INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM PROTOZOA-LIKE MULTICELLULAR ANIMALS (PROMETAZOA)

PHYLUM SPONGES (PORIFERA)

Aquatic, predominantly marine, solitary or colonial organisms that lead a sessile lifestyle and feed by filter feeding. Over 5,000 extant species are known (39 species in the freshwaters and seas of Ukraine).

The body of Sponges consists of various types of Cells and intercellular matrix, which is the product of their secretion. Nerve and specialized Muscle cells are absent. Sponges possess a Skeleton. Their body is usually colored by specific pigments in shades of yellow, brown, red, green, or violet. Their lifespan ranges from a few days to many years.

For a long time, sponges were considered organisms of uncertain status and were classified among zoophytes; their animal nature was recognized only in 1825.

Morphology. Solitary sponges are predominantly goblet-shaped, while colonies resemble bushes, crusts, spheres, or chalices. In a solitary goblet-shaped sponge, one distinguishes a base (or holdfast) by which it attaches to the substrate, and an osculum at the opposite end of the body.

The entire diversity of sponge Organization reduces to three main types: ascon, sycon, and leucon (Fig. 56). In asconoid sponges, the body wall consists of two Cell layers: the pinacoderm, which covers the exterior, and the choanoderm, which lines the paragastric cavity (Fig. 57). The mesohyl lies between these layers, containing various cells and their secretory products, as well as skeletal elements. The thickness of the body wall is pierced by channels connecting the paragastric cavity with the external environment. On the body surface, they open via pores (hence the Latin name of the phylum: porus - pore, ferre - to bear). Water enters the paragastric cavity through the pores and exits outward via the osculum.

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Fig. 56. Sponge body wall of various structural types (from Hausmann): a - ascon; b - simple sycon; c - complex sycon; d - leucon

Fig. 57. Structure of an ascon-type sponge (from Hadorn):

1 - osculum; 2 - spicules; 3 - choanocytes; 4 - pore; 5 - amoebocyte; 6 - porocytes;

7 - pinacocytes; 8 - collencytes; 9 - paragastric cavity

In syconoid and leuconoid sponges, the paragastric cavity is lined with pinacoderm, just like the outer surface, whereas the choanoderm is restricted to flagellated canals (sycon) or flagellated chambers (leucon).

In syconoid sponges, the mesohyl expands, and Regions of the paragastric cavity invaginate into it, forming radial canals whose walls are lined with the choanoderm.

This structural type is found in many solitary sponges. The leucon type is characteristic of the majority of sponges: the body walls thicken even further, and the choanoderm concentrates in flagellated chambers located within the mesohyl. This creates a sophisticated irrigation system that directs and regulates the flow of water containing food particles. The number of flagellated chambers can be substantial. For instance, in a small sponge Leuconia aspera (up to 7 cm high and 1 cm thick), their number exceeds 2 million.

Cell types (Fig. 58). The pinacoderm comprises pinacocytes and porocytes. Pinacocytes are flattened or T-shaped, while porocytes are pierced by a pore canal and are capable of contracting to open and close the pore. The choanoderm is formed by collar cells, or choanocytes. These are cylindrical cells bearing a single flagellum, around the base of which about 20 microvilli extend from The Cell surface; under a Light Microscope, the ring of microvilli resembles a collar. The mesohyl consists of an intercellular matrix containing various cell types. The most common of these are amoebocytes (archaeocytes), which move via pseudopodia and feed by phagocytosis. They engulf old or dead mesohyl cells and distribute nutrients throughout the sponge body. Myocytes are specialized cells located around the osculum and large canals, possessing weak contractile abilities. Collencytes and lophocytes are cells that produce fibers of the supporting protein, Collagen. Spongocytes are specialized cells that produce the skeletal protein spongin, a substance chemically similar to silk. Sclerocytes are cells inside which the elements of the mineral skeleton—spicules—are formed. Almost all cell types, and amoebocytes to the greatest extent, are capable of transforming into other cell types.

Fig. 58. Sponge cells (from Malakhov): a - pinacocyte; b - porocyte; c - general view of a choanocyte; d - its internal structure;

e - myocytes; f - collencyt; g - amoebocyte; h - sclerocyte; i - lophocyte: 1 - Nucleus; 2 - digestive vacuole; 3 - canal in porocyte; 4 - flagellum;

5 - collar microvillus; 6 - spicule

Skeleton. The vast majority of sponges possess a skeleton that performs supporting and protective Functions. It can be mineral—calcareous (made of calcium carbonate) or siliceous (made of silicon dioxide)—or organic, consisting of collagen fibers and spongin. The composition and STRUCTURE OF THE Skeleton form The basis of sponge Classification. The mineral skeleton consists of numerous needles, or spicules, which vary in shape and are arranged differently within the sponge body. The skeleton is usually located in the mesohyl, though spicules sometimes protrude outward as a defensive adaptation. Spicules can be mono-, tri-, or polyaxial, or possess a more complex structure, such as stars, hooks, anchors, etc. (Fig. 59). Occasionally, spicules fuse into a continuous skeletal framework, which is most characteristic of Glass sponges. In some sponges, particularly archaeocyaths (most of which are fossilized), the skeleton is monolithic.

