bands, and the endostyle are placed in the usual positions. On
each side of the anterior end of the branchial sac, close to the
peripharyngeal bands, is a mass of rounded gland cells which are
the source of the phosphorescence. The alimentary canal is placed
posteriorly to the bronchial sac, and the anus opens into a large
peribranchial (or atrial) cavity, of which only the median posterior
part is shown (p.br.) in fig. 33. The reproductive organs are developed
in a diverticulum of the peribranchial cavity, and consist of a lobed
testis and a single ovum at a time. The development takes place
in a part of the peribranchial cavity (Fig. 32, em). The segmentation
Development of Pyrosoma.
is meroblastic, and an elongated embryo is formed on
the surface of a mass of yolk. The embryo, after the
formation of an alimentary cavity, a tubular nervous
system, and a pair of laterally placed atrial tubes, divides
into an anterior and a posterior part. The anterior part then segments
into four pieces, which afterwards develop into the first ascidiozooids
of the colony, while the posterior part remains in a rudimentary
condition, as the “cyathozooid”; it eventually atrophies. As the
four ascidiozooids increase in size, they grow round the cyathozooid
and soon encircle it (fig. 32, asc and cy). The cyathozooid
absorbs the nourishing yolk upon which it lies, and distributes it
to the ascidiozooids by means of a heart and system of vessels which
have been meanwhile formed. When the cyathozooid atrophies
and is absorbed, its original atrial aperture remains and deepens
to become the central cavity of the young colony, which now consists
of four ascidiozooids placed in a ring, around where the cyathozooid
was, and enveloped in a common test. The colony gradually
increases by the formation of buds from these four original ascidiozooids.
| |||||||||||||||||||
| (Partly after Savigny.) | |||||||||||||||||||
|
Fig. 32.—Part of a Longitudinal Section through wall of Pyrosoma, showing arrangement of ascidiozooids, magnified. | |||||||||||||||||||
|
| |||||||||||||||||
| (Partly after Keferstein.) | |||||||||||||||||
|
Fig. 33.-Mature Ascidiozooid of Pyrosoma, from left side. | |||||||||||||||||
|
Phylogeny
The accompanying diagram (fig. 34) shows graphically the probable origin and course of evolution of the various groups of Tunicata, Phylogeny. and therefore exhibits their relations to one another much more correctly than any system of linear classification can do. The ancestral Proto-Tunicata are here regarded[1] as an offshoot from the Proto-Chordata—the common ancestors of the Tunicata (Urochorda), Amphioxus (Cephalochorda) and the Vertebrata. The ancestral Tunicata were probably free-swimming forms, not very unlike the existing Apendiculariidae, and are represented in the life-history of nearly all sections of the Tunicata by the tailed larval stage. The Larvacea are the first offshoot from the ancestral forms which gave rise to the two lines of descendants, the Proto-Thaliacea and the Proto-Ascidiacea. The Proto-Thaliacea then split into the ancestors of the existing Cyclomyaria and Hemimyaria. The Proto-Ascidiacea gave up their pelagic mode of life and became fixed. This ancestral process is repeated at the present day when the free-swimming larva of the simple and compound Ascidians becomes attached. The Proto-Ascidiacea, after the change, are probably most nearly represented by the existing genus Clavelina. They have given rise directly or indirectly to the various groups of simple and compound Ascidians and the Pyrosomidae. These groups form two lines, which appear to have diverged close to the position of the family Clavelinidae. The one line leads to the more typical compound Ascidians, and includes the Polyclinidae, Distomidae, Didemnidae, Diplosomidae, Coelocormidae, and finally the Ascidiae Luciae or Salpiformes. The second line gave rise to the simple Ascidians, and to the Botryllidae and Polystyelidae, which are, therefore, not closely allied to the other compound Ascidians. The later Proto-Ascidiacea were probably colonial forms, and gemmation was retained by the Clavelinidae and by the typical compound Ascidians (Distomidae, &c.) derived from them. The power of forming colonies by budding was lost, however, by the primitive simple Ascidians, and must, therefore, have been regained independently by the ancestral forms of the Botryllidae and the Polystyelidae. If this is a correct interpretation of the of evolution of the Tunicata, we arrive at the following important conclusions. (1) The Tunicata, as a whole, form a degenerate branch of the Proto-Chordata; (2) the Ascidiae Luciae (Pyrosoma) are much more closely related to the typical compound Ascidians than to the other pelagic Tunicata, viz. the Larvacea and the Thaliacea; and (3) the Ascidiae Compositae, form a polyphyletic group the sections of which have arisen at several distinct points from the ancestral simple Ascidians.
Fig. 34.
Bibliography.—(1) Cuvier, “Mém. s. les Ascidies,” &c., in Mém. d. Mus. ii. 10 (Paris, 1815); (2) Savigny, Mémoires sur les animaux sans vertèbres, pt. ii. fasc. i. (Paris, 1816); (3) Lamarck, Hist. nat. d. anim. sans vertèbres (1st ed., Paris, 1815-1823); (4) O. F. Müller, Zool. danica. (1806), vol. iv.; (5) Milne-Edwards, “Observ. s. les Ascidies Composées,” &c., in Mém. Acad. Sci. vol. xviii. (Paris, 1842); (6) Schmidt, Zur vergl. Physiol. d. wirbellos. Thiere (Brunswick, 1845); (7) Löwig and Kölliker, “De la Compos., &c., d. Envel. d. Tun.,” in Ann. Sc. Nat., 1846 (Zool.), 3rd series, vol. v.; (8) Huxley, Phil. Trans. (1851); (9) Kowalevsky, “Entwickel. d. einf. Ascid.,” in Mem. St Petersb. Acad. Sc. (1866), 7th series, vol. x.; (10) J. P. van Beneden, “Rech. s. l'Embryolog., &c., d. Asc. Simp.,” in Mém. acad. roy. belg. (1847), vol. xx.; (11) Krohn, in Wiegmann and
- ↑ By Dohrn and others their point of origin is placed considerably farther up on the stem of the Chordata, thus causing the Tunicata to be regarded as very degenerate Vertebrata (see 31).
