INVERTEBRATE ZOOLOGY - G. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOANS (EUMETAZOA)

SECTION DIBLASTICA, OR RADIATA

PHYLUM CNIDARIA, OR COELENTERATA

SUBPHYLUM MEDUSOZOA

CLASS SCYPHOZOA

Exclusively marine animals with the medusa stage dominating The life cycle, while the polyp generation is short-lived. The Class comprises about 200 species. Three species occur in the Black Sea and one in the Sea of Azov.

Scyphozoan medusae differ from hydromedusae in their significantly larger size—the bell diameter in medusae of the genus Cyanea can reach 2.5 m. They lack a velum and move by contractions of the bell walls. Scyphomedusae possess a more developed mesoglea, a more complex gastrovascular system and Sense Organs; their Nervous system features distinct ganglia. Gonads originate from the endoderm.

Morphology (Fig. 83). The quadrangular Mouth is situated in the middle of the subumbrella at the tip of the manubrium. Its corners are elongated into four groove-like processes, the oral arms, which serve to capture prey (crustaceans, larvae of various animals, fish fry, etc.).

Fig. 83. The scyphomedusa Aurelia aurita:

a - oral view (from Kükenthal); b - cross-section through the middle of the body (from Bayer):

1 - oral arms; 2 - mouth; 3 — tentacles; 4 - rhopalium; 5 - ring canal; 6 - radial canals; 7 - gonad;

8 - gastric filaments; 9 - Stomach; 10 - exumbrella; 11 - subumbrella; 12 - mesoglea

Gastrovascular system. The mouth leads into the endodermal stomach, which occupies the central part of the medusa's body. The Stomach forms four shallow, pocket-like extensions. Projecting into their sides are four ridges bearing gastric filaments, which serve to increase the digestive surface area. Gastric filaments are clusters of glandular and phagocytic epithelial Cells. Radial canals extend from the stomach and empty into a ring canal encircling the margin of the bell. In Aurelia aurita, for example, eight unbranched canals alternate with eight branched ones, and all their branches empty into the ring canal.

The main processes of extracellular and intracellular Digestion take place in the stomach, which is primarily concentrated with glandular and digestive cells, whereas the canal walls contain flat flagellated epithelial cells that mostly perform a transport function. Undigested food residues, as well as Metabolic waste products, are expelled through the mouth.

Nervous system. In addition to the nerve plexus located in the epidermis and gastrodermis, There are two nerve rings along the bell margin (outer and inner) and ganglia near the sense organs, the rhopalia (Fig. 84). There are usually eight rhopalia, which represent modified, shortened tentacles containing a single statocyst and several ocelli. Besides their sensory function, the rhopalia also regulate the rhythmic contractions of the bell Muscles.

Fig. 84. Rhopalia of a scyphozoan medusa (from Hyman):

a - side view; b - longitudinal section:

1 - eyespot; 2 - statocyst; 3 - cup-shaped ocellus; 4 - gastrodermis; 5 - "olfactory pit"; 6 - epidermis;

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

Scyphomedusae, like hydromedusae, are dioecious. The gonads develop in the gastrodermis of the stomach pockets.

Reproduction. Fertilization is external. The egg develops into a planula larva, which swims for some time before attaching to a substrate by its anterior end and metamorphosing into a solitary polyp, the scyphistoma (Fig. 85). The polyp grows, feeds, and reproduces by budding. Subsequently, a process called strobilation takes place. During this process, the body of the polyp elongates and divides through a series of successive transverse fissions into multiple discs, resembling a stack of plates. This stage is known as a strobilus.

Fig. 85. Life Cycle of the scyphozoan Aurelia (from Dogiel):

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

The disks formed As a result of strobilation are juvenile medusae, known as ephyrae, which detach from the strobila one by one and swim away. An ephyra resembles an eight-pointed star because the margin of its disk is cleft, forming eight lobes. The ephyra feeds intensively and grows; the notches gradually close, tentacles and a complex gastrovascular system develop, and the ephyra transforms into an adult medusa. Thus, scyphozoans exhibit a typical metagenesis, though the medusoid generation is dominant.

The most widespread medusae belong to the order Semaeostomeae, which inhabit all temperate and tropical seas and extend even into arctic regions, as exemplified by the moon jellyfish (Aurelia aurita). This order also includes a common inhabitant of northern seas: the giant lion's mane jellyfish, or cyanea (Cyanea arctica).

The order Stauromedusae comprises sessile medusae that live at shallow depths, attaching to a substrate (such as Algae or rocks) by means of a stalk formed on the exumbrella. The margins of the bell are extended into eight "arms" bearing short tentacles (Fig. 86). Between the "arms", some species possess modified capitate tentacles—rhopaloids—which they use to temporarily attach to the substrate while moving. In northern and temperate seas (including the Black Sea), Lucernaria (Lucernaria pulmo) and species of the genus Haliclystus are commonly found.

Fig. 86. Sessile medusa Haliclystus (from Dogiel):

1 - oral pole; 2 - tentacles; 3 - aboral pole; 4 - rhopaloid

Representatives of the order Coronatae inhabit predominantly great depths, with the genus Atolla being the best known among them. These deep-sea medusae do not actively pursue their prey; instead, they lure it by emitting light. They feed primarily on crustaceans.

In tropical seas, large, tentacle-free medusae of the order Rhizostomeae are the most abundant. Their oral arms are elaborately branched in a ROOT-like fashion (hence the order's name), forming folds, while the mouth opening closes, its function being replaced by numerous tiny pores located within the folds of the oral arms. Consequently, rhizostome jellyfish feed exclusively on microscopic planktonic organisms, drawing them into the stomach along with Water. Rhizostoma pulmo is found in the Black Sea (Fig. 87).

Fig. 87. Rhizostome jellyfish Rhizostoma pulmo (from Dogiel):

a - general view; b - longitudinal section: 1 - oral arms; 2 - stomach;

3 - canals leading to the stomach; 4 - pores

Several species of rhizostome medusae inhabit tropical seas (such as the edible jellyfish Rhopilema esculenta), which are salted and used for food in Japan and China (consumers effectively eat only the salted gelatinous mesoglea—known by the Chinese as "crystal meat").

Human encounters with jellyfish at sea can be rather unpleasant. In the Black and Azov seas, the rhizostome jellyfish (Rhizostoma pulmo) is common; its stinging cells are located On the surface of the bell, and touching it can cause a rather painful "burn".

An encounter with the so-called sea nettle (Chrysaora quinquecirrha) can result not only in Skin lesions but also in cardiovascular dysfunction and even Shock. However, the greatest danger to humans is posed by the sea wasp (Chironex fleckeri), found off the coast of Australia. Numerous cases are documented where death occurred within minutes of contact with this jellyfish.