Order I.—Ascidiacea.
The Ascidiacea include the great majority of species. With the exception of the one genus Pyrosoma, they lead a fixed or stationary life.
Sub-order 1.—Ascidiæ Simplices.
The Simple Ascidians are mostly solitary; in a few forms, however, colonies arise by budding from stolons, but each individual has a distinct test. The four families into which the sub-order is divided are chiefly characterised by the nature of the test, the number of lobes round the branchial and atrial orifice, and the character of the branchial sac.
In the family Molgulidæ the tough membranous test is often coated with sand; the branchial aperture is six-lobed, the atrial four-lobed, the branchial sac has long folds or pleats, and the stigmata are curved or arranged in spirals.
Molgula gigantea, which is one of the largest of the Ascidians, and which attains a length of over thirteen inches, forms a tough conical sac; the branchial and atrial orifices at the upper end have six and four lobes respectively. The test is leathery, smooth above, but coated with sand below. The exhibited specimen, which comes from the Straits of Magellan, has several specimens of the stalked Boltenia legumen attached to the lower part of the test.
The curious Molgula oculata (Fig. 10) has a soft oval or rounded body coated with sand. The branchial and atrial orifices have respectively six and four lobes. Specimens grow attached to the rocks and also live free in the sand. The surface of the test is provided with hairs, which adhere to the rocks and collect particles of sand. The adhesion not being very firm, specimens are easily detached by currents and collected into heaps by the eddies; when living in the sand only the two dark orifices are visible. The sand coating has been supposed to confer protection by mimicry of the environment; but Professor Lacaze Duthiers found, much to his chagrin, that the sandy tests of his specimens were of no avail in securing them from being devoured by crabs who seemed to scent their prey from afar.
Fig. 10.
Mogula oculata.
a, branchial; b, atrial orifice.
In the family Cynthiidæ the test is usually leathery, the branchial and atrial apertures four-lobed, and the branchial sac folded into longitudinal pleats.
The genera Boltenia and Culeolus include species in which the body is attached to a peduncle.
The large exhibited specimen of Boltenia pachydermatina is 28 inches in length, the head being 4 and the stalk 24 inches long. The two four-lobed apertures are along one edge, the branchial being the lower; the body is marked with long deep furrows, and the stalk with transverse wrinkles. Culeolus perlucidus, from 1600 fathoms in the Southern Ocean, is in the form of a small pear-shaped head on a slender stalk, the total length being 4½ inches. The branchial orifice forms a transverse slit with raised lips near the stalk, the slit-like atrial orifice being near the rounded end of the body. Culeolus moseleyi, another slender-stalked form, was obtained from 2425 fathoms in the Central Pacific.
Fig. 11.
A. Styelopsis grossularia on shell. B. Tadpoles of same, × 9. a, branchial; b, atrial orifice. (B, after Sir J. Dalyell.)
The little Cynthiid Styelopsis grossularia (Fig. 11), popularly known as the “Currant Squirter,” occurs in the form of bright red hemispherical blobs on stones and shells; when undisturbed, the branchial and atrial orifices expand and project upwards. The eggs are brilliant red in colour. Sir John Dalyell was the first to discover the tadpole form, which is about ⅒ inch long (Fig. 11, B), and to observe the tadpoles become fixed and develop into fixed Ascidians. He calls the active little swimming larvæ “Spinulæ,” from their resemblance to small pins.
The family Ascidiidæ includes forms with a gelatinous or cartilaginous test; the branchial and atrial orifices usually have 8 and 6 lobes respectively; the branchial sac is without folds.
Ascidia mentula, described above, belongs to this family.
Chelyosoma is characterised by the test forming tortoise-like horny plates on the upper surface. The exhibited specimen of C. macleayanum (Fig. 12) comes from Greenland; the upper hemispherical part of the test is divided into 8 plates; the branchial and atrial orifices are situated in the joints between the plates.
Fig. 12.
Chelyosoma macleayanum, slightly enlarged. a, branchial; b, atrial orifice.
The fine specimen of Phallusia mammillata from Naples consists of several individuals partly fused together; the branchial and atrial orifices are wide open, and the mantle can be seen through the thick knobby translucent test.
