INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION DIPLOBLASTICA, OR RADIATA

PHYLUM CTENOPHORA

Exclusively marine animals, the majority of which inhabit tropical seas. In polar regions, certain ctenophore species form large aggregations. They are generally free-swimming animals, though crawling and sessile species are also known. Their sizes range from 2–3 mm (Tinerfe cyanea) to 2.5 m (Venus's girdle, Cestus veneris). Most crawling forms are brightly colored.

Ctenophores differ from other diploblastic animals in their specific mode of locomotion using modified cilia—comb plates (ctenes). They possess a specialized aboral organ that regulates their movement. Another characteristic feature of these animals is the presence of specialized adhesive Cells (colloblasts) on their tentacles, which they use to capture and retain prey. During embryonic development, In addition to the ectoderm and endoderm, a mesodermal rudiment is laid down, from which Muscle cells are formed. The Gonads develop from the endoderm. Development is direct.

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Fig. 102. Ctenophores (from Dogiel): a - Pleurobrachia rhodopsis (order Cydippida); b - Coeloplana; c - Tjalfiella (order Platyctenida);

d - Beroe cucumis (order Beroida); e - Cestus veneris (order Cestida)

About 120 species are known; Pleurobrachia rhodopis inhabits the Black and Azov seas. Some 15 years ago, another species appeared in the Black Sea—Mnemiopsis leidyi—which became a serious competitor for all planktivorous marine inhabitants. However, an ecological catastrophe was averted due to the subsequent appearance of a third species, the active predator Beroe ovata, which preys upon mnemiopsis.

Anatomy and Morphology. The body of ctenophores is sac-like, oval, spherical, or pear-shaped, sometimes taking the form of a long ribbon (Fig. 102). The Mouth opening is located at the oral pole, and the aboral organ at the opposite, aboral pole. Eight meridional bands, or Ribs, run along the body surface (Fig. 103). They bear transverse comb plates formed by fused cilia (hence the name of these animals). The cilia are long—up to several millimeters in length. The aboral ends of the ribs taper and transition into ciliated grooves that merge in pairs (reducing to four) and enter the aboral organ, which is externally covered by a transparent conical dome made of fused cilia. This organ (Fig. 104) consists of a statolith (in the form of calcium phosphate concretions) resting on four curved elastic loops formed by interconnected flagella of sensory cells. The aforementioned four ciliated grooves lead to these loops. Together with the aboral organ and ciliated grooves, the comb rows constitute a unique locomotory system.

Fig. 103. Diagram of ctenophore anatomy:

a - general diagram (from Dogiel); b - cross section at The Stomach level (from Beklemishev):

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

Fig. 104. Structure OF THE ctenophore aboral organ (from Ospovat):

1 - ciliary dome; 2 - statolith; 3 - elastic loops; 4 - ciliated groove; 5 - dome pores

The statolith of the aboral organ oscillates continuously; its movements are transmitted via the loops and ciliated grooves to the rows of comb plates, inducing a wave of beating in them. Thus, the aboral organ not only Functions as an Organ of Equilibrium but also regulates the movement of the comb plates. The locomotory system of ctenophores functions largely independently of The Nervous system. The aboral organ is not a neural center, as was recently believed. Nervous control over locomotion is limited to a "startle" response that halts Ciliary movement. During locomotion, the comb plates strongly diffract light, causing iridescent flashes to seemingly ripple across them. Ctenophores swim with the aboral end forward.

Most species possess two long, branched tentacles capable of retracting into specialized tentacular sheaths. The tentacles bear numerous adhesive cells (colloblasts), which are unique to ctenophores. A colloblast is hemispherical and filled with vacuoles containing an adhesive secretion that binds planktonic organisms to it. It is attached to the tentacle by a spirally coiled filament that ensures prey retention (Fig. 105).

Fig. 105. Structure of a ctenophore colloblast (a) and Cytology/practical/72.html">Cross section of a tentacle (b) (from Naumov):

1 - adhesive tentacle Cell; 2 - spiral cord; 3 - straight cord; 4 - tentacle muscle cylinder

The external morphology of ctenophores combines a four- to eight-radiate Symmetry with a biradiate one (eight rows of comb plates, four ciliated grooves, and two tentacles).

The body of ctenophores consists of an outer cell layer, the epidermis, and an inner layer, the gastrodermis, separated by a well-developed mesoglea containing scattered amebocytes. The epidermis is formed by a layer of epithelial cells. Among them are cells with long cilia that form the comb plates; glandular adhesive cells concentrated on the tentacles; and pigment cells responsible for the coloration of certain ctenophores. The pharynx epidermis contains numerous glandular cells that secrete digestive Enzymes. The gastrodermis lines the gastovascular system, which performs the functions of Digestion and nutrient transport. The mesoglea is a non-cellular jelly-like substance containing isolated amoeboid and muscle cells. The mesoglea is often transparent, making the animal virtually invisible in the Water. Some deep-sea species possess a dense mesoglea.

