12 



THE VOYAGE OF H.M.S. CHALLENGER. 



Synopsis of the Genera of Thalassicollida. 



A. Alveoli neither within nor without the 

 central capsule. 



f Nucleus spherical (sometimes ellip- 



( soidal), not branched, . . 1. Actissa. 



/ Nucleus spherical, 



B. Numerous large alveoli within the central ) 



capsule (not in the calymma). "S Nucleus branched or covered with radial 



(, sacs, .... 



/ Nucleus spherical, 



C. Numerous large alveoli without the central J 



capsule, within the jelly- veil or calymma. "S Nucleus branched, or covered with radial 



\ sacs, .... 



2. Thalassolampe 



3. TJialassopila. 



4. Thalassicolla. 



5. Thalassophijsa. 



Genus 1. Actissa, 1 n. gen. 



Definition. T halassicollida with simple spherical nucleus, without any alveoli 

 (either within or outside the central capsule). 



The genus Actissa is the most simple and typical form of all Radiolaria, and may be 

 regarded as the common ancestral form of the whole class. The spherical body is com- 

 posed of a simple spherical capsule and a concentric spherical calymma or jelly-envelope. 

 Neither the former nor the latter contains alveoli. The central capsule possesses a strong 

 membrane perforated by small pores, and contains in the intracapsular sarcode numerous 

 small pellucid vacuoles, and in its middle a simple, concentric, spherical nucleus (often 

 with some nucleoli); sometimes also one or more oil-globules. The extracapsularium 

 forms a soft, voluminous, structureless calymma or enveloping jelly-sphere, perforated 

 by the numberless, fine pseudopodia, which radiate outwards from the matrix or 

 the thin granulated sarcode-layer, surrounding the capsule. Often (but not constantly) 

 xanthellse or yellow cells are scattered in it. Actissa differs from the following 

 skeletonless genera in the absence of all alveoli ; it has neither intracapsular alveoli (like 

 Thalassolampe) nor extracapsular alveoli (like Thalassicolla). The first observed species 

 of this genus is that which I found in 1866 at the Canary Islands, Actissa prototypus; 

 the second is that which Hertwig accurately described in 1879, from the Mediterranean 

 (Messina), Actissa primordialis ; the third I observed in 1881 at Ceylon, frequent 

 and sporiparous, Actissa princeps. A fourth species (Actissa radiata) exhibited a dis- 

 tinct radial segmentation of the capsule-contents. These four species are quite spherical. 

 Six other species, occurring in different preparations from the Challenger, are distinguished 

 by modifications of the spherical capsule-form and may represent three different subgenera 

 (or, perhaps better, genera ?) Actiprunum ellipsoidal, Actidiscus lenticular, Actilarcus 

 lentelliptical ; perhaps these are the ancestral forms of the three sections : Prunoidea, 

 Discoidea, Larcoidea. 



1 Actissa, = Radiant, 



