ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 13. GENERAL CHARACTERISTICS OF PHYLUM CHORDATA (Chordata)

3. General Characteristics of Subphylum Tunicata (Tunicates)

Subphylum Tunicata (Fig. 53) comprises about 1,100 species. Typical chordate features (notochord, dorsal nerve cord, and pharyngeal gill slits) are present only during the larval stage of development. Adult forms lack a notochord (except for appendicularians). The size of tunicates ranges from 0.3 cm (appendicularians) to 30 m (pyrosome colonies). The body is enclosed in a sac-like outer covering, or tunic (named after the ancient Roman garment), which gives the subphylum its name.

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Fig. 53. Salp (after Naumov, Kartashev, 1979):

1 — Mouth; 2 — ventral side; 3 — cloacal aperture.

Beneath the tunic lies the mantle, or body wall, consisting of a single-layered epidermis (ectoderm) and several (2–3) layers of longitudinal and transverse Muscle bands embedded in loose Connective Tissue. They are exclusively marine. Some species lead a sessile (attached) lifestyle, while others drift slowly through the Water Column.

The body of these animals (such as ascidians and salps) features a base for attachment to the substrate, as well as oral and cloacal apertures—siphons (Fig. 53)—which can open and close by means of circular Muscles. Water enters the oral siphon driven by the beating of cilia located there and passes through the Pharynx, which occupies most of the body cavity. At the entrance to the pharynx, there are oral tentacles (up to 30).

The pharynx is perforated by gill slits that filter water. The water current is generated by cilia located on the gill bars. Thus, these animals are passive filter feeders. From the bottom of the pharynx, a short Esophagus leads to an enlarged Stomach, whose walls contain glands that produce digestive Enzymes. The Stomach is followed by the intestine. The intestine opens via the anus into the body cavity near the cloacal siphon. Undigested waste is carried out of the body cavity through the cloacal siphon by the water current.

The excretory system consists of small vesicles located on the walls of the intestine and mantle. Uric acid accumulates in these "accumulation Kidneys" and is not excreted from the vesicles during the animal's lifespan. In colonial species, metabolic waste is excreted as gaseous ammonia, while uric acid accumulates in the "accumulation kidneys."

The pharynx also Functions as a respiratory organ. Along the DORSAL SIDE OF the pharynx runs the nerve cord, or dorsal nerve cord, which features a compact cluster of Nerve Cells—the nerve ganglion (dorsal ganglion)—at its upper end.

The Circulatory system is open. The Heart is a short tube; from one end, a vessel branches into the walls of the pharynx, while vessels from the other end lead to the Internal Organs (stomach, intestine) and the mantle, where Blood flows into small cavities called lacunae. The heart contracts periodically, pumping first in one direction for several minutes, and then reversing to pump in the opposite direction. Consequently, blood is directed alternately to the internal organs and mantle, and then to the pharyngeal walls, where it is oxygenated.

Tunicates are hermaphroditic. The paired Ovaries, which resemble long sacs filled with eggs, are located in the body cavity and attached to the mantle wall. Short, tubular oviducts open into the body cavity near the cloacal siphon. The Testes, consisting of numerous oval lobes, are also located on the mantle walls; their short ducts open into the body cavity. Self-Fertilization does not occur because Gametes in an individual do not mature simultaneously, meaning the animal functions alternately as a male and a female. Fertilization takes place externally in the water, and thus larval development also occurs outside the parent Organism.

The Development of the fertilized egg results in a tailed larva, which differs significantly in Structure from adult animals (ascidians). It has a small oval body and a relatively long tail. A small oral opening leads to the pharynx, which is initially not perforated by gill slits. The pharynx is followed by a blind-ended intestine. Separation from the ectoderm gives rise to the nerve tube, the anterior end of which expands into a sensory vesicle. An eyespot and a statocyst form within this sensory vesicle. The sensory vesicle opens via a pore into the anterior part of the pharynx. Behind the pharynx begins the notochord, which extends along the body and tail. The nerve cord is positioned above the notochord. Muscle cells lie on both sides of the notochord, and their contractions drive the movement of the tail. After hatching from the egg membranes, the larva enters the water and swims by beating its tail. On the dorsal side of the body, behind the sensory vesicle, paired invaginations form, which eventually fuse around the pharynx (forming the body cavity). Simultaneously, gill slits (from 2 to 6) break through the pharyngeal walls. At this stage, the larva possesses the Characteristic Features of Chordates (notochord, dorsal nerve cord, and pharyngeal gill slits), but it does not feed.

The free-swimming larval stage lasts only a few hours. Ectodermal outgrowths form at the anterior end of its body, developing into an adhesive papilla. The adhesive papilla secretes a sticky mucus, which the larva uses to attach itself to underwater objects (rocks, shells, substrate). After attachment, the tail (including the notochord, nerve cord, and muscle cells) gradually degenerates, the pharynx expands as the number of gill slits increases, the digestive tract differentiates, and its terminal end opens into the expanding body cavity. Simultaneously, the circulatory system develops, Gonads form, the oral and cloacal siphons shift, and the body assumes the sac-like appearance characteristic of the adult. During this metamorphosis, the eyespot and statocyst disappear, and the nerve Cells of the sensory vesicle walls condense into a compact nerve ganglion—the dorsal ganglion.

In addition to sexual reproduction, a significant number of species (such as ascidians) are also capable of asexual reproduction by budding. An outgrowth (sometimes several) forms at the lower part of the animal's body, into which extensions of all internal organs grow. Swellings—buds—develop at the end of the outgrowth, and through complex differentiation, the organs of an adult, sexually mature individual form within each bud. The animals produced by budding either remain with the parent organism (colonial ascidians) or detach from it, settle on the substrate, attach themselves, and lead an independent life.

Tunicates include both solitary and colonial forms. Colonies are formed by budding, and all related individuals share a common tunic.