A guide to the shell and starfish galleries

by British Museum . Department of Zoology; F. J. Bell; R. Kirkpatrick; E. A. Smith

TUNICATA.

Wall Case to left of main entrance to Shell Gallery.

The Tunicata are marine animals, the majority of which live, in their adult stage, a stationary life, fixed to the rocks or sea-bottom, but a comparatively small number are free-swimming.

Fig. 1.
Ascidia mentula from the right side. at, atrial aperture; br, branchial aperture; t, test.
[After Herdman: Tunicata, Encyc. Britannica.]

They occur in the form of cartilaginous or leathery sacs, fleshy incrustations, solid fleshy masses, free-swimming, barrel-shaped animals, solitary or united into chains or hollow cylinders; or, lastly, of minute free-swimming tadpole-shaped organisms. To explain briefly the structure of a Tunicate, Ascidia mentula (Fig. 1), is selected. The animal, which lives on a muddy bottom, in from five to twenty fathoms, resembles a conical sac fixed by the broader end, of grayish green colour and about 4 inches in height. At the narrower end are two orifices, one terminal—the branchial orifice or mouth, and the other a little lower—the atrial orifice: the former has eight lobes and the latter six.

When the Ascidian is undisturbed, the orifices are wide open, and currents enter by the branchial and leave by the atrial orifice. On the least alarm, the orifices close, jets of water being at the same time squirted out; hence the popular name “Sea-squirts” given to these animals.

Fig. 2.
Diagrammatic section of Ascidia representing the three sacs, and the branchial sac as the pharynx or throat.
a, branchial; and b, atrial orifice; c, tunic or test; d, mantle; e, branchial sac; f, gullet; g, stomach; h, anal orifice; i, dorsal lamina; dotted line indicates the endostyle.

The Ascidian is orientated as follows: hold the animal with the branchial orifice pointing forwards and the atrial upwards; the branchial orifice will be anterior and the opposite end posterior; the atrial orifice will lie on the upper or dorsal aspect, the opposite aspect being lower or ventral, and the sides right and left. The aspects, in fact, correspond with those of a vertebrate animal. A vertical section roughly shows the animal to be formed of three concentric sacs (Figs. 2, 3). The outermost, which is tough and membranous, is called the Test or Tunic, the whole group owing its name to the presence of this protective covering.

Fig. 3.
Diagrammatic dissection of A. mentula.
at, atrial orifice; br, branchial orifice; a, anal orifice; brs, branchial sac; dl, dorsal lamina; end, endostyle; m, mantle; ng, nerve ganglion; oea, orifice of gullet; pbr, peribranchial cavity; st, stomach; t, test; tn, tentacles.
(After Herdman: Tunicata, Encyc. Britannica.)

The middle sac, termed the Mantle, which almost corresponds in shape to the outer, is composed of connective tissue, muscle-fibres, blood-vessels, etc.; in spirit specimens, the mantle is shrunk away from the test except at the orifices and at a point behind, where vessels enter the test.

The innermost or Branchial Sac is attached behind the branchial orifice and along the ventral edge, but otherwise hangs free in the interior, the space around and outside of the sac being termed the atrial or peribranchial cavity.

The delicate walls of the branchial sac, which resemble fine muslin, are perforated by innumerable vertical slits, termed stigmata, arranged in transverse rows (Fig. 4).

Fig. 4.
Ascidia mentula. Part of wall of branchial sac showing stigmata. Magnified.

The margins of the stigmata are lined with cilia which set up currents; and the water which enters by the branchial orifice, passes through the stigmata into the atrial cavity, and thence out through the atrial orifice. The walls of the branchial sac are chiefly composed of a sieve-like meshwork of fine blood-vessels arranged in transverse and longitudinal rows. The currents of water passing through the stigmata aërate the blood in the vessels. Besides the stigmata, the branchial sac has two relatively large orifices, viz., the branchial orifice or mouth, and, at the opposite end, the opening into the gullet. The branchial sac is, in fact, a capacious throat or pharynx (Diagram Fig. 2 and Fig. 14). Inside the branchial orifice is a circle of fine tentacles, which guard the entrance to the branchial sac. The food of the animal consists of minute animal and vegetable organisms.

It may be wondered how this food is secured, seeing that the currents of water are continually passing through the sieve-like walls of the branchial sac to the exterior again. Within the branchial orifice and above the branchial sac are two circular ciliated ridges with a groove between, which is full of viscid secretion; the cilia on the ridges direct particles into the groove where they are retained by the mucus.

Passing backwards along the ventral edge of the branchial sac is a thick-lipped furrow, which appears like a rod in the thin-walled sac, and hence is called the endostyle. This organ secretes the mucus which is carried up by ciliary action to the circular groove in front of the branchial sac, and thence to the gullet along a fold or crest, termed the dorsal lamina, situated along the dorsal edge of the branchial sac.

The gullet opens into a large stomach situated posteriorly on the left side of the branchial sac. The stomach opens into the intestine, which, after forming a loop, terminates in the anal orifice or vent opening into the atrial cavity.

The tubular heart lies below the stomach, a remarkable feature in the circulation consisting in the periodic reversal of the blood current. An elongated nerve ganglion is situated between the branchial and atrial orifices.

Fig. 5.
Ascidian Tadpole with part only of the tail C. Magnified section.
N, nervous system with enlarged brain in front and narrow spinal cord behind n; N′, cavity of brain; O, the single cerebral eye lying in the brain; a, auditory organ; K, pharynx; d, intestines; o, rudiment of mouth; ch, notochord or primitive backbone.
(From Gegenbaur’s ‘Elements of Comparative Anatomy.’)

Ascidia mentula is hermaphrodite. The egg develops into a minute tadpole-like larva which swims about by means of its tail. Water entering by the mouth passes out through the gill-slits. A nerve-tube extending along the back and tail is swollen in front into a brain-vesicle; and underneath the long nerve-tube behind the brain is a stiff skeletal rod or axis—the notochord—which constitutes the rudiment of a backbone. Inside the brain are two unpaired sense organs, an eye and an organ of hearing (Fig. 5). After swimming freely for a few hours, the larva settles down head foremost and fixes itself by papillæ on the anterior end (Figs. 6, 7). Presently the tail becomes absorbed, and the posterior end of the nerve-tube, and the brain with its eye and hearing organ, undergo atrophy, the nerve ganglion of the adult alone representing the cerebrospinal axis of the larva. The branchial sac and intestines develop greatly, and growth proceeds in such a manner that the mouth is pushed round to a position opposite to the fixed area, and gradually the animal becomes the adult ascidian.

Fig. 6.
Degeneration of Ascidian Tadpole to form the adult. The black pieces represent the rock or stone to which the Tadpole has fixed its head.

Fig. 7.
Very young Ascidian with only two gill-slits.
(Figs. 6, 7, from Lankester’s ‘Degeneration.’)

This wonderful metamorphosis presents a striking example of DEGENERATION resulting from the adoption of a fixed mode of life. The active free-swimming larva with its brain, eye, hearing organ, and muscular tail becomes transformed into a comparatively inert sac.

Fig. 8.
Tadpole of Frog and Ascidian. Surface view. (Lankester’s ‘Degeneration.’)

Fig. 9.
Tadpole of Frog and Ascidian. Diagram representing the chief internal organs. (Lankester’s ‘Degeneration. A chapter in Darwinism.’)

The tadpole of an Ascidian resembles that of a frog (Figs. 8, 9), not merely superficially, but also in its general structure and mode of development. The Tunicata are now generally regarded as a degenerate offshoot from the ancestral stock of the Vertebrata, in that the larva possesses a skeletal rod (rudimentary backbone) separating the dorsally situated nerve-tube (cerebrospinal axis) from the ventrally situated intestinal tube, the existence of the cerebral eye in the Ascidian tadpole further tending to confirm the truth of this theory. Apart from a knowledge of the course of their development, Tunicata would have been classed among the Invertebrata, but the structure of the larva clearly reveals the affinities of the group to the backboned animals.

Ascidia mentula belongs to the group of Simple Ascidians which are all fixed, and are either solitary or joined into colonies in which each individual or ascidiozooid has a distinct test of its own. In the Compound Ascidians, which form colonies by budding, the ascidiozooids are buried in a common investing mass and have no separate tests. In a third group, the Salpa-like Ascidians, the ascidiozooids are united to form free-swimming colonies shaped like hollow cylinders open at one end. The above three groups belong to one great Order—the Ascidiacea. A second Order, Thaliacea, includes the free-swimming Salpa and Doliolum, which exhibit alternation of generations in their life history. A third Order Larvacea, includes very minute free-swimming forms which possess a tail in the adult stage. There are sixteen families of Tunicata.

The following is a tabular view of Prof. Herdman’s classifications:—

Order I. Ascidiacea.	Sub-order 1.	Ascidiæ	Simplices, 4 Families.
	Sub-order 2.	„	Compositæ, 7 Families.
	Sub-order 3.	„	Salpiformes, 1 Family.
Order II. Thaliacea	3 Families.
Order III. Larvacea	1 Family.