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#. ---- 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.