Page:Scientific results HMS Challenger vol 18 part 2.djvu/20

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Cystidina, Haeckel, 1883, Sitzungsb. Jena Ges. für Naturw., Februar 16.

Definition.— without skeleton. The soft body composed of a monopylean central capsule (with porochora and podoconus) and of a surrounding jelly-veil or calymma.

The family comprises the simplest and most primitive forms of, the only group which is entirely without a skeleton. The central capsule is therefore perfectly free and naked, enveloped by the calymma only, as in the among the, in the Phæodinida among the. Probably these naked and skeletonless must be regarded as surviving remnants of the common ancestral group of this legion; but the possibility is not excluded that the few observed forms are either young  which have not yet secreted a skeleton, or older  which have lost their original skeleton.

We distinguish in this small family two genera only: Cystidium with hyaline, not foamy calymma, without extracapsular alveoles, and Nassella, with a very voluminous foamy calymma, including numerous large alveoles; the former corresponds to Actissa and the latter to Thalassicolla among the or the skeletonless. But in these two latter genera, as in all, the central capsule is perforated everywhere by innumerable small pores; the two former genera, however, exhibit the same characteristic podoconus in the central capsule, and the same porochora at its base, as all the other. The pseudopodia are protruded from the central capsule through the porochora only.

The Central Capsule is in the two observed genera either ovate or nearly spherical, usually slightly tapering towards the basal mouth. Its transverse section is constantly circular. The membrane of the capsule is usually rather thick and double-contoured, and bears on the truncate basal pole a circular "porochora" or area porosa, through which the pseudopodia are protruded. The porochora is either quite simple, circular, or in some species trilobed, with three equal circular lobes, each of which is surrounded by a girdle of small granules. The podoconus, or "pseudopodial-cone," arising vertically from the horizontal basal porochora, is half as long as the central capsule, or longer, simply conical and finely striped longitudinally. The nucleus lies usually in the uppermost part of the central capsule, above or behind the podoconus, and is either spherical or ovate, sometimes kidney-shaped. It includes one or more nucleoli.