozi through the Mesozoic. There are no spicules, but a calcareous skeleton forms an outer perforated wall and also the walls of interior chambers.
Class Glass sponges - Hyalospongia
Class Hexactinellida, or Hyalospongiae
Representatives of this class are commonly knows as glass sponges. The name Hexactinellida is rived from the fact that the spicules include a hexaxon, or six-pointed type. Furthermore, some of the spicules often are fused to form a skeleton that may be lattice-like and built of long, siliceous fibers. Thus, they are called glass spong The glass sponges, as a whole, are the most syi metrical and the most individualized of sponges—that is, they show less tendency to ft interconnecting clusters or large masses with oscula. The shape is usually cup-, vase-, orun and they average 10 to 30 cm in height. The oring in most of these sponges is pale. There is well-developed atrium, and the single osculum is sometimes covered by a sieve plate—a gratelikej covering formed from fused spicules. Lattice-like skeletons composed of fused spicules in sperieJ such as Venus's-flower-basket (Euplectella) n the general body structure and symmetry of thehvl ing sponge and are very beautiful; the white, filmy] skeleton looks as if it were fashioned from wool . Basal tufts of spicule fibers implanted in sand or sediments adapt many species for living on soft bottoms.
The histology of hexactinellids is very different from that of other sponges. All surfaces exposed to water are covered not by pinacoderm but by a syncytial layer (trabecular syncytium), through which long spicules may project. Another syncytium, containing flagella with collars, lines the flagellated chambers. Archeocytes are one of the few discrete cell types. The flagellated chambers are commonly thimble shaped and oriented at right angles in parallel planes to the body wall and central antrium. Hexactinellids are thus somewhat syconoid in structure.
In contrast to the Calcarea, the Hexactinellida are chiefly deepwater sponges. Most live between depths of 200 and 1000 meters, but some have been dredged from the abyssal zone. Although found throughout the world, hexactinellids are the dominant sponges in the Antarctic.
Species of Euplectella, Venus's-flower-basket, display an interesting commensal relation with certain species of shrimp (Spongicola). A young male and a young female shrimp enter the atrium and, after growth, are unable to escape through the sieve plate covering the osculum. Their entire life is spent in the sponge prison, where they feed on plankton brought in by the sponge's water cur¬rents. A spider crab (Chorilla) and an isopod (Aega) are also found as commensals with some species of Euplectella.
This large class contains 90 per cent of sponge species and includes most of the common and familiar forms. These sponges range in distribution from shallow water to great depths.
Coloration is frequently brilliant because of pigment granules located in the amebocytes. Different species are characterized by different colors, and a complete array of hues is encountered.
The skeleton of this class is variable. It may consist of siliceous spicules or spongin fibers or a combination of both. The genus Oscarella is unique in lacking both a spongin and a spicule skeleton. These Demospongiae with siliceous skeleton differ from the Hexactinellida in that their larger spicules are monoaxons or tetraxons, never hexaxons. When both spongin fibers and spicules are present, the spicules are usually connected to, or completely embedded in, the spongin fibers.
All Demospongiae are leuconoid, and the majority are irregular, but all types of growth patterns are displayed. Some are encrusting; some have an upright branching habit or form irregular mounds; others are stringlike or foliaceous. There are also species, such as Poteiion, that are goblet or urn shaped, and others, such as Callyspongia, that are tu¬bular. The great variation in the shapes of the Demospongiae reflects, in part, adaptations to limitations of space, inclination of substrate, and current velocity. Large upright forms can exploit vertical space and use only a small part of their sur-face area for attachment. Encrusting forms, although they require more surface area for attach¬ment, can utilize vertical surfaces and very confined habitats, such as crevices and spaces be¬neath stones. The largest sponges are members of the Demospongiae; some of the tropical loggerhead sponges (Spheciospongia) form masses over a meter in height and diameter.
Several families of Demospongiae deserve mention. The boring sponges, composing the family Clionidae, are able to bore into calcareous structures, such as coral and mollusk shells, forming channels that the body of the sponge then fills. At the surface the sponge body projects from the channel opening as small papillae. These papil¬lae represent either clusters of ostia opening into an incurrent canal or an osculum. Excavation, which is begun by the larva, occurs when special amebocytes remove chips of calcium carbonate. The amebocyte begins the process, etching the margins of the chip by digesting the organic framework material and dissolving the calcium carbonate (Pomponi, 1979). The chip is then undercut in the same manner, the amebocyte en¬veloping the chip in the process. Eventually, the chip is freed and is eliminated through the excurrent water canals. Cliona celata, a common boring sponge that lives in shallow water along the Atlantic coast, inhabits old mollusk shells. The bright sulfur yellow of the sponge is visible where the bored channels reach the surface of the shell. Cliona lampa of the Caribbean is red, and it commonly overgrows the surface of the coral or coralline rock that it has penetrated as a thin encrusting sheet. Boring sponges are important agents in the decomposition of shell and coral.