articulation is the result of a vital action in some of these cells, whereby they are deprived of the power of selecting a calcareous coating from the surrounding water, their energies being entirely devoted to the function of growth.

I now proceed to describe the structure of some of those corals which consist of a series of branches like the Nullipores, but all of which exhibit cells for the lodgment of polypes. When divided transversely or longitudinally, a branch of coral will always present a porous character, the pores being continuous with the polype-cells; but when examined microscopically, many of the parts which appear to be composed of solid lime will be found to exhibit traces of a cellular structure, as shown in Fig. 87. The cells, when present, will be found to differ very materially from those of any Nullipore, and all the calcareous material is

FIG. 87.

contained within them. The presence of cells is not always so plainly discernible as is represented in Fig. 87, for changes are continually going on, both in the cell-walls and in the calcareous material which they enclose, the former, as before stated in page 5, being absorbed, whilst the latter undergoes a species of crystallization. I have made a very great

number of sections of the stony skeletons of corals, and have repeatedly failed in detecting any trace of cellular structure in every specimen; those parts newly formed should be selected in order to see them to advantage. I have already spoken of the development of the stony axes of the Gorgoniada; these appear to differ from the true Corals in having a spicular origin, which spicula, in process of growth, coalesce and become converted into a solid mass. Thus then, independently of the minute structural difference between a Nullipore and a Coral, there is also this striking fact, that both are developed by an organic basis, the one having the calcareous material external to the basis, whilst in the other it is always internal to it.

LECTURE XI.

SKELETON OF ZOOPHYTES.

HAVING now examined the structure of the skeleton in the orders Hydroida, Asteroida and Helianthoida of the class Anthozoa, I proceed to the class Polyzoa, which is divided by Dr. Johnston into two orders, the Infundibulata and Hyppocrepia. The Infundibulata (Cilio brachiata of Dr. A. Farre) are all marine animals with compound polypes, each of which has a mouth surrounded by filiform retractile tentacula provided with ciliated arms, by which they are distinguished from the Hydroida. The Hyppocrepia, on the contrary, are all lacustrine, or natives of fresh water; but in them the mouth is also surrounded with ciliated retractile tentacula. In the Anthozoa the polype is either naked, as the Hydra, or enclosed in a horny skeleton formed by soft animal matter continuous

with the polypes, but not by the polypes themselves. In the Asteroida the skeleton, as in the Gorgoniæ, is also internal, whilst in the Helianthoida it is again external; but in the present class, Polyzoa, the skeleton forms a portion of the polype itself, hardened by the deposition either of horny tissue or calcareous

matter.

In the Anthozoa we found the polypes developed from a common central mass, but in this class each polype is distinct and enclosed in its own peculiar cell. In the Anthozoa, as in the case of the Hydræ and Actiniæ, the polypes occur in a separate or naked form, but, according to Johnston, in the Polyzoa there is always some kind of case or skeleton to protect them; a tunic, which after investing the body of the polype as in a pouch, is reflected over the aperture of the cell, the reflected portion becoming external and solidified either by calcareous depositions in its texture, or by a mutation of its thin membranous tissue into a horny investment, better suited to the office it has to perform of protecting the sentient body from too rough a contact with the medium in which the animals live, and from other accidents.

When the polype retires within its cell, the mouth of the cell is closed by that portion of the tunic which they push before them when they protrude themselves, and some species of Flustra, are provided with a tendinous apparatus for lengthening the body, which is doubled up in the interior of the cell, when they are at rest.

The order Infundibulata is divided by zoologists into seven families, viz.: Tubuliporida, Crisiada, Eucratiada, Celleporida, Escharide, Vesiculariadæ and Pedicellinæ. Of the two first of these I possess no specimens, but of the third, or Eucratiada, I have one of great interest-the Anguinaria spatulata, or Snake's Head coralline of Ellis-which, from possessing a higher organization than the other members of the family, and from the presence of striped muscular fibre, should be classed with the Bryozoa among the Mollusca ; but as its external form and the structure of its skeleton somewhat resembles that of the Hydraform Zoophytes, I shall describe it here. This Zoophyte, shown at c in Fig. 88, attaches itself to pieces of sea-weed, along which it creeps in a spiral manner, sending up here and there a curved tube, with its free extremity dilated into a polype-cell resembling the head of a snake, on which account the animal has received the name of Snake's-head Coralline. The tubular processes containing the polypes, are composed of calcareous matter; the part corresponding to the body of the snake is surrounded by a series of rings, whilst the dilated portion, or head, exhibits a minutely hexagonal or granular structure, as if made up of cells of that figure. When acted on by dilute acid, the calcareous matter is removed, and a very transparent horny membrane is left, which retains much of the original structure.

The best description of this Zoophyte is that given by Mr. Busk in the second volume of the "Transactions

VOL. II.

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