The musculature is well developed and consists of independent muscle cells organized into circular and longitudinal fibers located in the mesoglea beneath the epidermis, as well as around the pharynx and stomach, with the tentacular musculature being the most developed.

The gastrovascular system begins with the mouth opening, which leads into a voluminous ectodermal pharynx that opens into the endodermal stomach lined with gastrodermal cells. Three tiers of gastrodermal canals extend from the stomach. One canal is directed toward the aboral pole and branches at its end into four branchlets, two of which end blindly, while two open to the outside via pores. The second tier consists of two canals near the animal's equator, which branch twice to form eight equatorial canals; the latter empty into eight meridional canals running along the body beneath the rows of comb plates and ending blindly near the poles. The third tier comprises the pharyngeal canals, which run parallel to the pharynx and end blindly near the oral pole.

Food is captured by the mouth or tentacles, or, in some creeping species, by the pharynx, which is capable of everting outwards. Unlike other animals, in ctenophores food is digested primarily in the pharynx under the action of enzymes secreted by the glandular cells of its walls. Ctenophores feed on small planktonic organisms: crustaceans, salps, small fish, and other ctenophores.

Nerve Cells are present in both the epidermis and the gastrodermis; their numerous processes interconnect with one another and with muscle processes, forming a nerve net, or diffuse plexus. Nerve cells are concentrated around the mouth, along the comb plates, and most densely at the aboral pole, where the aboral organ is located. There are also sensory cells capable of perceiving changes in illumination, chemical stimuli, and the like. Ctenophores are capable of responding to light. For instance, comb jellies (order Beroida) change their coloration from milky white to pinkish-violet in the light due to the presence of specialized cells in the epidermis known as chromatophores.

Many ctenophore species (genera *Cestus*, *Mnemopsis*, *Beroe*) are capable of fluorescence associated with the presence of specialized cells called photocytes. The light produced by a single *Beroe* comb jelly is sufficient for reading.

Reproductive System. Ctenophores are hermaphrodites; gonads develop from the endoderm and are located on either side of the meridional canals of the gastrovascular system: a sausage-shaped Ovary lies on one side of each canal, and a similar Testis lies on the other. Germ Cells are discharged through the mouth, body ruptures, or, less commonly, through specialized pores.

Reproduction. Fertilization is external. Self-fertilization is also possible, in which case the early Selection/3.html">Stages of development take place within the gastral cavity. Cleavage is total and unequal. Gastrulation occurs via epiboly. Some endodermal cells migrate into the space between the ectoderm and endoderm. This is regarded as the rudiment of the third germ layer—the mesoderm; however, in ctenophores, these cells do not form major Tissues, but only tentacular Muscles and sparse mesogleal cells (Fig. 106).

Fig. 106. Embryonic development of ctenophores (after Ivanova-Kazas, modified):

a - Cleavage of the ovum; b — gastrulation and onset of Organogenesis; c - larva:

1 - macromeres; 2 - micromeres; 3 - ectoderm; 4 - endoderm;

5 - mesodermal rudiment; 6 - gastral cavity; 7 — pharynx

Development is direct, without metamorphosis. A young individual hatches from the egg, resembling simply structured Representatives of the order Cydippida,

which is why it is referred to as the cydippid larva. Its subsequent development is accompanied by an Increasing complexity of Organization.

Some creeping forms exhibit a peculiar form of parental care: brood chambers form within the maternal Organism, where egg development takes place. Asexual reproduction has been noted only in certain creeping ctenophores (genera *Coeloplana*, *Planoctena*,

*Vallicula*), which exhibit The phenomenon of laceration.

The phylum Ctenophora is divided into two classes: Tentaculata and Atentaculata. The first class includes the majority of ctenophore species, among which are free-swimming forms (the ribbon-like Venus's girdle, lobate ctenophores which have a delicate consistency yet are voracious predators swallowing even small fish), creeping forms (such as representatives of the order Platyctenida), and even a single species, *Tjalfiella tristoma*, which has adapted to a sessile lifestyle (Fig. 102). The second class comprises a small group of planktonic species found in all seas, sometimes in great Abundance. The best-known representatives belong to the order Beroida (comb jellies). The greater part of their body volume is occupied by a large bell-shaped pharynx leading into a massive mouth opening. Its inner margin bears ciliated structures that trap prey (salps, jellyfish, other ctenophores). The largest in size are Arctic and Antarctic species. Comb jellies serve as a food source for commercial fish species (haddock, cod).