In Rhodosoma the test is modified so as to form stiff plates recalling the valves of a bivalve shell. One plate is attached to the rocks, the other closing against the first like a lid; the anterior end of the animal with its branchial and atrial orifices is visible only when the lid is open. The Mediterranean species R. callense (Fig. 13) grows attached to the rocks. The little exhibited specimen is on a fragment of shell in front of a black patch. The figure shows specimens with the lid open and closed.
Fig. 13.
Rhodosoma callense, × 10. A, “valve” open; B, shut. a, branchial; b, atrial orifice. (After Lacaze Duthiers.)
Family Clavelinidæ. The body is attached to a creeping stolon or mass of stolons, from which new individuals arise by budding. The other three families of Simple Ascidians included solitary forms, but the Clavelinidæ are social, and form colonies wherein each individual has its own test.
Clavelina lepadiformis (Fig. 14) forms graceful crystal vases about an inch in height. The figure shows one individual, but usually the processes at the base extend out as stolons whence other individuals arise.
Diazona violacea, from Cornwall, forms beautiful purple disk-shaped colonies in which the ascidiozooids arise from a basal mass of stolons. Sometimes the ascidiozooids die down, leaving only a smooth violet pad, which in due time produces a new crop of ascidiozooids.
Fig. 14.
Clavelina lepadiformis; diagrammatic, showing the anatomy. The oval bodies are the eggs; at lower end lies the tubular heart; the root-like processes at the base grow into stolons, whence other ascidiozooids arise.
Fig. 15.
Perophora listeri; A, slightly, B, further magnified. Ascidiozooids in right, left, and lateral aspects.
a, branchial; b, atrial orifice.
The remarkable Rhopalæa neapolitana, from Naples, may be roughly compared to an hour-glass with a very long constriction. The test is smooth in the upper part, but knobby and encrusted with foreign bodies below. The upper or thoracic end contains the branchial sac, and the lower or abdominal portion the stomach, heart, and reproductive organs, the gullet and intestine traversing the whole length of the narrow central region. Although from its general structure Rhopalæa is a Clavelinid, it is not certainly known to produce buds.
Perophora listeri (Fig. 15) occurs in the form of little jelly-like transparent blobs rising by short stalks from a silvery thread-like stolon. Owing to their small size and transparency, it is possible to examine specimens alive under the microscope, the currents passing through the stigmata in the walls of the branchial sac, and the beating of the heart being distinctly visible. The rapid motion of the cilia surrounding stigmata gives the appearance of dark wheels all rotating in the same direction. The heart beats so as to drive the blood current so many times in one direction, and then after a short pause, in the reverse direction.
The exhibited specimen growing on an oyster shell, is from Plymouth.
Sub-order 2.—Ascidiæ Compositæ.
The Compound Ascidians are fixed forms, which give rise to colonies by budding, the individuals being immersed in a common mass and not possessing separate tests.
Although reduced to an extremely small size each individual or ascidiozooid of a colony possesses the same organs as a large Simple Ascidian, excepting that the former does not possess a separate test. Frequently the individuals of a colony are grouped into systems, in which the atrial orifices open into a common cloaca. The little ascidiozooids vary greatly in shape in the different families. In the Polyclinidæ, for instance, they are long, the organs being so to speak, drawn out, and being arranged in three regions, the thoracic, abdominal and post-abdominal, the first region containing the branchial sac, the second the stomach, and the third the heart and reproductive organs. In the Distomidæ, the body exhibits two regions, thoracic and abdominal, the heart and reproductive organs lying alongside of the stomach. The Botryllidæ comprise only one region, the stomach and the other organs being situated by the side of the branchial sac.
The Compound Ascidians include seven families which are characterised chiefly by the method of bud formation, and by the arrangement of the organs into one, two, or three regions.
It is only possible, from limits of space, to refer to a few interesting forms.
The species of Botryllus are those most commonly met with. They form richly coloured gelatinous incrustations on rocks and seaweeds. B. violaceus (Figs. 16, 17, and 18 D) is blue with white lines; B. smaragdus, green; B. marionis, brown with white and carmine; B. castaneus, purple, and so on. The individuals are arranged in circular systems with the branchial orifices round the circumference and the atrial orifices opening into a common central cavity (Fig. 17), the whole colony being composed of groups of systems.
Fig. 16.
Botryllus violaceus on seaweed. (After H. Milne-Edwards.)
