Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/487

 participate in its gentle movements, and also in the benefits accruing from the currents in the water developed by the cilia of the Polyzoon.

It was evident that the ornamentation of the hydroid was mimetic of that of the Polyzoon.

Dr. Allman suggests (op. cit.) that the ornamentation and the calcareous test do not resemble the structure of the covering of Bimeria, that the opening on the distal end of the tentacles was too small for the passage of a well-developed soft tentacle, and that the chambered base is anomalous amongst the class. He considers that the form should be associated with the Rhizopoda.

The lately discovered specimens indicate the manner of the growth and development.

In specimens consisting of a short stem and broad base without the capitulum the stem does not appear to have been fractured, and it is tapering in form and has a narrow superior orifice. Another specimen (Pl. XIV. fig. 1) resembles those just mentioned, but is taller. The stem has a branch which has been fractured. Other specimens (figs. 2 & 3) show that the upper part of the capitulum close to or at the metastome is projected (1) in the form of a closed prominence, or (2) as a stem-like continuation with a small aperture on its distal end.

When this stem-like continuation has grown to a considerable length, the appearance of the whole organism is that of a long stem with a central whorl of tentacles.

Many specimens have very large bases. These cover many cells of the Fenestella and the intermediate calcareous tissue. It is very difficult to arrive at a satisfactory conclusion whether or not the base is itself divided into cellular compartments. After due consideration I feel disposed to believe that the wide and dactylose base. incrusts the Fenestella, and that there is a space in the base which is uneven and very irregular in shape, but not cellular *. The shape and general complexity of this space are determined by the outline of the Polyzoon beneath; and there do not appear to be involutions or septum-like processes of the calcified periderm of the Hydroid.

It would appear from the examination of the specimens that the form grew from the base at first without a hard capitulum and ten- tacles ; that it branched in this condition, or had an offshoot destined either to continue the same trophosomal structures, or give rise to those of a different kind and belonging to the gonosome.

The opening in the distal end of the nascent stem could admit of the passage of a polypiform mass with all its tentacles. Under such circumstances, the calcareous stem would be the periderm of the hydrocaulus. The branch would terminate in the same manner as the stem, or else would, under the theory of the Hydroidean affinities of the animal, be the periderm of the gonosome.

With growth, the more or less bell-shaped polypite surmounting

due to the cells of the Fenestella.
 * The cellular appearance shown in Phil. Trans. 18G9, plate lxvi. fig. 4, is

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