ANATOMY OF THE BOTRYLLIDIE. - A de scription of the form and structure of the test of these compound Ascidians has been already given. The in- and out-lets of the aggregated individuals, as we have already seen, have either separate external openings, or the several vents of a " system " of animals unite to form one large common cloaca and external anal orifice.
All these external orifices are more or less irritable and contractile. When the little animals dilate their branchial orifice, a part of their body is raised so as to slightly emboss the general surface of the mass, and they pro trude a membranous circlet or ring, the free border of which is cut into, generally six, regu lar lobes, but sometimes into eight. In Poly clinunz, the mouth is very contractile, and is surrounded by six little digitiform processes. Parascidia has eight of these tentaculiform bodies. Frequently, after death, as in Lepto clinum, these orifices contract, and their bor ders sink in so as to be discerned with diffi culty. In the interior of this orifice, and towards the base of the collar, a series of minute tentacular filaments can be perceived by the aid of a lens, which are directed, like the spokes of a wheel, towards the centre of the opening, or have a more or less curled ap pearance, in a similar manner to the tentacles occupying the same position in the simple Ascidians and in the Clavellinidte. The ten tacles vary from four to twelve or more, but nine or ten is their usual number; they differ materially in length, some being nearly ru dimentary, whilst others, alternating with the first, are long enough to meet across the open ing. In Amaroucium Nordmanni there are six long and six short tentacles within the bran chial tube. In Botryllus snzaragdus, in which the buccal orifice admits of considerable dila tation, a circlet of four longish filiform ten tacles may be seen. The tentacles in B. aureus are more numerous than usual, but both in this species and in B. violaceus they are almost rudimentary. Diazona has fifteen or sixteen simple tentacles, and in Sigillina there are twelve.
Around the base of the denticulated rim of the branchial orifice in Anzarourium argus (fig. 782.) there are seen four minute pinkish spots, which are probably rudimentary organs of sight. Similar, but more numerous, pigment spots are observed in Leptoclinum Listeri, Pa rascidia, and other genera.
The separate anal orifices are frequently destitute of the crenulated or denticulated margin that surrounds the branchial tube. This condition has afforded a distinguishing character for some of the minor groups in Milne Edwards' classification of the Bo tryllidce. In Polyclinum, Amarouciunz, Bo tryllus, and other genera, the anus, instead of opening directly outwards, empties itself into a common cloaca' cavity, that belongs to a number of individuals, and is in the form of a large canal hollowed out of the common tegumentary mass. This is frequently rami fied interiorly, as in Botrylloides, and termi nates at the opposite extremity by a simple excretory orifice, not occupying the centre of the " system," but placed nearly at one of the extremities, and communicating with the in dividuals situated at the other end of the group by means of the interior canal.
In Amaroucium the fircal or anal orifices, by which the common cloacm open externally, are wide open during life, and easily per ceived ; their form is round and their border thick ; but after death they contract, and are so much sunk in that they are demonstrated with difficulty.
The anal orifice in some genera is sur mounted by a membranous languet, either simple or trifid (fig. 783. i').
In out the natural groups of the associated Ascidians we have spoken of the division of the body into distinct portions, as the thorax, and super- and post-abdomen in Antaroucium (fig. 782.), and in Polyclinum, where they correspond to the three vertical chambers of the animal's cell ; and into the thorax and abdomen in Didemnum. The latter arrangement resembles that of the Cla vellizu e ; but in the Botryllians, on the other hand, no distinct separation is observable, the viscera being pushed up by the side of the branchial sac, as in the Perophorce and the simple Ascidians (fig. 783.). The thorax is generally more or less cylindrical, sometimes hemispherical (fig. 769.) or subglobose, and contains the branchial organs. In Botryllus violaceus there are two little glandular tuber cles, one on the right and one on the left of the buccal orifice, and situated at a nearly equal distance from the superior extremity of the ventral sinus and the dorsal nucleus or nerve-ganglion.