4. Polyphyletic hypothesis, deriving the different groups of NASSELLARIA from different skeletonless Nassellida, by development of simple siliceous skeletons in different ways. Among the numerous polyphyletic hypotheses which are possible, one of the simplest would be the supposition that three different fundamental forms of skeleton may have arisen independently one from another: (1) a simple sagittal ring as original form of the #Stephoidea# and #Spyroidea# (A); (2) a simple basal tripod as original form of the #Plectoidea# (B); (3) a simple latticed cephalis as original form of the #Botryodea# and #Cyrtoidea# (C). This triphyletic hypothesis is supported by R. Hertwig (1879, loc. cit., p. 136); he assumes that the original skeletonless Nassellida (Cystidium) have produced three different branches, his "Acanthodesmida" (= #Stephoidea# and #Spyroidea#) with a primary ring, his "Plagiacanthida" (= #Plectoidea#) with a primary tripod, and his Cyrtida (= #Botryodea# and #Cyrtoidea#) with a primary cephalis. This hypothesis seems rather probable on the first view; but it meets with the greatest difficulties in view of the fact that these three original elements of the skeleton are more or less evidently combined in the great majority of NASSELLARIA. The greatest difficulty arises from the fact that often among very similar and closely allied forms the first exhibits all three elements (A, B, C) combined, whilst the second has a combination of A and B, the third of B and C, the fourth of A and C; and there are other forms, very similar to the former, in which one element only is recognisable. Another difficulty arises from the fact that the intimate structure of the cephalis in the majority of #Cyrtellaria# is not perfectly known, and often exhibits structures which are difficult to explain with regard to the three elements A, B, C. Under these circumstances further researches on the numerous imperfectly known NASSELLARIA are required, and chiefly accurate observations on their more minute structure and on their important ontogeny.

{895}We divide the immense legion of NASSELLARIA into two large orders, the #Plectellaria# without complete lattice-shell, and the #Cyrtellaria# with a complete lattice-shell or a "cephalis," including the central capsule; the latter, of course, have arisen from the former. The #Plectellaria# comprise three different suborders, the #Nassoidea# (without skeleton), the #Plectoidea# (with a tripodal skeleton, without ring), and the #Stephoidea# (with a primary sagittal ring, with or without tripod). The #Cyrtellaria# again also comprise three different suborders, the #Spyroidea# (with bilocular cephalis and a sagittal constriction), the #Botryodea# (with multilocular and lobate cephalis, exhibiting two or more constrictions and three or more lobes), and the #Cyrtoidea# (with simple, unilocular cephalis, without constriction).

Synopsis of the Orders and Suborders of NASSELLARIA.

I. Order {No skeleton, 1. #Nassoidea#. PLECTELLARIA. { Nassellaria without {Skeleton with a basal tripod, complete lattice-shell. { without ring, 2. #Plectoidea#. { {Skeleton with a sagittal ring { (usually without tripod), 3. #Stephoidea#.

{Cephalis bilocular, with a { sagittal constriction, 4. #Spyroidea#. II. Order { CYRTELLARIA. {Cephalis multilocular, with Nassellaria with a { two or more constrictions complete lattice-shell. { and lobes, 5. #Botryodea#. { {Cephalis simple, without { constriction and lobes, 6. #Cyrtoidea#.

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Order V. PLECTELLARIA, Haeckel, 1883.

Definition.--NASSELLARIA without complete lattice-shell, usually with an incomplete skeleton, formed of a ring, a tripod, or an irregular framework.

Suborder I. NASSOIDEA, Haeckel.

Definition.--NASSELLARIA without skeleton. Single family Nassellida.

{896}Family XLV. #NASSELLIDA#, Haeckel.

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

Definition.--NASSELLARIA 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 #Nassellida# comprises the simplest and most primitive forms of NASSELLARIA, 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 #Colloidea# among the SPUMELLARIA, in the Phæodinida among the PHÆODARIA. Probably these naked and skeletonless MONOPYLEA 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 NASSELLARIA which have not yet secreted a skeleton, or older NASSELLARIA 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 #Colloidea# or the skeletonless SPUMELLARIA. But in these two latter genera, as in all PERIPYLEA, 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 MONOPYLEA. 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.

Synopsis of the Genera of Nassellida.

Calymma hyaline, without alveoles, 382. Cystidium.

Calymma foamy, with numerous large alveoles, 383. Nassella.

{897}Genus 382. Cystidium,[1] R. Hertwig, 1879, Organismus d. Radiol., p. 87.

Definition.--#Nassellida# with hyaline calymma, without extracapsular alveoles.

The genus Cystidium is the simplest and most primitive among all NASSELLARIA, and may be regarded as the common ancestral form of this legion, for which it has the same high importance as Actissa for the SPUMELLARIA. The central capsule is quite simple, ovate or nearly spherical, and included in a voluminous hyaline calymma, which contains no large alveoles. Cystidium differs from Actissa, its probable ancestral form, in the possession of the "podoconus" and its basal "porochora," characteristic of all NASSELLARIA or MONOPYLEA.

1. Cystidium princeps, n. sp. (Pl. 91, fig. 1).

Central capsule ovate, one and a third times as long as broad. Podoconus with simple circular porochora, about half as long as the capsule, surrounded at its base by red granules of pigment. Nucleus spherical. Three equal oil-globules in the endoplasm. No pigment in the voluminous calymma, which includes numerous xanthellæ.

Dimensions.--Length of the central capsule 0.1, breadth 0.075; nucleus 0.035.

Habitat.--Indian Ocean, Maldive Islands (Haeckel), surface.

2. Cystidium lecythium, n. sp.

Central capsule ovate, one and a half times as long as broad. Podoconus with simple circular porochora, three-fourths as long as the capsule. Nucleus ellipsoidal. No oil-globules in the endoplasm. No pigment and no xanthellæ in the calymma.

Dimensions.--Length of the central capsule 0.12, breadth 0.08; nucleus 0.04.

Habitat.--Central Pacific, Station 271, surface.

3. Cystidium inerme, R. Hertwig.

Cystidium inerme, R. Hertwig, 1879, Organismus d. Radiol., p. 87, Taf. vii. figs. 1-1_b_.

Central capsule subspherical, a little longer than broad. Podoconus about two-thirds as long as the capsule, with trifid porochora, which is composed of three equal circular lobes. Nucleus spherical. Numerous oil-globules in the endoplasm. The calymma includes numerous xanthellæ and brown pigment around the mouth.

Dimensions.--Length of the central capsule 0.06, breadth 0.05; nucleus 0.03.

Habitat.--Mediterranean, Messina (R. Hertwig), surface.

{898}Genus 383. Nassella,[2] nov. gen.

Definition.--#Nassellida# with foamy calymma, containing numerous large extracapsular alveoles.

The genus Nassella differs from the preceding Cystidium, its probable ancestral form, in the development of numerous large alveoles in the extracapsular calymma, and therefore exhibits the same relation to it that Thalassicolla bears to Actissa among the SPUMELLARIA. The foamy calymma is very voluminous, and includes numerous symbiotic xanthellæ.