24 THE VOYAGE OF H.M.S. CHALLENGER. 



development was given in 1876 by Eicliarcl Hertwig in liis Histologie der Eadiolarien 

 (pp. 12-42, Tafn. i., ii.). A number of other very remarkable forms of Collozoum liave 

 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 Eadiolaria by the definition " Skeletonless Radiolarian Colonies." These 

 occur floating on the surface of all warmer seas, often in astonishmg 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 Tlialassicolla, 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 ca2isule remains either spherical, or it becomes 

 ellipsoidal or discoidal, rarely polyhedral or amoeboid. In Collozoum as in all colonial 

 Eadiolaria, 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. 



Genus 6. Collozoum^ Haeckel, 1862, Monogr. d. EadioL, p. 522. 



Definition. — Skeletonless colonies of Eadiolaria. 



The genus Collozoum, as abeady mentioned, is the only representative of its family, 

 and comprises all Eadiolaria living associated in colonies, and having no skeleton. 

 Therefore Collozoum possesses all the peculiarities described above. Although the float- 

 ing 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 Collodiniuin (or Collozoum sensu restrict© !) wiU be characterised by the 

 spherical form of its central capsules, Collojymnum by the ellipsoidal form (PL 3, fig. 9), 

 Collophidium by the cylindrical, very prolonged form (figs. 2, 3), CoUodiscus by the 

 lenticular or discoidal form, and Collodastrumi by the indefinite, polyhedral, or 

 amoeboid form (figs. 4, 5). 



Subgenus 1. Collodinium, Haeckel. 



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

 ellipsoidal, or cylindrical. 



' CoHozoum = Jelly-animal ; kiMm, Z^ma. 



