ANATOMY OF CLAVELLINID/E.—CiatTaina.
— At the superior extremity is the buccal orifice, which is circular, looking directly upwards (fig. 768. c), and furnished with a very thin, prominent, cylindrical margin ; to the interior of which is attached a ring or circlet of simple tentacular filaments, about thirty in number, of which about ten are long enough to reach to the centre of the cavity of the tube, whilst the others are very short, and situated between the first. Near this first orifice, and toward the supe rior part of the dorsal aspect of the thorax, is the second or anal orifice, which is also cir cular, with an entire rim. The external tunic or tegumentary membrane (test) of the body is thin, but subcartilaginous. It adheres feebly to the other parts, except around the two orifices. At its base, or inferior extre mity, there are a variable number of radici form prolongations, which serve to fix the animal, and some of which often bear at in tervals little pyriform tubercles, which be come developed into new individuals.
The test is so transparent and colourless, that the interior structure of the animal may be easily studied without having recourse to dissection. There is observable, however, certain yellowish lines traversing the transpa rent test, which have a granular appearance, and correspond, according to M. Milne-Ed wards, to the lines of junction of certain parts of the interior. Two of these bands descend vertically, very near one another, throughout the medial line of the ventral surface of the thorax, and are separated by a linear, colour less, semi-opaque space. A third line rises on the right and left of the test, at the superior part of the thorax, and is directed horizon tally backwards, describing a circle around the base of the oral orifice. A fourth line, also annular, encircles the inferior extremity of the thorax. A fifth surrounds the anal ori fice, and is prolonged upwards and forwards, almost to the posterior margin of the oral orifice. There is generally a sixth line, much paler than the others, and more deeply situ-• ated, at some distance from the dorsal surface of the thorax, descending vertically from the superior to the inferior ring.
Towards the middle of the abdominal por tion of the body the stomach appears, as a small, orange-yellow, oval mass, bearing four yellow vertical lines, like those of the tho rax. Lastly, close to the stomach, is another
orange-coloured spot, formed by a coloured portion of the intestine, and, still lower, the whitish gland-like mass, comprising the gene rative organs (fig. 768. p).
The mantle is suspended within the first tunic ; it is membranous and extremely deli cate. Superiorly it is attached around the two orifices, and inferiorly it terminates in a cul-de-sac. It often presents inferiorly some tu bular prolongations, sometimes simple, some times ramified ; which descend towards the base of the animal, and sometimes project into the interior of the root-like processes of the external tunic. Its surface is traversed with divers muscular fibres, some of which arel circular, and constitute the sphincters around the mouth and anus ; whilst others, to the number of nine or ten pairs, arise from a kind of tendinous collar surrounding the mouth, and descend vertically to the inferior extremity of the abdomen. These last muscles serve to shorten or bend the body, and it would appear that it is by the elasticity of the external tunic that they are lengthened again, after having been so contracted, for there do not appear to be any traces of transverse muscular fibres proper to act as antagonists to the vertical fibres.
In all the thoracic portion of the body a third tunic is present. This, like the pre ceding, is membranous, and is suspended in a kind of sac formed by the latter ; it adheres to it by the rim of the anal orifice and along the collar around the base of the mouth. Inferiorly this membranous pouch is united in its extent to the two orifices of the diges tive canal, where for the most part it is con tinuous with the wall of that tube. Its cavity constitutes the thoracic chamber. It encloses the branchial organs, and presents on the dorsal side a free space, which forms a kind of cloaca, ending at the anal orifice of the ex ternal tunic. Along the medio-ventral line a vertical groove is observable, and at the point where it adheres to the second tunic, between the mouth and the anus, we can perceive a minute tubercle, which is the nervous gan glion. This thoracic chamber exactly re sembles the great cavity of the Salpians, and would scarcely differ at all, if the cloaca were shorter, and the anal orifice more distant from the mouth, and directed backwards.