ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 7. SUBKINGDOM MULTICELLULAR ANIMALS (Metazoa). PHYLUM CNIDARIA (or Coelenterata)

3. Characteristics of the Main Classes of Coelenterates

The Class Hydrozoa includes about 4,000 species that lead sessile or free-swimming lifestyles and range in size from a few millimeters to 1 m. A typical representative of this class is the freshwater hydra. This diploblastic aquatic animal exhibits radial Symmetry. Its body is sac-like, elongated, and up to 1.5 cm long (Fig. 20).

Fig. 20. Freshwater hydra (after Dogiel, 1981).

General view (a) and internal Structure (b):

I — longitudinal section; II — cross section; III — part of the cross section under high magnification; IV — epitheliomuscular Cell; V — tip of a tentacle with discharged stinging Cells; VI — VII — stinging cells;

1 — tentacles; 2 — endoderm; 3 — ectoderm; 4 — budding young hydra; 5 — female gonad — Ovary and egg;

6 — basal disc with aboral pore; 7 — mesoderm; 8 — gastric cavity; 9 — spermatozoa; 10 — male Gonads — Testes;

11 — Mouth.

The lower end of the body is the basal disc (sole), which contains gland cells in the ectoderm that secrete an adhesive substance. In addition, there is an aboral pore in the basal disc. This acts as a kind of sucker that, together with the adhesive substance, firmly attaches the basal disc to the substrate (stones, plants).

At the opposite end of the hydra's body is the mouth, surrounded by tentacles (from 5 to 12) with stinging cells. The mouth leads into the coelenteron, or gastric cavity (from the Greek gastros — Stomach).

The outer layer of the hydra's body, the ectoderm, consists of Different types of cells:

✵ epitheliomuscular cells, the outer part of which forms the integument, while the inner part is elongated into a contractile Muscle fiber; these fibers enable tentacle movement and body contraction, allowing the hydra to move by slowly gliding on its basal disc or "walking", alternately stepping on its basal disc and tentacles;

✵ between the epitheliomuscular cells, especially on the tentacles and around the mouth, there are stinging cells, or cnidocytes, which serve for defense and attack; each cell contains a capsule filled with a toxic substance; next to it is a spirally coiled thread with a sensory trigger — a cnidocil; when the cnidocil is stimulated, the capsule contracts, discharging a stinging thread covered with spines that penetrates the victim's body, paralyzing it with venom;

✵ beneath the epithelium are star-shaped Nerve Cells interconnected by their processes; this forms a primitive diffuse Nervous system that coordinates the animal's reflex activities;

✵ interstitial cells are small, unspecialized cells responsible for replacing lost cells (regeneration). Hydras possess an extraordinary capacity to regenerate lost body parts. Even 1/200th of the animal's body can give rise to a new Organism.

Beneath the ectoderm lies the non-cellular mesoglea.

The inner cell layer, the endoderm, consists of:

✵ interstitial cells, which are responsible for replacing lost cells (regeneration);

✵ digestive cells, which have: a) muscle processes that enable the hydra's movements; b) flagella that facilitate the movement of food within the gastric cavity; digestive cells are capable of forming pseudopodia to engulf small food particles, which are then digested inside The Cell within digestive vacuoles (intracellular Digestion);

✵ gland cells that secrete digestive Enzymes into the gastric cavity, where digestion partially takes place (extracellular digestion); thus, the freshwater hydra combines both intracellular and extracellular digestion.

The hydra is a predator, feeding on small aquatic animals (planktonic Protozoans, small crustaceans, rotifers, worms, and fish fry), which it catches with its long tentacles. Metabolic waste products are osmotically released into the gastric cavity, from which, along with undigested food remains, they are expelled through the mouth.

The hydra breathes oxygen dissolved in Water, absorbing it through its entire body surface.

Hydras reproduce both asexually and sexually. Under favorable conditions (in summer), asexual reproduction—budding—prevails: 1–3 buds form on the hydra's body, which eventually detach from the parent organism. In autumn, hydras reproduce sexually. Reproductive glands, or gonads (from Greek gonao meaning "to give birth"), develop in the ectoderm of the hydra's body: male gonads are testes, in which spermatozoa mature, and female gonads are Ovaries, in which egg cells mature. Consequently, these animals are bisexual (hermaphrodites). Fertilization is cross-fertilization, effected by foreign spermatozoa swimming in the water. The fertilized egg—the zygote—develops into a two-layered larva with the rudiments of an intestinal cavity, known as a planula, which moves about in the water and overwinters at the bottom of the water body. The adult hydra dies in autumn. In spring, a new polyp develops from the fertilized egg.

The most common species of hydra found in water bodies include the stalked, common, slender, and green hydras.

In addition to the hydra, hydrozoans include small hydroid medusae (such as Gonionemus) and marine colonial polyps (the Portuguese man-of-war, or Physalia).

The Class Scyphozoa, or scyphozoan medusae (Scyphozoa), comprises 200 species that live exclusively in marine environments. Medusae have a more complex structure than polyps, an ADAPTATION TO A free-swimming, active lifestyle (Fig. 21). Their body is transparent, bell-shaped (ranging from a few centimeters to 2 m in diameter), with tentacles along the margins.

