THE RADIOLARIA 117 



Brandt therefore considers that the outer pseudopodia upon con- 

 tact with certain stimuli (wave-motion and heat) contract and 

 transmit the stimulus to the subjacent alveolar protoplasm. This 

 in turn contracts and the surface vacuoles collapse. When this 

 process has been continued for a certain time the specific gravity of 

 the animal is raised and a slow descent follows. Equilibrium 

 is again established, the vacuoles are re-formed, and the animal rises 

 again to the surface. 



The calymmal spheres do not, however, monopolise the hydro- 

 static function. The flotation of Radiolaria is determined by 

 extension of its surface as well as by the lowering of its specific 

 gravity, and in this sustentative adaptation the outer pellicle 

 and the skeleton play the chief role. The skeleton of the 

 Acantharia is composed of a radiating series of tent-poles upon 

 which the ectoplasm can be raised and tightened by the elastic 

 filaments that pull up the baggy ectoplasm, which upon inflation 

 by vacuolar water expands, and so raises the animal to a higher 

 zone of water ; or again contracts, followed by deflation and sinking 

 of the whole mechanism. 



Again, in Phaeodaria we have a still more elaborate skeleton, 

 the appendicular parts of which are related to the formation and 

 support of the ectoplastic membrane. In an impressive variety of 

 sustentative adaptations the ectoplasm of Eadiolaria deposits silicic 

 acid or strontium sulphate ; and the attempt now being made 

 to trace a correlation between the variation of this support, 

 the extent and thickness of the outer membrane, and the density 

 and viscosity of various tracts of water inhabited by widely 

 varying forms, has already met with some success (Hacker [35]). 

 Racial forms occur. Aulacantha scolymantha, for example, only 

 attains a diameter of 2 '3 mm. in warm surface waters; its ecto- 

 plastic membrane is soft and its spicules small and simple ; whereas 

 in deep, cold water (400-1000 metres) it reaches 7 mm. and 

 consists of a much tougher envelope supported by more numerous 

 spicules. Circoporus sexfuscinus and other Phaeodaria are also 

 dimorphic and exhibit a similar differential relation to the surface 

 and abyssal Avaters in which they occur. 



The ectoplasm rarely contains assimilates or other inclusions. 

 Oil -globules, however, occur in the large Collodaria ; pigment 

 (blue, black, brown, or red) in the Thalassicollidae, Sphaeroidea, 

 Discoidea, and some Acantharia ; and concretions (probably 

 proteid) in some Thalassicollidae. Yellow cells are generally 

 present in the ectoplasm, and the only large division in which they 

 are unknown is that of the Phaeodaria. In the Acantharia, 

 however, they occur almost constantly in the endoplasm. A 

 further account of these cells is given below. 



The myonemes are peculiar modifications of the basal ends of 



