INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 1 - G.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION EUMETAZOA

PHYLUM CTENOPHORA

CLASS CTENOPHORA

Subclass Cyclocoela

Representatives of this subclass are characterized by a system of interconnecting channels—anastomoses—between the gastral canals near the oral pole.

Order Beroida

Unlike other ctenophores, members of this order lack tentacles at all stages of their life cycle. A large bell-shaped Pharynx occupies most of the body, accessed by a massive Mouth opening. Its inner edge bears specialized ciliary structures known as macrocilia, which serve to capture prey such as large salps, jellyfish, and other ctenophores (Fig. 126, d).

Arctic and Antarctic species attain the largest sizes. Beroids serve as a food source for commercial fish species, including haddock and cod.

Order Cestida

Representatives of this order possess tentacles, though in adults they are housed within tentacular sheaths. Their bodies are flattened not along the oral-aboral axis, as in benthic ctenophores, but laterally. These ctenophores swim through sinusoidal movements of their ribbon-like bodies, powered by the alternating contraction of Muscle bands running along the body margins. The beating of the comb rows Functions solely to maintain the horizontal orientation of the principal body axis. The largest representative of this order is Venus's girdle (Cestus veneris). Cestids feed on planktonic crustaceans (Fig. 126, e).

Order Lobiferida

The common name of this order derives from the presence of lobes of various SHAPES AND SIZES around the mouth or along the entire body, which assist in capturing planktonic prey. Although delicately gelatinous in consistency, they are voracious predators. For instance, the muscular Ocyropsis crystallina swims rapidly through the Water Column, utilizing its oral lobes like wings to overtake and swallow even small fish.

These are extremely fragile creatures recently discovered in the Sargasso Sea. Their dome-shaped bodies resemble jellyfish; they feed passively, gathering plankton with their mouths as the organisms adhere to the inner surface of the dome. The mouth is situated on a specialized projection, toward which various sections of the dome converge.

In terms of their level of Organization, ctenophores are closely related to Cnidarians and were previously classified together with them in a single phylum. However, ctenophores differ significantly from cnidarians in possessing an aboral organ, moving primarily via ciliary structures, lacking a sessile stage in their life cycle, and possessing colloblasts (sticky Cells) instead of nematocysts. Ctenophores are the only group of Multicellular animals that have retained a primarily planktonic lifestyle and originated from free-swimming ancestors. Cnidarians, by contrast, transitioned to a sessile lifestyle, losing the aboral organ and the capacity for ciliary locomotion, which is now retained only in their larvae. Thus, although ctenophores and cnidarians share common ancestors, they have evolved along distinct evolutionary pathways.

Ctenophores merit special attention from researchers because their body plan is frequently used to address key evolutionary questions. In the late 19th century, the Swiss zoologist A. Lang proposed the ctenophore hypothesis for THE ORIGIN OF Flatworms, based on structural similarities between turbellarians and benthic ctenophores. In his view, modern flatworms descended from ancient ctenophore-like ancestors. Today, this hypothesis is of historical interest only. The renowned researcher I. I. Mechnikov drew attention to the similarities between ctenophores, the tornaria larva of hemichordates, and the dipleurula larva of Echinoderms, suggesting that deuterostomes (echinoderms, hemichordates, and Chordates) originated from ancient ctenophore-like ancestors.