In the cases so far considered, the body of the avicularium is fixed. The highest differentiation acquired by these structures occurs in cases like Bugula, where they are borne on flexible stalks, which may even exceed the avicularia in length.[[535]]
Fig. 240.—Bugula turbinata, showing avicularia (a, a'). The figure is explained on p. [468].
In Bugula turbinata (Fig. 240) each zooecium is provided with one of these appendages, attached to the base of the outer of the two spines which border its orifice. The avicularia of the two edges of the flattened branch are much larger than those of the more internal zooecia. The upper jaw is strengthened by a kind of buttress, or thickening of the ectocyst, which passes on each side across the avicularium to the hinge-line of its mandible. The upper part of the beak is strongly hooked, while the tip of the mandible bears a prominent spike, which fits inside the upper beak when the jaw snaps. A great part of the head is filled with a strong muscle, whose fibres exhibit a distinct transverse striation, and converge into a median tendon. The latter is inserted into the middle of the mandible. The muscle serves to close the jaws, and is the representative of the muscles by which the operculum is closed in an ordinary zooecium. The lower jaw is opened by means of a pair of muscles which are situated immediately under the ectocyst of the avicularium, and pass into the mandible close to its hinge.
Fig. 241.—Illustrating the transition from avicularia to vibracula. A, Microporella ciliata Pall., Scilly Is., × 62; a, avicularium with short mandible (closed); a', avicularium with vibraculoid mandible (open); m.p, median pore; o, ovicell: B, Mastigophora dutertrei Aud., Shetland Is., × 47; s, sinus of orifice; v, seta of vibraculum (or vibraculoid avicularium).
Within the jaws, in the region which we may term the palate, is a rounded knob, which bears a tuft of delicate sensory hairs, which doubtless enable the avicularium to recognise the presence of any foreign body. The closure of the mouth may, indeed, be instantaneously induced by touching it with the point of a needle. It has been suggested that a small mass of cells which bears these hairs may represent the rudiment of the polypide.
The "vibraculum" (Fig. 242) is regarded as an avicularium in which the mandible has become elongated, so as to form a thin, chitinous "seta," which from time to time moves through the water. The part of the vibraculum which represents the zooecium commonly bears a tubular rootlet, used for attaching the colony to the substance on which it is growing (Fig. 254, p. [517]).
In Microporella ciliata (Fig. 241, A) the avicularia are very variable, and in some cases take on a "vibraculoid" character. But in the fully-developed vibraculum (Fig. 242) there is usually no such compromise of characters. It may, however, be noted that Scrupocellaria scabra (Fig. 254), which belongs to a genus characterised by its highly differentiated vibracula, possesses structures (v.z) which could hardly be distinguished from avicularia were it not for the presence of the rootlet (r).
In the course of some observations which I had the opportunity of making on Bugula calathus at Naples, a fine hair offered to a small colony was seized with such force by the avicularia that the entire colony was lifted out of the water by the hair. The same colony had captured (1) a small Nereis, which it held with several of its avicularia; (2) an Anisopod Crustacean, 2½ mm. long; and (3) a small Amphipod, which was held by one of its antennae. The Anisopod was held by the tip of one leg with one avicularium, and by the penultimate joint of one of its chelae with an avicularium of another branch. It was captured in such a way that its chela, the "hand" of which was about half as long as the avicularium, actually closed on to the avicularium without being able to effect its escape. A little later the other chela was caught by another avicularium. Curiously enough, however, an avicularium did not necessarily close even when part of a captured animal was actually in its mouth. The avicularia made no attempt to place themselves in an advantageous position for catching fresh parts of the Nereis, which they might easily have done. The avicularia which had captured prey remained motionless. The others moved backwards and forwards (cf. the various positions of the avicularia shown in Fig. 240) ten times in ¾ to 1 minute, snapping their jaws perhaps once in that time. The two Crustacea were still retained by the avicularia two days later. On the next day they had both disappeared; but the colony had again caught the Nereis, which had previously effected its escape with the loss of nearly all its tentacular cirri.