Invertebrate Zoology: Study Guide - T. A. Dauda 2014

Metazoa
Sponges

General characteristics

The phylum Porifera encompasses about 5,000 species of lower multicellular aquatic animals, inhabiting aquatic environments primarily within the tropical and subtropical Zones of the World Ocean. Due to their colonization of diverse habitats (ranging from ocean depths of 50 m to 11 km, intertidal zones, to muddy substrates of freshwater bodies, etc.), the external appearance of Sponges varies greatly (Fig. 10), and their body length ranges from a few millimeters to 1.5 meters. Almost all of them are colonial, or more rarely solitary, sessile polyps.

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Fig. 10 Sponges:

1 — Leucosolenia; 2 — Sycon; 3 — freshwater sponge (Spongilla); 4 — bath sponge; 5 — Aplysina colony.

Fig. 11 Various structural types of sponges (arrows indicate the direction of Water currents):

A — asconoid type; B — syconoid type; C — leuconoid type; 1 — osculum (outlet aperture); 2 — pores; 3 — flagellated chambers; 4 — incurrent canals; 5 — excurrent canals; 6 — choanocytes (collar Cells); 7 — spongocoel (central cavity).

The body of sponges (Fig. 11) is pierced by numerous pores and formed by two Cell layers: the outer ectoderm and the inner endoderm. Between them lies the mesohyl, a layer of a specialized amorphous substance containing scattered individual cells. The ectoderm consists of flat surface cells capable of slight contraction. The endoderm is represented by flagellated collar cells (choanocytes) that capture food particles and digest them, while the beating of their flagella creates a water current. Various groups of cells reside in the mesohyl: amoebocytes involved in Digestion, reproductive cells, and skeletal-forming cells. The Skeleton is built either of an organic substance, spongin, or of monaxon, triaxon, or tetraxon calcareous and siliceous spicules located in the mesohyl. The mesohyl mechanically and chemically connects all cell types.

The internal Organization of sponges features 3 main structural types — ascon, sycon, and leucon (Fig. 11). Sponges of the ascon type have a radially symmetrical, vase-like body. A thin wall encloses the central cavity. Its outer surface is lined with flat surface cells, and the inner surface with flagellated cells. The wall is pierced by numerous pores. The continuous beating of flagella drives water from the central cavity out through the osculum, drawing fresh portions of water into the sponge's body through the pores. The ascon type is found in a small number of calcareous sponges or in the juvenile stages of other species whose Structure becomes more complex upon reaching adulthood.

The increased complexity in The structure of sycon-type sponges is manifested in the thickening of the body walls, which form numerous cylindrical invaginations of the central cavity. In this arrangement, flagellated cells are located exclusively within these pockets, forming flagellated chambers.

Water enters the flagellated chambers through pores and canals, then passes into the central cavity, and from there exits through the osculum. This structural type is found in small calcareous sponges of the genus Sycon and certain others. Most sponges exhibit a more complex organization. Elongated flagellated chambers are subdivided into smaller spherical ones, arranged in multiple layers within a significantly thickened body wall. All flagellated chambers are connected to the external environment by a complex system of incurrent canals, and to the central cavity by a system of excurrent canals.

Sponges reproduce in two ways: asexually and sexually. Asexual reproduction takes the form of budding. A bulge forms On the surface of the sponge, which grows until an osculum ruptures at its free end; subsequently, the bud either detaches from the parent body to lead an independent lifestyle or remains attached to the parent Organism, thereby forming a colony. In the freshwater sponge Spongilla, internal budding occurs. By autumn, spherical cell aggregates known as gemmules form within the sponge's body. In winter, the adult sponges die off, while the gemmules drop to the bottom and overwinter. In spring, new sponges develop from them.

Sexual reproduction in sponges involves The formation of male (spermatozoa) and female (ova) Gametes from amoeboid cells, their fusion, and The Development of a zygote, which forms a larva. The larva swims freely for a time before attaching to a substrate and gradually metamorphosing into a sponge. Among sponges, there are both dioecious and hermaphroditic forms—i.e., capable of producing both male and female reproductive products.

Sponges possess a highly developed capacity for regeneration—the restoration of the body from its fragments. If a sponge is macerated or even pressed through a sieve, the resulting slurry, consisting of dissociated individual cells or cell clusters, proves capable of regenerating an entire organism.

In nature, sponges play a vital role as biofilters. Settling in water bodies with significant organic pollution, they participate in biological water purification. Sponges have few predators and are inedible, as most of them emit an unpleasant odor caused by the secretion of toxic substances.

The practical value of sponges is relatively minor. Toxins are extracted from certain marine forms. Sponges known as bath sponges, after drying, are used for body washing. The freshwater sponge Spongilla, which forms growths on submerged objects, is used in folk medicine as a topical anti-rheumatic remedy.

The Classification of the phylum Porifera is based on The chemical composition of the skeleton. There are 3 classes: Calcarea (Calcareous sponges), Hexactinellida (Glass sponges), and Demospongiae (Demosponges).