INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION ARROW WORMS (CHAETOGNATHA)

PHYLUM CHAETOGNATHA (CHAETOGNATHA)

CLASS ARROW WORMS (SAGITTOIDEA)

About 150 species have been described, of which three are found in the Black Sea (Sagitta euxinica is endemic to this sea) and one in the Azov Sea. Most species inhabit the Water Column at various depths, remaining in constant motion, while only species of the family Spadellidae occur near the bottom in shallow waters—maneuvering among Algae and rocks—as well as at greater depths (down to 1 km).

The size of arrow worms ranges from 0.5 to 9 cm. The smallest species are benthic, whereas the largest are northern nektonic forms.

As a rule, chaetognaths are colorless and therefore difficult to spot in the water. Reddish, orange-brown, and yellow-green arrow worms are found only among benthic-dwelling species.

Anatomy. In body shape, arrow worms resemble a pointed arrow with fins at the posterior end, which is how they got their name. The body consists of a HEAD, trunk, and tail, and is bordered by paired lateral fins and a caudal fin (Fig. 416). At the anterior end of the head are long, sickle-shaped curved bristles and shorter Teeth that form a specialized grasping apparatus. Muscles are attached to the Base of the bristles, moving them during prey capture. Behind the bristles lies the so-called hood (capor)—a unique organ found only in arrow worms (Fig. 416, a). This is a fold of the head Skin that can instantly flip back to expose the grasping apparatus and just as quickly close it. During any movement, the hood covers the bristles, thereby increasing body streamlining, while at the moment of prey capture, it makes a sharp backward motion, allowing the bristles and teeth to seize the prey.

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Fig. 416. Arrow worm Flaccisagitta inflata (after Kasatkina): a - anterior end; b - diagram of internal anatomy

1 - rows of teeth; 2 - bristles; 3 - eyes; 4 - hood; 5 - ciliated loop (visible through the body wall); 6 - intestine;

7 - ventral ganglion; 8 - Ovary; 9 - anus; 10 - trunk-tail septum; 11 - Testis;

12 - Seminal Vesicle with male genital opening; 13 - caudal fin

One or two pairs of lateral fins primarily function as a rudder, while the main locomotory organ is the caudal fin at the end of the body. As already mentioned, a special substance, elaeoidin, is found in the fin rays.

Integument. The body is covered by a thin cuticle, beneath which lies a single-layered, so-called plaster epithelium (Fig. 417). Its Cells have undulating edges, joined such that the convexity of one Cell fits into the depression of the neighboring one. Such cell interlocking imparts stretchability and flexibility to the epithelium, preventing rupture of the integument during sudden Movements of the animal while hunting. Certain areas of the epithelium, particularly above the Brain and at the border of the trunk and tail, become multilayered, which is necessary to cushion the impact against prey.

Fig. 417. Integument of chaetognaths (after Kasatkina):

a - "plaster epithelium"; b - multilayered epithelium in a wider region: 1 - thickness of the epithelium

Beneath the integumentary epithelium lies the basement membrane—an elastic plate that determines body shape. It acts as an antagonist to the longitudinal musculature during dorsoventral body bending and also serves as its Skeleton, with muscles attaching to the basement membrane.

Body cavity. Adult chaetognaths possess a capacious body cavity not lined with peritoneal epithelium; it is divided by two transverse mesodermal septa into three compartments corresponding to the body regions. In addition, the trunk region contains a longitudinal septum that suspends the intestine. During embryonic development, a pair of coelomic sacs is formed, which quickly lose their lumen, and their wall cells contribute to The formation of muscles and other mesodermal Organs.

Musculature is well developed: the head region contains numerous specialized muscles that move the hood, bristles, teeth, etc. In the trunk and tail regions, the muscles are grouped into four longitudinal bands. All muscles are striated.

The Digestive System begins with an oral funnel—a depression in front of the Mouth opening, whose integument secretes mucus that facilitates the swallowing of prey. The mouth leads into an expanded muscular Pharynx, which then narrows and transitions into the endodermal part of the intestine. At its very beginning, some species have paired pocket-like outgrowths that presumably protect the intestine from rupture when swallowing voluminous prey. Before the trunk-tail septum, the midgut transitions into a short hindgut that opens via the anus far from the posterior end of the body.

Arrow worms feed mainly on small crustaceans, fish larvae, and their own juveniles. Sometimes the prey is significantly larger than the arrow worm itself, in which case it is swallowed gradually.

