ZOOLOGY OF INVERTEBRATES IN THREE BOOKS - BOOK 1 - H.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE METAZOANS (EUMETAZOA)

PHYLUM COMB JELLIES (CTENOPHORA)

CLASS COMB JELLIES (CTENOPHORA)

The body of ctenophores is sac-like, oval, rounded, or pear-shaped. At one of its poles — the oral pole — lies the Mouth opening, and at the opposite pole — the aboral pole — is the aboral organ. The main axis of the body passes through both poles (Fig. 122).

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Fig. 122. Diagram of The Structure of a ctenophore — General Overview (a), cross-section at The Stomach level (b): 1 — aboral organ; 2 — aboral canal (acrogaster); 3 — tentacle; 4 — equatorial canals; 5 — tentacle sheath; 6 — meridional canals; 7 — Pharynx; 8 — mouth; 9 — pharyngeal canal; 10 — comb plates; 11 — stomach

On the body surface of ctenophores, in a meridional direction (from the oral to the aboral pole), there are eight ridges, or Ribs (hence the Ukrainian name of the phylum and class), bearing transverse comb plates formed by fused cilia. These are the longest cilia in the animal kingdom (several millimeters in length), which, moreover, possess three microtubules in their center instead of two. When beating, the comb plates disperse light, creating a shimmering rainbow effect as the animal moves. The comb plates serve as the primary Organs of locomotion; their action propels the animal forward with the oral pole leading.

Most ctenophores have two tentacles, sometimes significantly longer than the body, which can be retracted into specialized tentacle sheaths. One of the surfaces of the tentacles is branched and covered with specialized adhesive Cells found exclusively in ctenophores (Fig. 123).

Fig. 123. Structure of ctenophore adhesive cells:

1 — hemispheres containing droplets of sticky secretion; 2 — spiral filament; 3 — muscular strand of the tentacle

Each Cell is hemispherical in shape and connected to the tentacle by an elastic straight strand (a modified Nucleus) and a strand spirally wound around the straight one (a modified flagellum). The Cell produces an adhesive substance that traps planktonic organisms, allowing the tentacle to pull the prey toward the mouth. If the prey attempts to escape, the strands cushion the shocks experienced by the ctenophores.

The body of ctenophores, much like that of Cnidarians, consists of an epidermis (ectoderm) and a gastrodermis (endoderm) separated by a well-developed mesoglea. The mesoglea is often crystal clear, making the animal virtually invisible in the Water. Such species are difficult to collect because their delicate bodies are easily damaged. However, certain deep-sea ctenophores possess a dense mesoglea. Most creeping forms are brightly colored.

The Muscular System of ctenophores is well developed, especially in creeping forms. The musculature consists of independent Muscle cells, similar to those in stony and soft corals. It is located within the thickness of the mesoglea and comprises longitudinal and circular fibers beneath the epidermis, similar fibers surrounding the pharynx and stomach, as well as radial fibers extending from the stomach to the body wall. The musculature is most highly developed in the tentacles. Muscles facilitate the capture of food by the mouth or tentacles and perform a protective function — upon contraction, they cover the aboral organ. In some ctenophores, the musculature also serves a locomotory function, particularly in species that lead a creeping lifestyle or swim via serpentine movements of their ribbon-like body or by means of oral lobes.

The Digestive System begins with a mobile mouth opening that leads into a voluminous ectodermal pharynx, where food is primarily digested through Enzymes secreted by its walls, unlike in most other animals. In some ctenophores, the pharynx can be everted outward.

It opens into an endodermal stomach, from which three tiers of channels belonging to the gastrovascular system branch out. The first tier (acrogaster) extends toward the aboral pole as a single canal that branches into four short branches. Two of these end blindly, while the other two open to the exterior via pores. The second tier consists of two canals located near the equator, perpendicular to the axis of the stomach. They branch twice to form eight equatorial canals that empty into eight meridional canals. The meridional canals lie beneath the comb plates. The third tier is formed by two pharyngeal canals that extend from the stomach along the pharynx toward the oral pole. Depending on body shape and The Development of additional canals, branches, and anastomoses, the STRUCTURE OF THE gastrovascular system can vary significantly from the description above.

The pharynx in ctenophores is flattened into a tube-like shape, and the plane in which it is flattened is called the pharyngeal plane. The stomach is also flattened, but in a plane perpendicular to the pharyngeal plane; the two tentacles (if present) are situated in this same plane, which is therefore called the tentacular plane.

