252 



THE PROTOZOA 



organisms captured by the pseudopodia and passed into the body, 

 to be digested in this region. In the Tripylaria an aggregation of 

 food-material and excretory substances produces a characteristic 

 greenish or brownish mass concentrated round the main aperture 

 of the central capsule, and known as the phceodium, whence this 

 suborder is sometimes known as the Phaeodaria. 



2. The calymma is composed for the most part of a great number 

 of vacuoles containing fluid, the function of which is hydrostatic ; 

 the contents of the vacuoles are stated to be water saturated with 

 carbon dioxide, causing the animal to float at the surface, and 



enabling it to regulate its position 

 in relation to conditions of environ- 

 ment. In rough weather the vacuoles 

 burst or are expelled from the body, 

 and the animal sinks into deeper and 

 quieter layers of water; there fresh 

 vacuoles are formed, enabling it to 

 return again to the surface if the 

 conditions are favourable. Contractile 

 vacuoles of the ordinary type are not 

 present. 



In addition to the vacuoles, the 

 calymma contains numerous " yellow 

 cells," generally regarded as sym- 

 biotic organisms of vegetable nature, 

 and named " zooxanthellaa " or 

 zoochlorellae," according to their 



FIG. 106. Lithocircus productus, 

 Hertwig, showing a bilaterally 

 symmetrical skeleton consisting 

 of a simple siliceous ring pro- colour. Absent in the Tripylaria, 

 longed into spicular processes, these yellow cells are found, as a 

 sk., Skeleton ; ex., central cap- , , . , 



sule ; pf., pore-area, surmounted mle > m tn e calymma, but m 

 by a conical structure (c.), the Acantharia they occur in the intra- 

 #~ Ca nucleusT 0!! otl-gtobule! capsular protoplasm (Fig. 105, x). 

 After Biitschli, Leuckart and The nature of the yellow cells of 

 " Zoologische Wand. Acantharia has been much disputed, 

 and many observers have regarded 

 them as an integral part of the organism itself ; this view has 

 recently been revived by Moroff and Stiasny, who bring forward 

 evidence to prove that the yellow cells of Acantharia are a 

 developmental phase of the organism. Still more recently this 

 view has been extended by Stiasny to the colony-forming 

 Sphaerozoa in the first place, and then to Radiolaria generally. 

 The difficulty in the way of such an interpretation which arises 

 frojn the co-existence, in %halassicolla and other genera, of yellow 

 cells in the calymma, with an undivided nucleus in the host- 

 organism, is met by supposing - that in such cases developmental 



