. Elements of zoology, or, Natural history of animals / ed. by Reese. Zoology. ASCIDIOIDA THE HOWERBANKIA. r>i9 dition. A species lately discovered on the British shores, the Jiowerbankia densa, will afford an excellent illustration of the structure of the separate polypes. These arise like buds from a sort of creeping stem, which connects them all, and which has the power of extending the structure by its own growth; but no communication has been observed between the interior of this stem and the stomachs of the polypes, like that which exists in the Sertularia. Each is enclosed in a
. Elements of zoology, or, Natural history of animals / ed. by Reese. Zoology. ASCIDIOIDA THE HOWERBANKIA. r>i9 dition. A species lately discovered on the British shores, the Jiowerbankia densa, will afford an excellent illustration of the structure of the separate polypes. These arise like buds from a sort of creeping stem, which connects them all, and which has the power of extending the structure by its own growth; but no communication has been observed between the interior of this stem and the stomachs of the polypes, like that which exists in the Sertularia. Each is enclosed in a horny trans- parent sheath, the upper part of which is so flexible as to be capable of being drawn inwards by the action of muscles, thus closing the mouth of the cell. 937. The polype itself has, when protruded, the general form of the hydra, the mouth being surrounded by ten long and slender tentacula. There is this important difference, how- ever, that in the Bowerbankia and all the other polypes of this order, the arms are fringed with cilia, or little hair-like filaments; whilst in the Hydraform Polypes no such appendages exist. This difference is more important than it would at first sight appear, being connected with the whole economy of the animal. This nydra only obtains its food by grasping with its arms that which comes within their reach; but, in the animals under considera- tion, the rapid vibration of the filaments, which seem to take place at the will of the polype, produces currents in the water by which a regular stream is brought to- the mouth; and this stream contains many nu- tritive particles, from which a selection, adapted to the highly organized digestive apparatus, may be made. 928. The mouth opens into a wide tube, which may be regarded in the light of the oesophagus or gullet of higher animals; and this terminates, at its lower end, in a narrow orifice, leading to a globular cavity, which seems analogous to a gizzard, having thick muscular walls, line
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