The two tubes on the superior aspect of the mantle, which are directed towards, and protruding into, the two somewhat tubular orifices of the test, are more muscular than the rest of the sac, being surrounded with, generally very distinct, sphincters in addition to the fascicles of muscles that traverse them longitudinally, and are continued diagonally across the sac (fig. 780.). When open, the margins of these tubes are crenulate. The free extremity of the branchial orifice is some times quite entire, but occasionally terminates in a circle of regular tooth-like processes, which are regarded by Dr. A. Farre as the ana logues of the tentacles of the Bryozoa. By con traction, the tubes are thrown into 5 or 6 folds. At the exterior angles of these folds in some Ascidians (A. intestinalis, Cynthia ampulla, there is a minute red spot ; in A. manznzillata, the edges of the folds are more or less red dened by an increase and diffusion, as it were, of these little red granules. Internally each tube has frequently at its base some slight valve-like prominences or folds; and the in ner extremity of the branchial tube is always fringed with a circlet of simple or compound tentacular filaments. These, however, may more properly be said to belong to the bran chial sac.
The mantle becomes dusky and opaque, and its muscular tissue more distinctly seen, in preserved specimens. In its recent state it is more or less diaphanous, and usually of a sober tint ; but occasionally, in Ascidice, it is of a fine crimson (A. venosa), or variegated with crimson and white (A. virgi nea), or spotted with red (A. asperse); and in A. parallelogramma it is ornamented with rectangular reticulating white lines and occa sional bright yellow or crimson spots. In A. conchilega it is white, passing to blue ; and in A. arachnoidea it is dark blue.
The branchice of the Ascidiadce is a large bag of fine vascular network furnished with vibratile cilia, contained within the mantle of the animal, and lining the walls of that cavity (fig. 778.). Sometimes the branchial sac is oblong, oval, or rectangular ; and then, it occupies all the length and one of the sides of the cavity of the mantle (Cynthia). In Ascidia manzmillata and A. monacha it is very long; and, after having descended to the bottom of the mantle, it is bent upwards upon itself, extending half way up the cavity again.
A tube, differing in size in various species, which has its analogue, perhaps, in the retrac tile operculum of the Bryozoa, conducts the water from the buccal aperture to the respira tory sac. This tube or process of the mantle encloses the neck of the branchial sac. At the inner extremity of the tube, where it widens into, and is continuous with, the bran cilia] sac, are frequently observed about five small valvular folds ; and below these there is always a circle of fleshy filaments or tentacular appendages (fig. 778. and fig. 39 d. Vol. I. page 112. ANIMAL These are present also in the Clavellinidce and the Botryllidce. They vary from 6 to 26 in number, and are either simple, as in Phallusia, or branched, as in Cynthia, Bol tenia, &c. By detaching the anterior part of the respiratory sac of Cynthia, or others, and examining it from its inner and inferior aspect, one may perceive, at the inner mar gin of the tube, several arboriform append ages, somewhat large at the base, and having the brandies either swollen at their extremi ties, or finely laciniate and almost plumiform in Cynthia Dione they are somewhat bipinnate. The number of the tentacles is difficult to be arrived at, as smaller filaments appear amongst the six or seven larger ones. The branches
are generally directed downwards, towards the bottom of the sac, but sometimes are slightly curled upwards. Seen under the mi croscope, these filaments are hollow ; and the extremities and swellings of the branches are cul-de-sacs. The several tentacles intercom municate with each other by their hollow bases, and with the vascular network of the respiratory sac.
The walls of these tubular organs are very thin and transparent ; traces of muscular fibre have been detected in their substance, and a circulation ; but no trace of vibratile cilia, either on their outside or within their cavity. Van Beneden observes that a fluid traverses their cavity in a similar manner to the blood in the branchim of the Doris. In Actinia and Holothuria simple and ramified filaments occur, that offer considerable analo gies to the arborescent tentacles of Ascidice. They are all hollow ; and the fluid moving in their interior comes directly from the peri intestinal cavity ; this great cavity, commu nicating as in Biyozoa, with the tentacular appendages.
The internal surface of the respiratory sac is sometimes uniform (in Ascidia, Phallusia, &c.), and frequently longitudinally plicated and disposed in deep and regular folds, all following the curve of the cavity, and termi nating at a little smooth area above the pha rynx (Cynthia, Boltenia, &c.). The folds are from 8 to IS in number, and form the first indication of the four branchial laminae of the bivalve Acephalans. The structure of the membrane consists of an infinity of small, anastomosing vessels, generally cross ing each other at right angles, and forming quadrangular interspaces, which, under the microscope, are seen to be still more mi nutely subdivided in the same manner. Milne Edwards observes that in Phallusia each of the meshes of the respiratory membrane is occu pied by a minute spiracle, that allows of a communication between the interior of the branchial sac and the cavity of the mantle (the " thoracic chamber " of Milne-Edwards), the dorsal portion of this chamber being the cloaca, the base of which is occupied by the orifices of the digestive and generative tubes, and the summit of which opens externally by the anal aperture. In Cynthia ampulla the meshes are very irregular and almost inextri cable, some of the minute vessels having apparently a spiral arrangement ; and the vessels are rather more numerously disposed around the oesophageal orifice. A somewhat spiral or vermicular arrangement of the ex tremities of the branchial vessels exists also in Chelyosoma, as figured by Eschricht ;* and as seen in a very minute sessile Ascidia, from South America, in the collection of Mr. Bower bank. In the respiratory network of Cynthia the large longitudinal vessels are seen to be the most prominent of all. They form, with the large transverse vessels, square meshes, which three other shorter vessels subdivide into four transverse meshes; and these are further intercepted by extremely fine longitudinal vessels. The vertical vessels of this quadran gular network may be said to arise from the transverse vessels, which communicate by each extremity with two vertical trunks, placed at opposite sides of the sac, and representing respectively the branchial artery and vein. The latter, in C. nzicrocosmus and other As cidians, bears a longitudinal series of small transverse tentacular filaments. Similar ten tacles we shall have to describe as belonging to Chelyosoma (fig. 777.0.