blood, which ramify through the common test and serve to connect
the vascular systems of the various members of the colony, have
numerous large ovate dilatations, the ampullae, upon their terminal
twigs (fig. 29). Various functions have been assigned to these
ampullae in the past, and Bancroft has shown that in addition to
acting as storage reservoirs for blood, organs for the secretion of
test matrix, and accessory organs of respiration, they are also organs
for blood propulsion. The ampullae execute co-ordinated pulsations,
the co-ordination being due to variations in the blood-pressure.
It was actually found that the ampullae could keep up the circulation
for some time in a portion of a colony independently of the
hearts of the ascidiozooids. All the hearts in a colony of Botryllus
contract simultaneously and in the same direction. The reversal
of the circulation may be regarded as due to the engorgement of
the ampullae in the superficial parts of the colony. These when
distended overcome the resistance of the heart's action, and cause
it to stop and then reverse.
Family VII., Polystyelidae.—Ascidiozooids not grouped in systems. Branchial and atrial apertures four-lobed. Branchial sac may be folded; internal longitudinal bars present. The chief genera are: Thylacium (Carus), with ascidiozooids projecting above general surface of colony; Goodsiria (Cunningham), with ascidiozooids completely imbedded in investing mass; and Chorizocormus (Herdman), with ascidiozooids united in little groups which are connected by stolons. Several of the species show transitions between the other Polystyelidae and the Styelinae amongst simple Ascidians.
Gemmation and Growth of Colonies.—A number of new observations have been made in recent years upon the budding of compound Ascidians, some of which are very puzzling and contradictory in their results. Metschnikoff, Kowalevsky, Giard, Hjort, Pizon, Seeliger, Ritter, van Beneden and Julin have all in turn added to our knowledge of the details of development and life-history, of the various processes of gemmation and of the formation of colonies. It is impossible as yet to reconcile all the conflicting accounts, but the following points at least seem pretty clear.
Gemmation may be very different in its details in closely related compound Ascidians. There are, however, two main types of budding, to one or other of which most of the described methods may be referred. There is first the “stolonial” or “epicardiac” type, seen in the Chalarosomata, typically in Distomidae and Polyclinidae, and comparable with the gem mat ion in Clavelinidae, Pyrosomidae and Thaliacea outside this group. Secondly, there is the “parietal” or “peribranchial” type, seen in the Pectosomata, typically in the Botryllidae. The remarkable process of gemmation seen in the families Didemnidae and Diplosomidae may probably be regarded as a modification of the stolonial type. The double embryo in the Diplosomidae is probably to be intenzpreted as precocious budding (rather than as embryonic fission), due to acceleration in development (tachygenesis). The type of budding, and even details such as the length of the stolon, have much to do with differences in the nature and appearance of the colonies produced. The stolon, which has a wall continuous with the body-wall of the parent, contains an endodermal element in the form of the so-called “epicardium,” and also a prolongation of the ovary, or at least a string of migrating germ-cells, so that the reproductive elements are also handed on. Still, it is clear from recent researches that the development of the bud (blastozooid) and that of the embryo (oozooid) do not proceed along parallel lines. It is impossible to harmonize the facts of gemmation with the germ-layer theory, and attempts to explain budding in Ascidians as a process of regeneration, by which the organs of the parent or their germ-layers give rise to the corresponding organs in the bud, have signally failed.
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(After Pizon.) | ||||||||||
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Fig. 30.-Young buds of Botryllus sectioned to show the separation of the branchial (vb) from the peribranchial (cp) cavities. | ||||||||||
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Figs. 29 and 30 show the buds in the Botryllidae, after Pizon, who has followed day by day the changes of growth in young colonies of Botryllidae, tracing the rise of successive generations of buds and the degeneration of their parents. The buds are parietal, arising from the walls of the peribranchial cavities (fig. 29), and at an early period they acquire the structure shown in fig. 30, where there are two vesicles undergoing further subdivision and differentiation, but investigators still differ as to whether the inner, which gives rise to the branchial sac and alimentary canal, is not produced along with the outer from the ectoderm of the parent.
A remarkable case of polymorphism has been found by M. Caullery in the buds of the compound Ascidian Colella. Some of the buds Reproduction by Gemmation and the Formation of Colonies. have an abundant store of reserve materials in their outer layer of cells, while others are without this supply. The former are placed deeply in the stalk, develop slowly, and probably serve to regenerate the colony when the head portion has been removed or has died down. In these cases where the ectoderm has taken on the function of storing the reserve material, it is found that all the organs of the bud are formed from the cells of the endodermic vesicle. The first ascidiozooid of the colony produced by the tailed larva does not form sexual reproductive organs, but reproduces by gemmation so as to make a colony. Thus there is alternation of generations in the life-history. In the most completely formed colonies (e.g. Botryllus) the ascidiozooids are arranged in groups (systems or coenobii), and in each system are placed with their atrial apertures towards one another, and all communicating with a common cloacal cavity which opens to the exterior in the centre of the system (fig. 28, D).
Sub-Order 3.-Ascidiae Luciae.
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Fig. 31.—Pyrosoma elegans. |
| A, Side view of entire colony. |
| B, End view of open extremity. |
Free-swimming pelagic colonies having the form of a hollow cylinder closed at one end. The ascidiozooids forming the colony are Ascidiae Luciae. embedded in the common test in such a manner that the branchial apertures open on the outer surface and the atrial apertures on the inner surface next to the central cavity of the colony. The ascidiozooids are produced by gemmation from a rudimentary larva (the cyathozooid) developed sexually.
This sub-order includes a single family, the Pyrosomidae, containing Structure of Pyrosoma. one well-marked genus, Pyrosoma (Péron), with half a dozen species. They are found swimming near the surface of the sea, chiefly in tropical latitudes, and are brilliantly phosphorescent. A fully developed Pyrosoma colony may be from an inch or two to upwards of twelve feet in length. The shape of the colony is seen in fig. 31. It tapers slightly towards the closed end, which is rounded. The opening at the opposite end is reduced in size by the presence of a membranous prolongation of the common test (fig. 31, B). The branchial apertures of the ascidiozooids are placed upon short papillae projecting from the general surface, and most of the ascidiozooids have long conical processes of the test projecting outwards beyond their branchial apertures (figs. 31, 32 and 33). There is only a single layer of ascidiozooids in the Pyrosoma colony, as all the fully developed ascidiozooids are placed with their antero-posterior axes at right angles to the surface and communicate by their atrial apertures with the central cavity of the colony (fig. 32). Their dorsal surfaces are turned towards the open end of the colony. The more important points in the structure of the ascidiozooid of Pyrosoma are shown in fig. 33. A circle of tentacles, of which one, placed ventrally (fig. 33, tn), is larger than the rest, is found just inside the branchial aperture. From this point a wide cavity, with a few circularly placed muscle bands running round its walls, leads back to the large branchial sac, which occupies the greater part of the body. The stigmata are elongated transversely and crossed by internal longitudinal bars. The dorsal lamina is represented by a series of eight languets (l). The nerve ganglion (on which is placed a small pigmented sense organ), the subneural gland, the dorsal tubercle, the peripharyngeal