Two mutually perpendicular planes — the pharyngeal and tentacular planes — can be passed through the body of a ctenophore, meaning ctenophores exhibit biradial Symmetry. However, most organs (rows of comb plates, radial canals, Gonads) are arranged around the main axis, with ctenophores possessing eight of each such organ. Thus, the overall structural plan of ctenophores is characterized by a combination of Two Types of symmetry: biradial and octoradial.

Ctenophores feed on various small planktonic organisms, particularly small crustaceans, and prey on salps, small fish, and other ctenophores. Some species are commensals (from French *commensal* — table companion), meaning animals that live off others without harming them. They inhabit The surface of sea stars, corals, and Sponges.

The Nervous system resembles a nerve net, with small clusters of Nerve Cells around the mouth and along the comb plates. Nerve cells are most abundant at the aboral pole, which houses a specialized sensory structure known as the aboral organ (Fig. 124).

Fig. 124 Structure of the ctenophore aboral organ:

1 — statolith; 2 — elastic loops; 3 — ciliated grooves; 4 — epidermal cushion; 5 — ciliary cap

It consists of a statolith—a small sphere of calcareous grains positioned on four curved, elastic arches formed by fused flagella. The cells to which they belong perceive Changes in the statolith's pressure when the ctenophore's body orientation shifts. Above the statolith is a specialized dome made of fused cilia. Four ciliated grooves extend from the arches, branching out and reaching the ends of the comb rows. A Condensation of the nerve plexus lies beneath the aboral organ. The aboral organ Functions as an equilibrium receptor and regulates the beating of the comb rows. Ctenophores are responsive to light. For instance, the coloration of sea gooseberries (order Beroida) shifts from milk-white to pink-violet upon exposure to light due to the action of specialized chromatophore cells in the epidermis.

Many ctenophores are capable of Bioluminescence, which is associated with specialized cells called photocytes located in the gastrodermis of the meridional canals. Among them, the most notable are Venus' girdle (Cestus veneris), Mnemiopsis, Beroe, and Pleurobrachia. Beroe exhibits the strongest luminescence—the light emitted by a single specimen is sufficient for reading. The luminescence of ctenophores, like that of other animals, results from the interaction of the substrate luciferin with the enzyme luciferase, which emits light. Ctenophore luminescence is coordinated with an escape-and-defense response. Strong mechanical stimulation triggers a cessation of ciliary beating, a sharp body contraction, and a brilliant flash of light.

Ctenophores are hermaphrodites. Gonads develop from the endoderm along the sides of the meridional canals of the gastrovascular system, with a sausage-shaped Ovary lying on one side of the canal and an identical Testis on the other. Germ Cells are released into the water through the mouth or via body ruptures, and more rarely through specialized ducts. Self-Fertilization is possible, in which case the early Selection/3.html">Stages of development occur within the gastric cavity.

The life cycle of ctenophores is simple, without metamorphosis. The fertilized egg undergoes total but unequal Cleavage, resulting in smaller cells (micromeres) forming at one pole of the embryo (the animal pole) and larger cells (macromeres) at the other (the vegetal pole). The ectoderm develops from the micromeres, while the endoderm forms from the macromeres. Gastrulation proceeds via epiboly, with micromeres overgrowing the macromeres. Some endodermal cells migrate into the space between the ectoderm and endoderm. Scientists regard these cells as a third germ layer—the mesoderm; however, in ctenophores, it does not give rise to major Tissues, but rather forms the Muscles of the tentacles and sparsely scattered cells within the mesoglea. A young individual hatches from the egg, resembling structurally the simplest ctenophores of the order Cydippida, and is therefore termed a cydippid larva. Subsequently, its anatomy grows more complex, reaching the adult level of Organization (Fig. 125).

Fig. 125. Embryonic development of ctenophores: egg cleavage (a), gastrulation and early Organogenesis (b), larva (c): 1 — macromeres; 2 — micromeres; 3 — ectoderm; 4 — endoderm; 5 — mesodermal rudiment; 6 — gastric cavity; 7 — pharynx

In certain pelagic species, The phenomenon of disogony has been described—a unique form of reproduction at the larval stage (neoteny) where, upon hatching, the larva begins to produce small eggs that develop into similar larvae, which then grow and transform into normal-sized adults. Some benthic forms exhibit parental care. Brood chambers develop within the maternal body, where the eggs mature.

Asexual reproduction is absent in most ctenophores. Notable exceptions include certain benthic forms (Coeloplana, Planoctena, Vallicula) that reproduce via laceration, much like cnidarians.

The modern Classification of ctenophores is based on the structure of the gastric canals at the oral pole of the body. This system is structured as follows: