Page:Scientific results HMS Challenger vol 18 part 1.djvu/232

24 development was given in 1876 by Richard Hertwig in his Histologie der Radiolarien (pp. 12-42, Tafn. i., ii.). A number of other very remarkable forms of Collozoum have been observed by me during the last few years, and partly figured in Pl. 3.

Collozoum, as the only representative of this family, is sufficiently distinguished from all other Radiolaria by the definition "Skeletonless Radiolarian Colonies." These occur floating on the surface of all warmer seas, often in astonishing masses, and may be easily confounded, owing to their external resemblance, with the jelly-like egg-masses of certain Mollusca. Collozoum is derived either from Actissa or from Thalassicolla, simply by multiplication of the unicellular body and by reunion of the associated capsules in one common calymma or jelly-veil; this is constantly alveolated, as in Thalassicolla. As in Actissa, the form of the central capsule remains either spherical, or it becomes ellipsoidal or discoidal, rarely polyhedral or amœboid. In Collozoum as in all colonial Radiolaria, the original central nucleus commonly undergoes cleavage very early into numerous small nuclei, whilst its place is usually taken by a central oil-globule. This peculiarity may serve often (but not constantly) for the distinction of isolated capsules of Collozoum from Actissa.

Definition.—Skeletonless colonies of Radiolaria.

The genus Collozoum, as already mentioned, is the only representative of its family, and comprises all Radiolaria living associated in colonies, and having no skeleton. Therefore Collozoum possesses all the peculiarities described above. Although the floating colonies of this genus occur in enormous masses on the surface of all warmer seas, nevertheless the number of different species in this genus is not great, and amounts only to thirteen. If this number increase by further investigations, the subgenera distinguished in the following description can be advanced to the range of genera; in which case Collodinium (or Collozoum sensu restricto) will be characterised by the spherical form of its central capsules, Colloprunum by the ellipsoidal form (Pl. 3, fig. 9), Collophidium by the cylindrical, very prolonged form (figs. 2, 3), Collodiscus by the lenticular or discoidal form, and Collodastrum by the indefinite, polyhedral, or amœboid form (figs. 4, 5).

Definition.—Form of the central capsules spherical or subspherical, never polyhedral, ellipsoidal, or cylindrical.