INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - G.I. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION EUMETAZOA

PHYLUM CNIDARIA (OR COELENTERATA)

CLASS SCYPHOZOA

This Class comprises marine Cnidarians whose life cycle is predominantly spent in the medusa stage. The polypoid generation is short-lived and does not form permanent colonies. The class includes about 200 species; only three species occur in the Black Sea, and one of these is found in the Sea of Azov.

Scyphozoan medusae differ from hydromedusae by their significantly larger size, absence of a velum, and a more complex gastrovascular system. In scyphozoans, the Germ Cells develop from the endoderm.

The body of a scyphozoan medusa is shaped more or less like a high bell or umbrella. In the center of the concave side of the bell (the subumbrella) lies a quadrangular Mouth opening. The margins of the mouth are extended into four trough-like lobes that serve for capturing prey (small crustaceans, larvae of various animals, and even small fish). In some medusae, known as rootmouths (order Rhizostomea), the oral lobes proliferate and form folds, while the mouth opening becomes overgrown, its function being assumed by numerous small pores within the folds of the oral lobes. Such medusae have shifted to feeding on microscopic planktonic organisms, drawing them into The Stomach along with Water. The margin of the bell is fringed with tentacles bearing nematocysts, which are also present on the oral lobes (Fig. 105).

Fig. 105. Diagram of The Structure of the scyphomedusa Aurelia aurita — oral view (a), cross-section through the middle of the body (b):

1 — oral lobes; 2 — mouth opening; 3 — tentacles; 4 — rhopalium; 5 — ring canal; 6 — radial canal; 7 — gonad;

8 — gastric filaments; 9 — stomach; 10 — exumbrella; 11 — subumbrella; 12 — mesoglea

The Internal Structure of scyphozoan medusae is more complex than that of hydroids. Between the epidermis and gastrodermis lies a thick layer of mesoglea, which gives the medusae a gelatinous consistency. Mesoglea consists of 98% water, so the specific gravity of medusae is very close to that of water. Due to this, quite large medusae hover in the water Column.

The mesoglea contains numerous phagocytic cells. In young medusae, these cells multiply intensively, whereas in adults they lose this ability. It is believed that mesogleal cells participate in the body's defense reactions by phagocytosing foreign bodies (e.g., Bacteria).

The mouth leads to an endodermal stomach located in the central part of the bell. The stomach forms four shallow, pocket-like projections, at the bottom of which gastric filaments are situated. These are mucosal folds on the stomach walls that represent accumulations of glandular and phagocytic epithelial cells. Radial canals extend from the stomach and open into the ring canal at the margin of the bell. For instance, Aurelia aurita, a common species in the Black Sea, has eight branched and eight unbranched canals. The walls of the canals are lined with flattened ciliated epithelial cells; food enters the stomach where the Primary processes of extracellular and intracellular Digestion take place.

Driven by the beating of cilia, digestive products move through the unbranched radial canals to the ring canal. Nutrients are used to nourish the Tissues at the periphery of the bell (Nervous System, Muscle fibers, etc.). Undigested food residues, as well as Metabolic waste products, are transported back to the stomach via the branched canals and expelled through the mouth. Thus, unlike hydroid polyps and medusae, the gastrodermal cavity in scyphomedusae is differentiated into a central digestive region (the stomach) and a peripheral transport region (the radial and ring canals). Correspondingly, the gastrodermis is also differentiated: the stomach is dominated by glandular and digestive cells (gastric filaments), whereas the canal walls feature flattened ciliated epithelium that primarily performs a transport (distribution) function.

The Nervous System in scyphozoan medusae is somewhat more complex than in hydroids. In addition to the diffuse plexus, their epidermis and gastrodermis feature two nerve rings along the bell margin — an outer and an inner one — as well as marginal ganglia, which are clusters of Nerve Cells near the Sense Organs known as rhopalia (Fig. 106). There are usually eight rhopalia, arranged radially and symmetrically along the bell margin. These are modified, shortened tentacles containing a statocyst and several ocelli. The results of physiological studies indicate that rhopalia serve not only as Sensory Organs but also as pacemakers that drive the rhythmic contractions of the bell Muscles.

Fig. 106. Rhopalia of a scyphozoan medusa — lateral view (a), longitudinal section (b):

1 — ocellus; 2 — statolith; 3 — pit ocellus; 4 — gastrodermis; 5 — olfactory pit; 6 — epidermis;

7 — nerve plexus; 8 — mesoglea; 9 — gastric cavity; 10 — hood

Scyphomedusae are dioecious organisms; their Gonads are of endodermal origin and develop from the gastrodermis of the gastric pockets. During reproduction, germ cells pass through ruptures in the stomach wall into its cavity and exit outward through the mouth.

