82 



COMPARATIVE ANATOMY. 



are merely represented by several separate columns, or lamellas, 

 which have wide openings between them (Fig. 25), and the cavity of 

 the chamber itself is less than the various connections between two 

 chambers, and where, finally, all the neighbouring chamber-spaces 

 communicate with one another, and the whole " test " is thereby 

 traversed by a cavitary system which communicates in all directions, 

 then the character of an external shell is completely lost. And so, 

 since the protoplasm is in all cases able to draw itself over the outer 

 surface of the shell, the shells in the Foraminifera ought to be 

 regarded as internal ones, and grouped accordingly with the skeletons 

 of the Radiolaria. 



§ 65. 



The " central capsule " must be noted as an organ of support 

 common to all the Radiolaria, although it is not very apparent. 

 It is a capsular-closed organ, various in form, placed in the middle 

 of the body, and formed of a membrane which closely resembles 

 chitin. In addition to fat-globules and structures which are re- 

 garded as nuclei, it always contains a quantity of protoplasm, which 

 is apparently continuous with the extra-capsular protoplasm by means 

 of fine pore-canals. In most Radiolaria (not in Thalassicolla, 

 Thalassolampe, and Collozoum) there is, in addition to this capsule, a 



skeleton, ordinarily formed 

 of silica, which traverses the 

 capsule when most fully de- 

 veloped (Fig. 26). Several 

 spicules then radiate from a 

 common centre, and these 

 may be connected with one 

 another by means of a con- 

 centric highly fenestrated 

 framework (Fig-. 26). In 

 some (AcanthometridEe) the 

 organic base of the frame- 

 work predominates, and is 

 but slowly replaced by silica. 

 Separate spicular bits 

 of silica, lying freely in 

 the protoplasm outside the 

 central capsule, are the 

 earliest indications of this 

 firm skeleton in the Collidas 

 and Polyzoa. In some they 

 become arranged in a radiate 

 manner,withoutbeing firmly 

 connected together. Circular skeletons, forming an open network, 

 arc formed by the radial spicula being connected together at equal 

 distances by rods placed perpendicularly to them. When a very 

 fine supporting reticulum is arranged around the radial spicula in 



Fig. 26. Skeleton of a Radiolarian (AcHnomma 



asteracanthion). Two concentrically-arranged 



fenestrated shells are broken through at one 



point, to show a third one (after E. Hiickcl). 



