Chapter V: Introduction: Outline of History—structure of a Typical Ascidian—embryology (3)

Free-swimming pelagic forms which exhibit alternation of generations in their life-history, and in the sexual condition form colonies. The body is more or less fusiform, with the long axis antero-posterior, and the branchial and atrial apertures nearly terminal and opposite. The test is well developed but transparent. The musculature of the body-wall is in the form of a series of transversely-running bands which do not usually form complete independent rings as in the CYCLOMYARIA. These partially-encircling muscles in the Salpidae (see Fig. 61, _m.b_) are probably to be regarded as modified branchial and atrial sphincters which have spread over the intervening body. The branchial and peribranchial (cloacal) cavities form a continuous space in the interior of the body, opening externally at the ends by the branchial and atrial apertures, and traversed obliquely from the dorsal and anterior to the ventral and posterior end by a long narrow vascular ciliated band, which represents the dorsal lamina, the dorsal blood-sinus, and the neighbouring parts of the dorsal edge of the branchial sac of an ordinary Ascidian. The alimentary canal is placed ventrally. It may either be stretched out so as to extend for some distance anteriorly, or, as is more usual, be concentrated to form along with the testis a rounded opaque mass near the posterior end of the body, known as the visceral mass or "nucleus." The embryonic development is direct, no tailed larva being formed. The embryo is united to the parent for a time by a "placenta."

This sub-order contains, in addition to its typical members, the SALPIDAE, another still somewhat problematical family the OCTACNEMIDAE, including a single very remarkable deep-water genus (_Octacnemus_), which in some respects does not conform with the characters given above, and exhibits a certain amount of affinity with the primitive fixed forms from which Salpidae have been derived.

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OCCURRENCE AND REPRODUCTION.—The family SALPIDAE[108] includes the single genus _Salpa_, Forskål, which, however, may be divided into two well-marked groups of species—(1) those such as _S_. (_Cyclosalpa_) _pinnata_, in which the alimentary canal is stretched out ("ortho-enteric" condition) along the ventral surface of the body, and (2) those such as _S. runcinata-fusiformis_, in which the alimentary canal forms a compact globular mass (Fig. 61, _v_), the "nucleus" ("caryo-enteric" condition), near the posterior end of the body. About fifteen species altogether are known; they are all pelagic in habit, and are found in nearly all seas. Each species occurs in two forms (Fig. 61, A and B), the solitary asexual (_proles solitaria_), and the aggregated sexual (_proles gregaria_), which are in most species quite unlike one {103}another, the aggregated form being usually more rounded, ovoid, or fusiform (Fig. 61, A), and the solitary more quadrangular, and often provided with conical processes or projecting points.

The solitary form gives rise, by gemmation at the posterior end of the endostyle (Fig. 63), to a complex tubular stolon, containing processes from the more important organs of the parent-body, which give rise to an endodermal tube, two peribranchial tubes, a neural tube, two blood-sinuses and mesoblast cells, a genital cord, and over all the ectodermal covering (see Fig. 64). This stolon becomes segmented (Fig. 63) into a series of buds or young "chain" individuals, of which there may be several hundreds. As the stolon elongates (Fig. 61, B, _st″_), the buds {104}undergo lateral shifting, and rotation round their longitudinal axis, so as to acquire the relations seen in the "chain," which then emerges from the tube in the test through which it has been growing, so as to project to the exterior near the atrial aperture. The buds at its free end which have now become far advanced in their development are set free in groups, which remain attached together by processes of the test, each enclosing a diverticulum from the body-wall (Fig. 62), so as to form "chains." Each member of the chain is a _Salpa_ of the sexual or aggregated form, and when mature may—either still attached to its neighbours or separated from them—produce one or several embryos (Fig. 61, A, _emb_), which develop into the solitary form of _Salpa_. Thus the two forms, different in appearance and structure and different in mode of origin, alternate regularly in the life-history of _Salpa_.

