BECENT EESEAKCHES ON THE RADIOLARIA. 141 



Gyrtida) it may have the form of a coue, -with its base provided 

 with three or four rounded processes. 



In no part of the sarcode do vpe find habitually contractile 

 spaces (as we do in the Heliozoa) ; but we may find in the intra- 

 or extracapsular sarcode, or in both, rounded spaces, called alveoli, 

 which are mere vacant spaces save for fluid contents, and are 

 devoid of any limiting membrane. They are often considerable 

 both in size and number. The sarcode surrounding some of these 

 alveoli may be so contractile as to occasionally obliterate them, 

 and so, by increasing the specific gravity of the organism, enable 

 it to sink. It is only the more superficially placed alveoli, how- 

 ever, which have been observed so to disappear. 



The intracapsular sarcode contains alveoli in Thalassicolla, Tha- 

 lassolampe, and Fhysematiwin only ; but, besides certain other for- 

 mations more or less frequently present, it constantly contains 

 one or both of two sets of structures, which structures, according to 

 the researches of Hertwig, bear to the whole capsule the relation 

 of many simple nuclei or of a single complex nucleus to a cell. 



The simple nuclei (the ^^ wasserhellen Bldschen^^ of authors) are 

 rounded homogeneous particles of denser sarcode devoid of in- 

 vesting membrane, and varying in size from about O'OOS millim. 

 to about 0"015 millim., the largest in size being found where the 

 substance is least. They may be so numerous as to fill the cap- 

 sule, and acquire by reciprocal pressure a polyhedral aspect ; or 

 they may be very few and grouped together towards the centre 

 of the capsule ; or, finally, when a complex nucleus is present, 

 they may be altogether absent. 



Fis. ;}. 



Nuclear vesicle of Myxohrachia iduteus. (After Haeckel.) 



