Ti Families - Thera phosidae (Avicularia, Poecilotheria).
The larger species of Bird's Nest Spiders (Avicularia), the opisthosoma of which is as large as a bantam's egg, undoubtedly attack young birds, and M'Cook gives an account of the capture in its web by an ordinary house spider of a small mouse.
Polymorphism usually occurs, certain individuals having the form of avicularia or vibracula.
- Zooecia of Umbonula pavonella, showing a pair of minute avicularia on either side of the orifice of each zooecium.
The most singular of the external appendages found in the Polyzoa are the avicularia and vibracula of the Cheilostomata.
The avicularia and vibracula give valuable aid to the systematic study of the Cheilostomata.
Avicularia of this type occur in the common Flustra foliacea, in various species of Membranipora, and in particular in the Onychocellidae, a remarkable family common in the Cretaceous period and still existing.
In the majority of Cheilostomes, the avicularia are, so to speak, forced out of the ordinary series of zooecia, with which they are rigidly connected.
Although at first sight the arrangement of the avicularia in Cheilostomes appears to follow no general law some method is probably to be made out on closer study.
They occur in particular in relation with the orifice of the zooecium, and with that of the compensation-sac. This delicate structure is frequently guarded by an avicularium at its entrance, while avicularia are also commonly found on either side of the operculum or in other positions close to that structure.
It can hardly be doubted that the function of these avicularia is the protection of the tentacles and compensation-sac. The suggestion that they are concerned in feeding does not rest on any definite evidence, and is probably erroneous.
But avicularia or vibracula may also occur in other places - on the backs of unilaminar erect forms, along the sutural lines of the zooecia and on their frontal surface.