STRUCTURE.—The more important points in the structure of a typical _Salpa_ are shown in Fig. 65. The branchial and atrial apertures are at opposite ends of the body, and lead into large cavities, the branchial and peribranchial sac respectively, which are in free communication at the sides of the obliquely-running dorsal lamina or "gill" (_d.l_). The transparent test is usually thick, and varies from a gelatinous to a stiff cartilaginous condition; it adheres closely to the surface of the mantle (ectoderm and body-wall). The muscle-bands (from 4 to about 20—usually 8 or 10) of the mantle do not in most cases completely encircle the body. They are present dorsally (Fig. 65, _mus.bds_) and laterally, but the majority do not reach the ventral surface. In many cases neighbouring bands join in the median dorsal line (Fig. 61). The muscle fibres are striated, and have rows of large equidistant nuclei. The anterior end of the dorsal lamina is in some cases prolonged to form a prominent tentacular organ, the languet or dorsal tentacle, projecting into the branchial sac, while near this opens a ciliated funnel corresponding to the dorsal tubercle, but having no connexion in the adult with either ganglion or subneural gland. The conjoined ganglion and subneural gland, the dorsal lamina, the peripharyngeal bands and the endostyle are placed in the usual positions. Eyes in the form either of a continuous horse-shoe-shaped pigmented ridge on the dorsal surface of the ganglion immediately below the ectoderm, or of one larger median and several smaller lateral ocelli are found in the various species of _Salpa_. These eyes have in {105}most cases a retina formed of elongated cells, and a pigment-layer placed upon the ganglion.

The so-called otocysts of _Salpa_ have been shown by Metcalf to be really glandular organs. They have been called lateral neural glands; they do not open at the dorsal tubercle, but separately into the pharynx. These lateral neural tubular glands have also been regarded as nephridia.

The large spaces at the sides of the dorsal lamina (often called the gill or branchia of _Salpa_), by means of which the cavity of the branchial sac is placed in free communication with the peribranchial cavity, are to be regarded as gigantic gill-slits formed by the suppression of the lateral walls and small stigmata of the branchial sac. The alimentary canal at the posterior end of the "gill" consists of oesophagus, stomach, and intestine, with a pair of lateral gastric glands or caeca. These viscera along with the reproductive organs, when present, make up the "nucleus" (Fig. 66, _v_).

ALTERNATION OF GENERATIONS.—Fig. 66 represents an aggregated or sexual _Salpa_, which was once a member of a chain, since it shows a testis and a developing embryo. The ova (always few in number, usually only one) appear at a very early period in the developing chain _Salpa_, while it is still a part of the gemmiparous stolon in the body of the solitary _Salpa_. This gave rise to the view put forward first by Brooks that the ovary {106}really belongs to the solitary stolon-bearing _Salpa_, which is therefore a female producing a series of males by asexual gemmation, and depositing in each of these an ovum, which will afterwards, when fertilised, develop in the body of the male into a solitary or female _Salpa_. This idea, if adopted, would profoundly modify our conception of _Salpa_ as an example of a life-history showing alternation of generations, but it seems to me to give a distorted view of the sequence of events. The fact that the stolon while in the solitary _Salpa_ contains, along with representatives of other important systems of the body, a row of germinal cells, does not constitute that solitary _Salpa_ the parent of the ova which these germinal cells will afterwards become in the body of an independent bud. We must regard as the parent the body in which the ova become mature and fulfil their function. The sexual or chain _Salpa_, although really hermaphrodite in its life-history, is usually[109] protogynous, _i.e._ the ova mature at an earlier period than the male organ or testis. This prevents self-fertilisation. The ovum is presumably fertilised by the spermatozoa of an older _Salpa_ belonging to another chain, and the embryo is far advanced in its development before the testis is formed. The development takes place inside the body of the parent, and is "direct"—no tailed larval form being produced.

