These observations, and others which have been recorded, do not, unfortunately, give any information as to the purpose of the movements of the avicularia and vibracula. It is obvious that they may be defensive in character; and it cannot be doubted that the avicularia can prevent inquisitive worms from straying at will over the surface of the colony. There is no evidence to show that animals are discouraged from interfering with a Bugula owing to the presence of its defensive weapons.
Fig. 242.—Caberea ellisii Flem., Norway. × 40. Back view of part of a branch. The large vibracular zooecia (v.z) occupy nearly the whole of the surface. s, Seta of vibraculum; z, zooecia.
It is not, indeed, certain what are the enemies against which the Polyzoa have specially to guard. Sea-urchins and certain Molluscs are known to browse on Polyzoa. Fresh-water Polyzoa, in which avicularia and vibracula are absent, are attacked by the larvae of Insects, and by Triclad Planarians. I have found the latter with their long pharynx everted and completely buried in a Cristatella colony. It is possible that some marine Cheilostomes may be saved from attacks of this kind owing to the existence of their armoury of avicularia and vibracula. It is also possible that these structures are of service by removing foreign particles which might otherwise settle on the colony, and tend to block up its orifices. It has further been suggested that animals seized by the avicularia may be held until they die, and that their disintegrating particles may then be carried to the mouths of the polypides by the ciliary currents of the tentacles; but proofs of this suggestion are wanting, and it must be admitted that the subject needs further elucidation.
The vibracula ordinarily remain stationary for some little time, every now and then giving a sweep through the water. In the majority of cases these structures, like the avicularia, act perfectly independently of one another, so far as can be made out; but in Caberea (Fig. 242) the vibracula move in unison, the simultaneous action of the whole series, after a period of quiet, being described as "positively startling."[[536]]
It has been stated by Busk[[537]] that the entire colony in Selenaria and Lunulites may be moved from place to place by the large vibracula which these forms possess.
Fig. 243.—Pedicellina cernua Pall., Guernsey. Entire colony. × 27. The colony has three growing ends, a; 1-8, individuals of colony; 1 and 8 are quite immature; and 7 (tentacles retracted) is still young; 2, is seen in longitudinal section; g, generative organ, and below it the ganglion; m, mouth; r, rectum; s, stomach; between g and r are three embryos in the brood-pouch; the tentacles are retracted; in 5 and 6 the tentacles are expanded; in 6 two embryos are seen within the circle of the tentacles, to the left of them is the rectum, and to the right the mouth; 3 is in the act of losing its calyx, and has already developed the beginning of a new polypide-bud; in 4 the primary calyx has been lost, and the new calyx is clearly marked off from the stalk.
Entoprocta.—The Entoprocta, although a very small sub-class, deserve special consideration, if for no other reason, from the fact that many writers regard them as the most primitive group of Polyzoa, and consequently as the forms which show most affinity to other classes of animals.
Their most obvious characteristic is, as we have already seen,[[538]] the position of the anus within the circle of tentacles. The individuals formed by budding always remain more separate from one another than those of most Ectoprocta.