The exhibited specimen of B. violaceus was grown in the tanks of the Biological Station at Plymouth. The red specimen of B. aurolineatus, from Naples, shows well the branchial and cloacal orifices. In Botrylloides, the individuals form elliptical or elongated systems.
Colella thomsoni was obtained near the Philippines at a depth of 10 fathoms. The specimen, which is about 7 inches in length, resembles an elongated head of clover on a thickened stalk. The individuals which compose the head are arranged in spiral lines, the atrial orifice of each ascidiozooid opening separately and not into a common cloaca.
Fig. 17.
A. Botryllus violaceus, magnified, showing two systems of 6 and 7 ascidiozooids. B. One ascidiozooid extracted.
a, branchial; b, atrial orifices; c, branchial sac; d, stomach.
(After H. Milne-Edwards.)
Colella quoyi (Fig. 18 A), from 25 fathoms off Kerguelen Island, forms a rounded head on a short peduncle, the total height being one inch. The ascidiozooids are arranged in vertical lines in the “head,” each line consisting of a double zigzag series.
Fig. 18.
Colonies of Ascidiæ compositæ, natural size. A. Colella quoyi. B. Leptoclinum neglectum. C. Pharyngodictyon mirabile. D. Botryllus.
(After Herdman, Challenger Report and Encyclopædia Britannica.)
Julinia ignota, from the Antarctic regions, forms long narrow colonies, which attain a length of nearly three feet. One end is attached, the rest of the colony apparently lying along the sea-bottom.
Amaroucium roseum from Naples forms translucent gelatinous masses; a slice is exhibited, showing the long slender ascidiozooids immersed in the mass.
Fig. 19.
Pyrosoma elegans, natural size. A. Side view of entire colony. B. End view of open extremity.
(Herdman: Tunicata, Encyclopædia Britannica.)
Pharyngodictyon mirabile (Fig. 18 C), from 1600 fathoms in the Southern Indian Ocean, resembles a small mushroom, and is about one inch in height. This species is one of the few deep-sea Compound Ascidians.
Leptoclinum albidum is a common and widely distributed species; it occurs in the form of thin white crusts. The glistening white appearance is due to the common test being densely crowded with minute stellate spicules of carbonate of lime.
The specimen of Leptoclinum neglectum (Fig. 18 B) encrusts a fragment of sponge.
Goodsiria pedunculata from the Straits of Magellan, forms a rounded cartilaginous mass attached by a short peduncle; sometimes several masses are attached to each other. Each of the small dark oval areas on the surface corresponds to the branchial and atrial orifices of one ascidiozooid.
Sub-order 3.—Ascidæ Salpiformes.
The Salpiform Ascidians comprise only one genus, Pyrosoma, which occurs in the form of free-swimming colonies shaped like hollow cylinders closed and rounded at one end and open and truncate at the other (Fig. 19). The wall of the cylinder is formed of a single layer of ascidiozooids (Fig. 20), so arranged that all the atrial orifices open into the interior of the cylinder, and all the branchial orifices on the exterior, the two kinds of orifices being at opposite ends of the body, and not close together, as in most simple and compound Ascidians.
Specimens vary in size from a few inches to upwards of four feet in length, and, as the name of the genus implies,[[22]] they are brilliantly phosphorescent. Sometimes they occur in innumerable multitudes, giving rise to a zone of greenish light extending for miles. Professor Moseley records that during the voyage of the Challenger in the North Atlantic a huge specimen of Pyrosoma spinosum, four feet in length, was captured. On tracing his name on its body, the word came out in letters of fire.
Fig. 20.
Section through wall of Pyrosoma, magnified, showing a single layer of ascidiozooids.
br, branchial; at, atrial orifice; tp, process of the test; br s, branchial sac.
(Herdman: Tunicata, Encyclopædia Britannica.)
In Pyrosoma elegans (exhibited), from Naples, the ascidiozooids are arranged in verticils, and the mouth of the cylinder is surrounded by a movable diaphragm; the outer end of each ascidiozooid is provided with a membranous spine. Six species of Pyrosoma are known. Pyrosoma atlanticum is found in the tropical Atlantic and Antarctic; P. giganteum in the Atlantic, Pacific, and Antarctic; and P. spinosum in the South Atlantic.