DEVELOPMENT AND LIFE-HISTORY.—The segmentation of the egg is holoblastic, and gives rise to a number of blastomeres, {107}which are for a time masked by the phenomenal activity of certain cells of extraneous origin, the "kalymmocytes," derived from the follicular epithelium surrounding the ovum. These follicular kalymmocytes migrate into the ovum, surround groups of blastomeres, and arrange themselves so as to reproduce the essential structure of the future embryo for which they form what may be termed a scaffolding or temporary support. After a time the blastomeres become active, proliferate rapidly, and finally press upon and absorb the kalymmocytes, and so eventually take their proper place in building up the organs. Some observers regard the kalymmocytes as being passive and nutritive only in function.

At an early period in the development a part of the surface of the embryo, on its ventral edge, becomes separated off, along with a part of the wall of the cavity ("oviduct"—a diverticulum from the atrium) in which it lies, to form the "placenta" (Fig. 67, _pl_) in which the embryonic and maternal blood-streams circulate in close proximity, and so allow of the conveyance of nutriment to the developing embryo by means of large migrating placental cells. At a somewhat later stage a number of cells placed at the posterior end of the body alongside the future nucleus become filled up with oil-globules to form a mass of nutrient material—the "elaeoblast" (Fig. 67, _ebl_)—which is used up later in the development. Many suggestions have been made as to the homology and meaning of the elaeoblast; but it may now be regarded as most probable that it is reserve food-material associated with the disappearing rudiment of the {108}tail found in the larval condition of most Ascidians. The development is direct; and it may be said, then, that this young asexual (solitary) _Salpa_ differs from the corresponding form in the life-history of _Doliolum_ (Fig. 60, A) in that its tail is no longer a locomotory organ, but is represented by a nutritive mass, the elaeoblast, while the body, in place of being free, is attached by its ventral surface to a special organ of nutrition—the "placenta"—in connexion with the blood-stream of the parent.

This embryo sexually produced inside the body of an aggregated form becomes a solitary _Salpa_ (such as Fig. 61, B), which differs in appearance, structure, and habits from its parent, and has no reproductive organs. After swimming for a time, however, it develops the ventral stolon on which buds form which are eventually sexual Salpae. These are set free from the solitary form in sets, still connected together, and they may swim about together for a time as a chain of aggregated Salpae before separating to become the adult sexual individuals (such as Fig. 61, A).

CLASSIFICATION.—_Salpa_ may be divided into the following subgenera:[110]—_Cyclosalpa_, Blainville, in which the alimentary canal is ortho-enteric, and the "chain" consists of individuals united in a circle; _Iasis_, Savigny, with several embryos formed at a time; and _Pegea_, Sav., _Thalia_, Blumenbach, and _Salpa_, Forskål, all with one embryo only, and differing from one another in the condition of the "gill" and other details: all except _Cyclosalpa_ have the alimentary canal caryo-enteric. _Cyclosalpa_ has three species, the best known of which is _C. pinnata_ of the Mediterranean, a form possessing light-producing organs like those of _Pyrosoma_, but placed along the sides of the body. _Salpa_ has four or five species, one of which, _S. runcinata-fusiformis_ (Fig. 61), has occasionally been found in British seas; _Thalia_ includes the species _T. democratica-mucronata_, which has been sometimes obtained in swarms in the Hebridean seas, or cast ashore on our southern or western coasts; _Pegea_ has the species _P. scutigera-confoederata_; and _Iasis_ contains the remaining half-dozen species, the best known of which is _I. cordiformis-zonaria_, the only other Salpian which has been found in British seas.

