HELIOZOA. 867 



said to be separated from the body by a space (?a homogeneous jelly). 

 There is some doubt as to its constitution. Three facts appear to point to 

 its organic nature : (i) the presence in Heterophrys of minute points or 

 processes of its free surface which contract under the action of carmine 

 staining fluid ; (2) its constant elevation round the bases of the pseudo- 

 podia ; (3) its occurrence in a loose and crumpled condition round an 

 encysted Sphaerastrum. On the contrary it has been said to consist in 

 Heterophrys of a coat of finely interlaced needles which resist the action 

 of strong sulphuric acid, and are consequently supposed to be siliceous. 

 An undoubtedly siliceous skeleton exists in the two remaining sections of 

 the class, the Chalarothoraca and Desmothoraca. In the former, it is com- 

 posed of loose parts : spherules in one to three layers (Pompholyxophrys 

 pumicea^Hyalolampe] ; round plates in a single layer (Pinacocystis rubi- 

 ciinda] ; leaf-like plates pointed at each end and pierced by pores (?) 

 (Pinaciophora fltiviatilis) ; spicules placed tangentially to the surface, 

 straight or curved, and pointed at both ends (Raphidiophrys] ; spines 

 radially placed, with basal plates, and either simple or forked at their 

 outer extremities, together with tangential spicules (Acanthocystis). These 

 loose structures are held together either by protoplasm derived from the 

 pseudopodia, or by a soft secreted material ; the fact that the parts retain their 

 position when the animal is contracted and encysted favours the latter 

 view. In Raphidiophrys the skeleton is so loose that it is raised into 

 eminences where the pseudopodia protrude. A clear interval may separate 

 the skeleton from the bocFy of the animal, whether a space or homogeneous 

 jelly is doubtful. In the Desmothoraca the skeleton is a lattice-sphere 

 through the apertures of which pass the pseudopodia. It turns brown 

 with age and is attached by a siliceous peduncle in Clathrulina ; it is free in 

 Orbulinella ; and in both instances it is larger than the body of the animal. 

 An adventitious skeleton of sand-grains occurs in the marine Lithocolla 

 and Raphidiophrys arenosa, of sand-grains mixed with diatom-shells in the 

 freshwater Elaeorhanis. 



The animal is attached by a chitinoid peduncle in the skeleton-less 

 Actinolophus, otherwise it is free though inclosed in a fixed shell in Cla- 

 thrulina. The protoplasm, which varies in consistence, and is rarely 

 coloured of a yellow tint, is clearly divisible into an ectosarc, and an 

 endosarc in Actinophrys, Actinosphaerium and Actinolophus, and probably 

 in all skeletogenous forms except the Desmothoraca. The ectosarc is more 

 or less granular, and the seat of digestion, the endosarc, homogeneous ; 

 in Actinosphaerium, however, the characters of the two parts are reversed. 

 In Actinolophus and Acanthocystis the endosarc is excentric and touches 

 the surface at one spot, in others it is central. There is no separating 

 membrane between the two parts, yet the transition from one to the other 

 is usually somewhat sudden. Some species are coloured : Actinolophus 



3 K 2 



