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.

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 PLECTELLARIA. Nassellaria without complete lattice-shell.		No skeleton,	1. Nassoidea.
		Skeleton with a basal tripod, without ring,	2. Plectoidea.
		Skeleton with a sagittal ring (usually without tripod),	3. Stephoidea.
II. Order CYRTELLARIA. Nassellaria with a complete lattice-shell.		Cephalis bilocular, with a sagittal constriction,	4. Spyroidea.
		Cephalis multilocular, with two or more constrictions and lobes,	5. Botryodea.
		Cephalis simple, without constriction and lobes,	6. Cyrtoidea.

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.

Family XLV. Nassellida, Haeckel.

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