AULOZOA. - The third subdivision of Po LYPIFERA is composed of a series of zoo phy tes very different in their org,anisation from those embraced by the two preceding. They have generally been named by natu ralists Tubular or Vaginated Polypes, and are distinguishable from the circumstance that their living substance, instead of being external to the hard polypary, is in them enclosed in a calcareous or corneous tube, sometimes simple, but more frequently rami fied, from which the polypes are protruded, either through a terminal aperture or from lateral cellules formed by the external en velope.
The Aulozoa are divisible into several groups, which we shall separately examine, beginning with the Tubularidce.
In the Tubularia (fig. 48), as in all polypes unprovided with a complete digestive canal, there is an organic portion which brings all the members of the colony into communica tion with each other, or forms in other words the body of the community, and from this common body buds are thrown out, from which ramifications are produced in all re spects resembling those met with in the vegetable kingdom ; these constitute the trunk and branches of a tree, which, instead of bearing flowers, produces poly.pes provided with tentacula, a digestive cavity, and most frequently a reproductive apparatus.
Tentacular apparatus.— In the Tubularidm the tentacula are situated, as in all polypes, around or in the immediate vicinity of the oral opening. Their number is very various, even in the same species, but the variations in their length are more apparent than real, for their contractile powers are such that they are constantly changing in their shape and dimensions, in which respect they resemble the Hydra described above. It is towards the extremity of the tentacle that this con tractile power is most remarkable ; and when the organ is not fully stretched out, it is enlarged or dilated near the end, insomuch that some authors have erroneously looked upon this part as performing the office of a sucker.
The disposition of the tentacula varies in different genera. The genus Eudendrium has a single row of tentacula, which are alternately placed a little more internally and externally. The genus Tubularia, properly so called, has a second row of shorter tentacles immediately surrounding the proboscidiform prolongation which constitutes its mouth, and in the genus Stipula (Sars) there is an additional row situated between these two, so that there are genera with one, two, or several rows of ten tacula ; the lower row is, however, always the longest, and it is these that are persistent when there is only one rank. The tentacula
are arranged in whorls in all the Tubularidw, except in the genus Syncoryna, in which they are distributed without regularity (fig. 47). No cilia are perceptible, either externally or in the interior of these tentacula, which, when highly magnified, appear to be entirely composed of transparent cells, closely agglo merated, and no traces of muscular fibre are by any means to be detected ; their move ments seem to depend entirely upon the con tractions of their component cellules, which are seen to diminish in size when the tentacle is shortened, and to expand during its elon gation, preserving nearly the same shape, whence it may be concluded that their pari etes are endowed with contractile powers. Professor Van Beneden indeed compares them to so many hearts placed end to end, which, by their dilatation elongate, or by their con striction shorten or bend, the tentacle of which they constitute the substance.
Digestive system.— In the Tubularidw there is seen, situated in the middle of the ten tacula, a sort of proboscidiform appendage, open in the centre, which is the entrance to the digestive cavity. The name- of pro boscis appears sufficiently applicable to this part, both on account of its situation and of the changes of form which it continually un dergoes. In its most usual condition it has the appearance of a protuberance provided at its summit with an orifice of very variable shape and size.* The cavity of the proboscis leads into that of' the stomach, but neither the one nor the other have parietes proper to themselves, but on the contrary seem, as in the Hydra, to be mere excavations in the substance of the polype. In the genus Coryne, the cavity of the stomach is circumscribed, so that each polype has a proper digestive cavity ; but in all the other genera belonging to this family, the stomachal receptacles of different indi viduals communicate mediately one with another, so that what is taken into the sto mach of one polype may pass into those of all the individuals composing the colony. Thus, what is eaten by a few individuals pro fits the whole community, seeing that what one swallows may pass into the stomachs of its neighbours.