Excretory, circulatory, and respiratory systems are absent; gas exchange occurs through the body surface.

The Nervous system is well developed (Fig. 418) and consists of an unpaired cerebral ganglion, or brain, on the DORSAL SIDE OF the head, a pair of vestibular ganglia located near the brain, a pair of smaller pharyngeal ganglia, and an unpaired large ventral ganglion in the trunk region. All ganglia are connected by connectives. Nerves extend from the ganglia to all organs.

Fig. 418. Central Nervous System of Sagitta (after Kasatkina):

1 - edge of the semi-transected hood; 2 - vestibular ganglion; 3 - pharyngeal ganglion;

4 - cerebral ganglion; 5 - eyes; 6 - ciliated loop; 7 - Stomach; 8 - ventral ganglion

Sense Organs are represented by eyes and the so-called ciliated loop. The eyes are located on the dorsal side of the head behind the brain and consist of five pigmented cups arranged in such a way that arrow worms perceive images not only from the dorsal side, but also from the ventral and lateral sides. Due to their transparent body, the field of view is 360°.

The ciliated loop is usually located behind the eyes (Fig. 416) and only in a few genera in front of them. It consists of two folds forming outer and inner rings. The outer ring is covered with cilia, while the Fine Structure of the inner ring varies among representatives of different genera and, in some of them, is involved in Fertilization, as discussed below. The main function of the ciliated loop is sensorimotor, similar to the function of the lateral line in fish. Through this organ, arrow worms perceive water vibrations—signals emitted by prey, conspecifics during the breeding season, and predators.

Reproductive System. Chaetognaths are hermaphrodites. A pair of Ovaries is located in the posterior part of the trunk, adjacent to seminal ducts that function as seminal receptacles, opening to the exterior on the sides of the body. A pair of Testes and Seminal Vesicles are located in the caudal region. In the seminal vesicles, spermatozoa are glued into spermatophores. Duct systems are absent: both eggs and spermatophores are released through ruptures of the body wall.

Reproduction. Fertilization is internal. Exchange of spermatophores between two arrow worms occurs during mating, when they position themselves so that the caudal fins of one specimen Touch the posterior lateral fins of the other. In this process, the spermatophore of one arrow worm is transferred to the lateral fin of the other; the fin envelops the spermatophore, and from there spermatozoa enter the seminal receptacles. In some species, the ciliated loop participates in this process, with cells of its inner ring secreting a substance that flows down the mid-dorsal/ventral line of the body and transports spermatozoa to the seminal receptacles. Self-fertilization is also possible in chaetognaths.

Fertilized eggs actively pass through the ovarian epithelium and further, rupturing the body wall, emerge to the exterior. Eggs are laid individually or glued by an adhesive secretion into clusters attached to underwater objects using the same adhesive secretion. In cold-water species of the family Eukrohniidae, eggs develop in a special pouch.

Cleavage is total, equal, and begins as spiral, later losing its regularity. Gastrulation proceeds by invagination. The blastopore of the gastrula closes, and much later the anus forms at this site, while the secondary mouth develops at the opposite (anterior) end of the embryo (Fig. 419). The coelom is formed enterocoelously. Subsequently, the coelom becomes obliterated, and the cellular material of its walls (mesoderm) is utilized, as mentioned earlier, for the formation of musculature and other mesodermal derivatives.

Fig. 419. Embryonic development of chaetognaths (after Dogiel): a - blastula; b - gastrula; c, d - Formation of the coelom and secondary mouth: 1 - ectoderm; 2 - primordial Germ Cells; 3 - endoderm; 4 - blastopore; 5 - archenteron; 6 - lateral pouches of the archenteron (coelomic pouch); 7 - secondary mouth

Development is direct, with young individuals hatching from the eggs resembling adults. They feed exclusively on fine food—Bacteria, unicellular algae, etc.—until the bristles and hooks of the grasping apparatus develop.

Arrow worms play a vital role in marine and oceanic food webs. During periods of mass reproduction, they can compete with the juveniles of planktivorous schooling fish; however, at certain times, adult chaetognaths constitute a dominant food source for such valuable commercial fish as salmon, herring, and cod.

Arrow worms are a rather ancient group of animals; their fossils are known from the Early Cambrian, suggesting that they appeared on Earth even earlier.