Fig. 59. Skeletal elements of sponges (after Malakhop): a - spicules in a spongin fiber; b - spicules connected by spongin;

c - types of spicules; d - spongin skeleton of a bath sponge

Feeding is passive, occurring through filtration. The beating of choanocyte flagella creates a continuous current of water flowing through pores and canaliculi into the paragastric cavity and further out through the osculum. Along with water, food particles enter the sponge's body and are phagocytosed by choanocytes. Food is digested within the choanocytes and amoebocytes, to which the food is transferred by choanocytes. Other cell types are also capable of phagocytosis: pinacocytes, sclerocytes, and lophocytes can capture food particles from the mesohyl matrix and digest them. Amoebocytes migrate through the mesohyl, performing the function of nutrient distribution. Undigested residues are passed on to pinacocytes, which expel them into the excurrent canals or to the exterior.

Sponges respire by utilizing dissolved oxygen in the water, which diffuses into the cells thanks to the continuous water current flowing through their bodies. Metabolic waste products are excreted either directly into the water or into the mesohyl matrix, from which they also diffuse into the water and are subsequently expelled through the osculum.

Irritability in sponges is limited. They are incapable of active movement or changing their body shape. Only the pores and the osculum can very slowly alter their diameter through the contractions of myocytes and porocytes. However, the immobility of sponges is compensated for by the water current flowing through their bodies.

Reproduction. Sponges reproduce both sexually and asexually. Asexual reproduction occurs via external or internal budding, longitudinal fission, and fragmentation. In external budding (Fig. 60), a bud forms on the parent Organism's body, into which all body layers grow, accompanied by an extension of the paragastric cavity. In other cases, the bud arises from an aggregation of amoebocytes. In solitary species (genera Ascetta, Sycon), the young individual detaches from the parent, whereas in colonial species, it remains attached to the colony, and the boundaries between individual specimens may disappear, causing the colonies to assume A wide variety of shapes (Fig. 61). Only the number of oscula indicates the number of individuals comprising the colony. Occasionally, due to the suppression of individuality in separate specimens, colonies begin to resemble solitary sponges. Such colonies acquire a goblet or cup-like shape (Fig. 61, c). In this case, the oscula are arranged regularly or even fuse into a single opening.

Fig. 60. External budding in sponges (after Koltun):

a - Anoxycalus ijimae; b - Polymastia mammillaris; c - Tethya aurantium

Fig. 61. Colonial and secondarily solitary sponges (after Koltun, modified):

a - Mycale ochotensis; b - Sycon ciliatum; c - Phakellia eribrosa

Internal budding is particularly characteristic of freshwater sponges (spongillids). A group of archaeocytes rich in nutrients detaches and becomes enclosed in a tough coat consisting of two spongin layers with a foamy air layer sandwiched between them. In some species, special skeletal elements—amphidiscs—are present, which prevent the collapse of the spongin layers, while the air layer provides protection against low temperatures. This is how the overwintering bud, or gemmule, is formed (Fig. 62). In autumn, when the spongillid colonies die off, the gemmules overwinter, and in spring, archaeocytes emerge from them to form a new colony.

Fig. 62. Gemmule of a freshwater sponge (after Koltun): a - general appearance; b - structural diagram:

1 - amphidiscs; 2 - foamy layer; 3 - amoebocytes

Some marine sponges exhibit reproduction via fragmentation: their body breaks apart into several pieces, each of which gives rise to a new organism. Sponges possess a remarkable capacity for regeneration—the restoration of body parts lost due to damage.

The vast majority of sponge species also reproduce sexually. Sponges can be either dioecious or hermaphroditic. Gametes arise anywhere within the mesohyl via Meiosis from amoebocytes or choanocytes. Spermatozoa are released into the water and penetrate through the pores into the mesohyl of another individual, where they fertilize the eggs that remain in place. Embryonic development takes place within the maternal organism.

The Development of sponges is accompanied by metamorphosis. In calcareous sponges, total equal Cleavage results in The formation of a flagellated blastula, which leaves the maternal organism and becomes a free-swimming larva. Subsequently, through immigration, its interior cavity becomes filled with amoeboid cells; such a larva is called a parenchymula (Fig. 63, a). In glass sponges and demosponges, Cleavage of the zygote is unequal, resulting in a single-layered larva known as an amphiblastula, in which one hemisphere consists of small flagellated cells and the other of large, nutrient-rich, non-flagellated cells (Fig. 63, b). The larva swims in the water Column, then settles on a substrate and transforms into an adult individual that initiates a colony.

a b

Fig. 63. Sponge larvae:

a - parenchymula; b - amphiblastula (after Koltun):

1 - flagellated cells; 2 - amoeboid cells; 3 - large non-flagellated cells; 4 - small flagellated cells

The Classification of Sponges is based on the chemical Composition and Structure of their skeleton. The phylum is divided into four classes.