INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 1 - H.I. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA)

PHYLUM SPONGES, OR PORIFERA (SPORGIA ABO PORIFERA)

CLASS GLASS SPONGES, OR HEXACTINELLIDS (HYALOSPONGIAE, OR HEXACTINELLIDA)

These are exclusively marine, predominantly deep-Water, fairly large (up to 1 m) Sponges with a siliceous Skeleton, usually solitary and rarely colonial. Their bodies are mostly goblet-shaped, tubular, or sac-like. The body is soft and tears quite easily, though it becomes hard and fragile when the skeleton is strongly developed. Sponges may be gray, white, yellowish, or brown. The mesohyl is almost entirely reduced, and all living Cells are essentially represented by a syncytium that forms the dermal membrane, the walls of the flagellated chambers, and a mobile network of strands and septa interconnecting all PARTS OF THE body. Choanocytes are absent in Glass sponges. Instead, they contain a continuous choanosyncytium from which individual collar complexes extend, each consisting of a flagellum and a surrounding basket of microvilli. Numerous lacunae (cavities) through which water circulates are present between the strands and septa. The irrigation system belongs to the syconoid type. The archaeocytes of glass sponges lack amoeboid activity and do not contain digestive vacuoles (phagosomes), which are found in the Cytoplasm of the choanosyncytium. Myocytes or any contractile elements are absent (Fig. 80).

The skeleton of glass sponges consists of various primary hexactinal (triaxial) spicules and their derivatives. Often, some rays are reduced, resulting in pentact, tetract, triact, or even monact spicules (Fig. 81). Spicules vary in length: large ones, or macroscleres (ranging from hundreds of micrometers to several tens of centimeters in size), and small ones, or microscleres (ranging from 10 to 100 µm). Microscleres appear as hexactinal stars or amphidiscs of various shapes. The skeleton of glass sponges is differentiated, with Different types of spicules occupying specific locations within the Organism. In most modern species, the spicules are fused to form a continuous supporting skeleton, although fossil sponges (from the Cambrian and later periods) with unfused individual spicules are also known.

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Fig. 80 Diagram of the body wall Structure of a glass sponge — cross-section through the body wall (a), walls of a flagellated chamber (b), spicules (c): 1 — dermal layer; 2 — skeleton; 4 — syncytial septa; 5 — flagellated chambers; 6 — collar complex

The uniqueness of glass sponges lies in their syncytial Organization. However, this is undoubtedly a secondary feature, since during embryonic development the egg of glass sponges undergoes complete Cleavage, and the individuality of separate cells is clearly expressed in the early Stages of Ontogeny.

The class Glass Sponges comprises two subclasses. The subclass Hexasterophora includes such well-known species as Venus' flower basket (Euplectella aspergillum), which has a cylindrical body, and the sac-like sponge Rossella. Representatives of this subclass possess hexactinal microscleres. The large spicules of these sponges often fuse together to form a lattice-like skeleton. In members of the second subclass, Amphidiscophora, the microscleres take the form of amphidiscs. A typical representative of this subclass is the glass-rope sponge (Hyalonema elegans). It possesses a goblet- or egg-shaped body supported by a stalk made of a bundle of long spicules with which the sponge anchors itself in the sediment (Fig. 81). Another species of remarkable structure is the giant glass-rope sponge (Monoraphis chuni), which features a perforated cylindrical body about a meter long, pierced by a thin spicule reaching up to three meters in length and 8.5 mm in thickness.