58 



ZOOLOGY 



SECT, 



Reproduction ordinarily takes place by binary fission, but 

 spore-formation also occurs. Actinosphserium, for instance, encloses 

 itself in a gelatinous cyst and undergoes multiple fission, forming 

 numerous spores each enclosed in a siliceous cell-wall. These 

 resting spores remain quiescent throughout the winter, and in spring 

 the protoplasm emerges from each and assumes the form of the 

 ordinary active Actinosphasrium. In Clathrulina spore-formation 

 takes place in the active condition, and the spores (Fig. 39, 3 b) are 

 tlagellulse, each being an ovoid body provided with two flagella. 

 Conjugation has been observed in some instances, but the precise 

 nature and significance of the process is still imperfectly known. 



ORDER 5. RADIOLARIA. 



The Radiolaria are a large and well-defined group of Rhizopods, 

 noticeable, in most instances, by the presence of a siliceous skeleton 

 of great beauty and complexity. They are all marine. 



General Structure. The most important characteristic of 

 the group is the presence of a perforated membranous sac, called 

 the central capsule (Fig. 40, cent, caps.), which lies embedded in the 

 protoplasm, dividing it into intra-capsular (int. caps, pr.) and extra- 

 capsular (e,:d. caps.pr.) regions. In the intra-capsular protoplasm is 

 a large and complex nucleus (nu.), or sometimes many nuclei : from 

 the extra-capsular protoplasm the pseudopods(^sc/.) are given off in 

 the form of delicate radiating threads, which in some cases remain 

 free, in others, e.g. Lithocircus, anastomose freely, i.e. unite to form 



networks. There is no con- 

 tractile vacuole, but in many 

 forms the extra-capsular pro- 

 toplasm contains numerous 

 large non-contractile vacuoles, 

 which give it the frothy or 

 bubbly appearance noticed 

 previously in Hastigerina. 

 The vacuolated portion of 



SKei.' yg^v*. ' K - fT the protoplasm has a gela- 



z ' l/\ \f tinous consistency, and is dis- 



tinguished as the cali/mna. 



The central capsule may 

 be looked upon as a chitinoid 

 internal skeleton, reminding 

 us of the shell of Gromia and 

 of the perforated calcareous 



shell of Hastigerina with its investment of vacuolated proto- 

 plasm. It is found in its simplest form in Thalassoplancta 

 (Fig. 41), in which it is spherical and uniformly perforated with 

 minute holes. In other forms, such as Litliocircus (Fig 40), it is 



nu 



Inl: caps.pr 

 cent caps 



FIG. 40. Lithocircus nnmilaris c,, it. caps. 

 central capsule ; ext. caps, j>r. extra-capsular 

 pn itoplism ; iiif. cnjix. jn: intra-capsular pro- 

 toplasm ; nu. nucleus ; /..<>/. pseudopods ; skel. 

 skeleton ; -. cells of Zooxanthella. (After 

 Butschli, from Parker's Bioloyy.) 



