far back on the ventral surface, one on each side of the
middle line, and lead into short funnel-shaped tubes which
open on the surface of the body behind the anus (fig. 18, at).
These tubes correspond to the right and left atrial involutions
which, in an ordinary Ascidian, fuse to form the peribranchial
cavity. The heart, according to Lankester, is formed of two
cells, which are placed at the opposite ends and connected by
delicate contractile protoplasmic fibrils. The large ovary and testis
are placed at the posterior end of the body. The remainder of the
structural details can be made out from figs. 18 and 19.
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| Fig. 18.—Semi-diagrammatic view of Appendicularia from the
right. | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Fig. 19.—Transverse Section of Oikopleura; anterior part of body and tail. | ||||||||||||||||||||
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The family Appendiculariidae comprises amongst others the following genera: Oikopleura (Mertens), and Appendicularia (Cham.), in both of which the body is short and compact and the tail relatively long, while the endostyle is straight; Megalocercus (Chun) containing M. abyssorum, a huge deep-sea form from the Mediterranean (30 mm. long); Fritillaria (Quoy and Gaimard), in which the body is long and composed of anterior and posterior regions, the tail relatively short, the endostyle recurved, and an ectodermal hood is formed over the front of the body; and Kowalevskia (Fol), a remarkable form described by Fol (14), in which the heart and endostyle are said to be absent, while the branchial sac is provided with four rows of ciliated tooth-like processes.
Order II.—Thaliacea
Free-swimming pelagic forms which may be either simple or compound, and the adult of which is never provided with a tail or Thaliacea. a notochord. The test is permanent and may be either well developed or very slight. The musculature of the mantle is in the form of more or less complete circular bands, by the contraction of which locomotion is effected. The branchial sac has either two large or many small apertures, leading to a single peribranchial cavity, into which the anus opens. Blastogenesis takes place from a ventral endostylar stolon. Alternation of generations occurs in the life-history, and may be complicated by polymorphism. The Thaliacea comprises two groups Cyclomyaria and Hemimyaria.
Sub-order 1.—Cyclomyaria.
Free-swimming pelagic forms which exhibit alternation of generations in their life-history but never form permanent colonies. The Characters of Cyclomyaria. body is cask-shaped, with the bronchial and atrial apertures at the opposite ends. The test is more or less well developed. The mantle has its musculature in the form of circular bands surrounding the body. The branchial sac is fairly large, occupying the anterior half or more of the body. Stigmata are usually present in its posterior part only. The peribranchial cavity is mainly posterior to the branchial sac. The alimentary canal is placed ventrally close to the posterior end of the branchial sac. Hermaphrodite reproductive organs are placed ventrally near the intestine.
This group forms one family, the Doliolidae, including three genera, Doliolum (Quoy and Gaimard), Dolchinia (Korotneff) and Anchinia (C. Vogt).
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Fig. 20.—Doliolum denticulatum, sexual generation, from the left side. Lettering as for fig. 18. | ||||||||||||||||
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Doliolum, of which about a dozen species are known from various seas, has a cask-shaped body, usually from 1 to 2 cm. in length. The terminal bronchial and atrial apertures (fig. 20) are lobed and the lobes are provided with sense organs. The test is very slightly developed and contains no cells. The mantle has eight or nine circular muscle bands surrounding the body. The most anterior and posterior of these form the bronchial and atrial sphincters. The wide branchial and atrial apertures lead into large branchial and peribranchial cavities, separated by the posterior wall of the branchial sac, which is pierced by stigmata; consequently there is a free passage for the water through the body along its long axis, and the animal swims by contracting its ring-like muscle bands, so as to force out the contained water posteriorly. Stigmata may also be found on the lateral walls of the branchial sac, and in that case there are corresponding anteriorly directed diverticula of the peribranchial cavity. There is a distinct endostyle on the ventral edge of the branchial sac and a peripharyngeal band surrounding its anterior end, but there is no representative of the dorsal lamina on its dorsal edge. The oesophagus commences rather on the ventral edge of the posterior end of the branchial sac, and runs backwards to open into the stomach, which is followed