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The family OCTACNEMIDAE includes the single remarkable genus _Octacnemus_, now known in a solitary and an aggregated form. It was found during the "Challenger" expedition, and was first described by Moseley. It is apparently a deep-sea representative of the pelagic Salpidae, and may possibly be fixed at the bottom. The body in the solitary form is somewhat discoid, with its margin prolonged to form eight tapering processes, on to which the muscle-bands of the mantle are continued. The alimentary canal forms a compact nucleus, which is attached to an apparently imperforate membrane which stretches across the body, separating the branchial from the atrial cavities. The endostyle is very short, and the dorsal lamina is also much reduced. The reproduction and life-history are entirely unknown. The aggregated form consists of a small number of individuals united by a slender cord composed of test, body-wall, and endodermal tissue. _Octacnemus_ has been found[111] in the South Pacific from depths of 1070 and 2160 fathoms, and off the Patagonian coast from 1050 fathoms. Two species have been described: _O. bythius_, Moseley, and _O. patagoniensis_, Metcalf. Metcalf, who has recently investigated the aggregated form (_O. patagoniensis_), considers that the genus is more nearly related to the Clavelinidae than to the Salpidae. Possibly its position might be best {110}indicated by a line diverging from near the point (3) in the phylogenetic diagram below.

GENERAL CONCLUSIONS.

The following diagram is a graphic representation of the genetic affinities, or what is now generally supposed to have been the probable course of phylogeny of the Tunicata. It will be noticed that it shows (1) the Proto-Tunicates arising from Proto-Chordata, not far from the ancestors of Amphioxus (see also, this vol. p. 112); (2) that the Larvacea are regarded as the most primitive section of the group; (3) that the Thaliacea (Doliolidae and Salpidae) are supposed to be derived not directly from primitive pelagic forms, but through the early fixed Ascidians, not far from (4) the ancestral compound Ascidians, which gave rise to the Pyrosomatidae; (5) that the Ascidiidae and other higher Simple Ascidians are derived, like the Compound Ascidians, from ancestral Clavelinidae; and (6), that the Ascidiae Compositae are polyphyletic, the Holosomata (Botryllidae and Polystyelidae) being derived from ancestral Simple Ascidians independently of the Merosomatous families.

↑
| Molgulidae
| Ascidiidae |
+––––––––Amphioxus (5) | |
P | +–––––––+–––––+–––––+––––––+––– Cynthiidae.
r | | | |
o | +– C | |
t | Larvacea. | l | (6) Polystyelidae
o | | | a |
c | (1) (2) | +– v Botryllidae
h +––––––––––––––+––––––––––––| e
o | Proto–Tunicata. | l
r | +– i Distomatidae
d | Doliolidae–+ | n |
a | | | i | Didemnidae and
t | | | d | | Diplosomatidae
a | | | a | |
| | (3)| e (4) | |
| +–––––––+–––––––+––––+––––––––––+––––Polyclinidae
| Salpidae–––+ |
| Pyrosomatidae

The Tunicata are remarkable for the variety in appearance, structure, and life-history which they present. No group illustrates in a more instructive manner so large a number of important biological principles and phenomena. They show solitary and colonial forms, fixed and free, pelagic and abyssal. The development is in some cases larval and with metamorphosis, in others abbreviated and direct. Persistent traces of ancestral characters are seen in the embryonic and larval stages, while the adults present the most varied secondary adaptations to littoral, {111}pelagic, and deep-sea, free-swimming and sessile modes of existence. In the details of their classification they demonstrate both stable and variable species, monophyletic and polyphyletic groups. They exhibit the phenomena of gemmation and of embryonic fission, of polymorphism, hibernation, alternation of generations, and change of function. They have long been known as a stock example of degeneration; but in fact they lend themselves admirably to the exposition of more than one "Chapter of Darwinism."

* * * * *

NOTE TO P. 78.—_Oligotrema_, Bourne (_Quart. J. Micr. Sci_. xlvii. Pt. ii. 1903, p. 233), a Molgulid from the Loyalty Islands, has a reduced branchial sac and greatly developed pinnate, muscular branchial lobes, probably used in capturing food.

{112}CHAPTER IV

CEPHALOCHORDATA