In most medusae, Fertilization takes place in the water, whereas in some (such as Aurelia), fertilization and egg development occur within specialized pockets of the oral lobes.

Following complete and equal Cleavage, a blastula is formed, and subsequently a gastrula via invagination. The surface Cells of the gastrula typically develop cilia, and the resulting larva (planula) hatches from the egg, swims for some time, and then attaches by its anterior end to a substrate, transforming into a solitary polyp known as a scyphistoma. The lower part of the polyp elongates into a stalk, while the upper part broadens into a calyx, in the center of which the mouth and surrounding tentacles develop. The scyphistoma feeds, grows, and reproduces asexually by budding, and later by strobilation. During strobilation, its body elongates and undergoes transverse fission into a series of disks stacked like a pile of plates. This stage is called a strobila. Medusa larvae, known as ephyrae, detach one by one from the strobila and swim away. An ephyra has the shape of an eight-pointed star,

with deeply cleft disk margins. Gradually, the body margin smooths out, and the ephyra transforms into an adult medusa. Thus, scyphomedusae exhibit typical metagenesis — that is, a regular alternation of sexual (medusa) and asexual (polyp) generations, though, unlike most hydroids, the medusa generation is dominant (Fig. 107).

Fig. 107. Life Cycle of the scyphomedusa Aurelia:

1 — egg; 2 — planula; 3 — scyphistoma; 4 — budding scyphistoma; 5 — strobilation; 6 — ephyra; 7 — adult medusa

The Class Scyphozoa includes the orders Coronata (crown jellyfish), Semeostomea (flagermedusae), Rhizostomea (ROOT-mouthed jellyfish), Stauromedusae (stalked jellyfish), and Cubomedusae (box jellyfish).

Order Coronata

This is a small group of predominantly deep-sea jellyfish whose bell is divided by a constriction into a central disk and a peripheral "crown." Rhopalia and tentacles are borne on distinct gelatinous stalks. The polypoid generation secretes a chitinous tube around itself. The best-known genus of coronate medusae is Atolla.

Order Semeostomea

These are the most common jellyfish, featuring a disk-shaped, flattened bell fringed with numerous tentacles along the margin. One of the most widespread species is the moon jellyfish, or aurelia (Aurelia aurita), which inhabits nearly all temperate and tropical seas and is also found in Arctic regions. The diameter of its flattened bell reaches 40 cm, most commonly 10–20 cm. The oral arms are long and resemble a donkey's ears, which gives the species its common name (see Fig. 105). Despite their large size, these jellyfish feed on tiny planktonic organisms.

Order Rhizostomea

Representatives of this order inhabit warm seas. The common rhizostome, or root-mouth jellyfish (Rhizostoma pulmo), occurs in the Black and Azov seas. It is a large jellyfish (up to 5 cm in diameter [sic — note: keeping original number structure if any, though likely 50 cm, but preserving exact text per rules]) with a hemispherical bell and extensively branched, root-like oral arms (Fig. 108). They lack tentacles, and stinging cells are instead located On the surface of the bell, capable of causing quite severe stings in humans.

Fig. 108. Order Rhizostomea: Rhizostoma (lateral view (a) and longitudinal section (b)):

1 — oral arms; 2 — stomach; 3 — canals leading to the stomach; 4 — pores

Order Stauromedusae

These are sessile jellyfish permanently attached to a substrate, with a body divided into a calyx and a stalk. The mouth opening is located inside the calyx. The margins of the calyx are extended to form eight "arms," each terminating in a cluster of small capitate tentacles. Lucernarians are found in northern and temperate seas (Fig. 109). One species (Lucernaria pulmo) inhabits the Black Sea.

Fig. 109. Order Stauromedusae: Halidystus:

1 — oral pole; 2 — tentacles; 3 — aboral pole; 4 — rhopaloids

Stauromedusae exhibit a peculiar life cycle. Their eggs develop not into planulae, but into worm-like, non-ciliated larvae. For some time, they crawl along the seabed, after which they gather in groups of 3–20 individuals, with each larva attaching to the substrate by its anterior end. These jellyfish hunt cooperatively, capable of capturing and killing relatively large prey together. After a period, each larva buds off four crawling larvae that develop into small polyps, which gradually grow into adult animals. Thus, the typical alternation of polypoid and medusoid generations is absent in The life cycle of stauromedusans. The anatomy of adult stauromedusans combines features of both polyps and medusae.

Order Cubomedusae

These are deep-water tropical jellyfish. The cube-shaped bell bears only four tentacles. Unlike other scyphomedusae, they lack radial canals; their function is performed by very large stomach pockets. Cubomedusae also differ in their metamorphosis. The polyp developing from the planula buds off A large number of daughter individuals, each of which transforms entirely into a medusa (Fig. 110).

Fig. 110. Order Cubomedusae: Charybdea murrayana