The other Thalassicollida differ from their common ancestral form, _Actissa_, mainly by the higher histological differentiation of the unicellular body. Whilst in _Thalassicolla_ and _Thalassolampe_ the nucleus remains a single sphere as in _Actissa_, it becomes branched or covered with radial blind saccules in _Thalassopila_ and _Thalassophysa_; also the intracapsular protoplasm develops here a great variety of peculiar different corpuscles, as oil-globules, pigment-granules, concentric concretions, crystals, &c. But the most striking peculiarity by which the other Thalassicollida differ from _Actissa_ is the development of large vesicular alveoli, either within or without the capsule; the unicellular body reaches by this inflation the extraordinary size of 5 to 10 mm. or more. {12}_Synopsis of the Genera of Thalassicollida._ A. Alveoli neither { Nucleus spherical within nor without { (sometimes ellipsoidal), the central capsule. { not branched, 1. _Actissa_. B. Numerous large { Nucleus spherical, 2. _Thalassolampe_. alveoli within the { central capsule (not { Nucleus branched or in the calymma). { covered with radial { sacs, 3. _Thalassopila_. C. Numerous large { Nucleus spherical, 4. _Thalassicolla_. alveoli without the { central capsule, { Nucleus branched, or within the jelly-veil { covered with radial or calymma. { sacs, 5. _Thalassophysa_. Genus 1. _Actissa_,[8] n. gen. _Definition._--#Thalassicollida# with simple spherical nucleus, without any alveoli (either within or outside the central capsule).