The walls of the medusa's body are formed of two cell layers. This resembles the hydra, with the difference that in the hydra both layers are separated by a very thin layer of mesoglea, whereas in the medusa the mesoglea constitutes a thick, gelatinous layer that forms the bulk of its body. This swollen, gelatinous layer contains so much water and so little solid matter that if a medusa is removed from the water, it dries into a thin, dry film upon evaporation. Scyphozoan medusae are generally much larger than hydrozoans.

Fig. 21. Structure of a scyphozoan medusa (after Dogiel, 1981):

1 — oral lobes; 2 — mouth opening; 3, 4 — mesoglea; 5 — stomach;

6 — gonad; 7 — radial canal; 8 — circular canal; 9 — rhopalium; 10 — tentacles.

Tentacles are located along the margin of the bell (for instance, in the Arctic jellyfish, the tentacles can reach up to 30 m in length). The mouth is situated on the underside of the bell, surrounded by oral lobes densely covered with stinging cells (nematocysts). The mouth leads into The Stomach, from which a system of radial canals radiates toward the bell's margins, connecting with a circular canal running along the bell's edge. The stomach, together with the canals, forms the gastrovascular system characteristic of medusae (from Greek gastros — stomach, and Latin vasculum — small vessel), which performs the Functions of both the digestive and circulatory systems. All medusae are carnivorous, feeding on planktonic organisms, including fish fry. However, deep-sea species also feed on detritus and dead organisms.

The Nervous System of medusae is characterized by The formation of clusters of nerve cells—ganglia (from Greek ganglion — a knot)—located near the Sense Organs. The sense organs are situated along the bell margin within rhopalia (from Greek rhopalon — a club)—shortened, modified tentacles arranged symmetrically (usually 8 in number). Each rhopalium bears one statocyst (from Greek statos — standing, kystis — a box), which serves as an Organ of Equilibrium, alongside several ocelli.

Medusae are characterized by so-called jet propulsion. The contraction of muscle fibers causes the edges of the bell to contract. Water is forced out from beneath it, propelling the medusa in the opposite direction.

During reproduction, medusae exhibit Morphology/12.html">ALTERNATION OF GENERATIONS: an asexual phase (polyp) and a sexual phase (medusa) (Fig. 22). The sexual generation is represented

by the medusa, in the ectoderm of which reproductive glands—gonads—are located. A free-swimming planula larva develops from the fertilized egg and eventually attaches to a substrate, transforming into a polyp that zoologists initially classified as a separate species and described under the name Hydra tuba (meaning "tube-hydra"). Today, this polypoid phase is called the scyphistoma (from Greek skyphos — a cup, stoma — mouth). Reproducing via transverse fission (strobilation), the scyphistoma gives rise to disk-shaped larvae known as ephyrae, which subsequently develop into adult medusae.

Fig. 22. Development of scyphomedusae (after Dogiel, 1981):

1 — egg; 2 — planula; 3–4 — development of scyphistoma; 5 — strobilation of scyphistoma; 6 — free-swimming ephyra; 7 — adult medusa.

The Black and Azov seas are inhabited by the following common species:

✵ Aurelia, or the moon jellyfish — 10–20 (up to 40) cm in diameter;

✵ Rhizostoma, or the ROOT-mouth jellyfish — larger than Aurelia, weighing 3–4 kg; tentacles are absent, replaced instead by large oral lobes densely studded with stinging cells.

The Class Anthozoa (corals and sea anemones) comprises 6,000 species of marine Coelenterates. They inhabit shallow tropical waters, with only a few species found in cold waters.

This class includes animals exclusively in the polyp form. There are two subclasses of anthozoans: octocorals (with tentacle counts in multiples of 8) and hexacorals (with tentacle counts in multiples of 6). The mouth leads into an ectodermal Pharynx lined with ciliated epithelium. The cilia are in constant motion, driving water into the intestinal cavity, which is divided by septa; octocorals possess eight such partitions, while hexacorals have six.

Anthozoans form large colonies with a shared calcareous Skeleton. Solitary forms, such as sea anemones (actinians), also exist among anthozoans and lack a skeleton entirely. In colonial forms, the lower end of the body is attached to the colony, whereas in solitary forms, attachment to the substrate occurs via a pedal disc.

Anthozoans feed on plankton and organic debris suspended in seawater. Sea anemones can also capture small fish and crustaceans as prey.

Anthozoans reproduce via budding or through sexual reproduction. A planula larva (from Latin planus — flat) develops from the fertilized egg, which eventually attaches to a substrate and transforms into a polyp. The life cycle of all anthozoans completely lacks a medusa stage.

Reef-building stony corals (madreporarian corals) are widespread in the tropical and subtropical Regions of the Atlantic, Indian, and Pacific Oceans. These are colonial forms that reproduce by budding, growing into massive structures and forming coral reefs. Reef-building corals grow only at shallow depths; therefore, the typical habitat for reef development is shallow waters near islands, where fringing reefs are formed. However, over hundreds of thousands or millions of years, the ocean floor along with the islands may subside, while the water level rises. In such cases, the corals continue building their colonies upward toward the surface, transforming the fringing reef into a barrier reef. If the island eventually submerges completely beneath the water, an atoll—a ring-shaped reef—is formed.