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Thaliacea and Larvacea

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Phylum Chordata
Subphylum Urochordata

I. Definition of the classes:

Chordate bilateria adapted for a filter-feeding plank­tonic existence, the outer body covering is composed of a cellulose-like mucopolysaccharide and proteins. Thaliaceans are barrel to spindle shaped, and may be free-livng or colonial.

Larvaceans are tadpole-shaped and construct specialized feeding structures out of mucus.

II. Extenal features:

Outer body covering is called the tunic and is com­posed of mucopolysaccharides and proteins secreted as a fibrous matrix by the single-layered epidermis. Amoeboid cells and blood cells may migrate from the mesenchyme into the tunic.

Salps and doliolid thaliaceans reproducing asexually trail a stolon derived from body tissues at the posterior of the endostyle. The stolon may attain a length of six feet.

Colonial thaliaceans form a cylindrical tube closed at one end.

Larvaceans have two gill clefts which open ventro­laterally to the outside and are called spiracles.

Larvaceans and thaliaceans are transparent, but the feeding structures and digestive systems of both may be made readily visible by the accumulation of micro-organisms.

III. Internal features:

1. Digestive system:

Consists of a mouth, pharynx, esophagus, stomach, intestine, rectum, and anus.

The esophagus is ciliated and moves food-laden mucus into the stomach.

The stomach secretes enzymes for extra-cellular digestion.

The intestine is the site of absorption.

The digestive tract in larvaceans is U-shaped, with the mouth opening at the anterior end, the intestine lying toward the rear of the animal, and the anus opening to the outside ventrally between the spiracles.

The digestive system in thaliaceans is very short and lies along the ventral body wall. The eso­phagus opens anteriorly into the pharynx and the anus opens posteriorly into the atrium.

2. Circulatory system:

A. The heart is a muscular tube which sends blood alternately ventrally and dorsally, beating 40-50 times before reversing direction. There are two excitation centers, one at each end of the heart.

The subendostylar sinus sends branches dorsally up the gill bars.

The dorsal sinus sends branches into the visceral mass, blood then drains into the dorsal end of the heart.

The blood contains specialized cells: lymphocytes, which develop into other cells; nutritive amebo­cytes; vanadium concentrating morula cells which may function in the production of the tunic; and storage cells which contain insoluble wastes and tend to collect in specific areas of the tunicate during its lifetime.

3. Nervous system:

A single cerebral ganglion sends nerves anteriorly
to the buccal siphon and musculature, and posteriorly sends nerves to the pharynx, visceral organs, and atrial musculature. There is an exocrine subneural gland which is considered to be homologous to the pituitary gland in vertebrates, although its function is not understood. In salps, a light-sensitive pig­ment cup lies above the ganglion. Other sense cells _are chemoreceptors and tactile cells around the buccal siphon and in the buccal and atrial siphons.

4. Excretory system:

Ammonia produced by the tunicate diffuses out.

Storage cells in the blood contain crystals of insoluble wastes.

5. Locomotion:

In thaliaceans, the body musculature consisting of muscle bands in the mesenchyme contract and relax, forcing water through the animal, result­ing in jet propulsion and feeding currents.

In larvaceans, the muscular tail is used to propel the animal or to generate currents.

6. Reproductive system:

Thaliaceans and Larvaceans are hermaphroditic, the gonads usually lie in close association with the intestine.

Thaliaceans gametes are released into the exhalant siphon.

In larvaceans, only the testes open to the outside.
The gonads arise as a rudiment, and it is unknown as to the tissue from which the gonads are formed.

IV. Reproduction:

Asexual reproduction

Thaliaceans only.

The genus Pyrosoma forms colonies by the budding of a stolon which undergoes strobilation.

Salps and doliolids bud new individuals by a strobi- lizing stolon.
Asexual generations alternate with sexual generations. Sexual'reproduction

A. Larvaceans release sperm into the water, and fertili‑
zation is internal.

B. Salps self fertilize, and the embryo develops intern­ally.

V. Embryology and development:

Cleavage is complete, slightly unequal, and determinate by the four to eight cell stage.

Gastrulation occurs by invagination, and is complete by the sixth to seventh cleavage.

No coelomic cavity is formed.

. The embryo takes the form of a tadpole larva.

. Metamorphosis occurs without settling.

F. Embryos which develop internally burst the body of the parent, or nurse. In thaliaceans, the new individual already has a partially formed stolon.

VI. Ecology and physiology:

Marine, planktonic from the surface to 1000 feet.

Filter-feeders on diatoms, flagellates, and dino­flagellates.

Thaliaceans filter water forced through their bodies with mucus produced by the endostyle. The mucus is collected by the esophagus and passed into the digestive system.

Larvaceans produce a mucus house secreted by special­ized cells on the trunk called oikaplast epithelium. The house has two incurrent channels crossed by fibers to limit the size of particles admitted. A current is set up by movements of the tail, forc­ing water back through a pair of feeding filters. The feeding filters are triple layered and collect particles from the seawater pasing through the layers. The animal sucks particles off the filters by action of the spiracles. The feeding current may propel the organism.

. These classes are an important link in the food chain between nanoplankton and fishes.

VII. Classification:

Class Thaliacea is divided into three orders.

Order Pyrosomida is composed of the genus Pyrosoma and contains colonial animals.

Order Doliolida contains free-living animals with two rows of gill slits.

Order Salpida contains free-living animals with only two gill clefts.

Class Larvacea has only one order.

Order Copelata is composed of tunicates which become sexually mature as tadpole-like animals similar to the larvae of ascidians.

VIII. Phylogeny:

A. Chordate features without presence of coelom or meta­merism indicate that tunicates departed from the mainstream of vertebrate evolution early.

B. Ascidians are considered to be the primitive members of the phylum. The morphology of Thalaceans and the neotenic character of Larvaceans are considered to be advanced traits.

(This material comes from ZOO 6207, "Advanced Invertebrate Zoology", at the University of Florida, Spring Semester 1982, class taught by Dr. Frank Maturo. The text was scanned and OCR'd by Wesley R. Elsberry in 2006. Typographic errors are likely due to the scanning process. This particular presentation was prepared by Wesley R. Elsberry for ZOO 6207.)