CO 



ZOOLOGY 



SECT. 



throughout the winter, and in spring the protoplasm emerges 

 from each and assumes the form of the ordinary active Actino- 

 sphaBrium. In Clathrulina spore-formation takes place in the 

 active condition, and the spores (Fig. 4*1, 3 b) are flagellulse, each 

 being an ovoid body provided with two flagella. 



Conjugation has been observed in some instances between two 

 or more individuals, which may separate again without any nuclear 

 changes taking place; or the conjugation may be followed by 

 a sexual process, comprising the coalescence of the protoplasm of 

 fhe two individuals and the coalescence of the nuclei (Fig. 42) 

 after each has given off a part of its substance (6), as in the 

 maturation of an ovum in multicellular animals (p. 19). 



ORDER 4. 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. 43, 

 cent. caps.\ which lies em- 

 bedded in the protoplasm, 

 dividing it into intra-capsular 

 (int. caps, pr.) and extra- 

 capsular (ext. caps.pr.) regions. 

 In the intra-capsular proto- 

 plasm is a large and complex 

 nucleus (nu.\ or sometimes 

 many nuclei: from the ex- 

 tra-capsular protoplasm the 

 pseudopods (psd.) are given 

 off in the form of delicate radi- 

 ating threads, which in some 

 cases remain free, in others, 

 e.g. Lithocircus, anastomose 

 freely, i.e. unite to form net- 

 works. In one large section the Acantharia the pseudopodia 

 contain firm axial rods similar to those in the pseudopods of the 

 Heliozoa. There is no contractile vacuole, but in many forms the 

 extra-capsular protoplasm contains numerous large non-contractile 

 vacuoles, which give it the frothy or bubbly appearance noticed 

 previously in Hastigerina. The vacuolated portion of the proto- 

 plasm has a gelatinous consistency, and is distinguished as the 

 calymma. 



The central capsule may be looked upon as a chitinoid 

 internal skeleton, reminding us of the shell of Gromia and of 



SKel. 



FIG. 43. Lithocircus annularis. cent. caps. 

 central capsule ; ext. caps. pr. extra-capsular 

 protoplasm ; iiit. caps. pr. intra-capsular pro- 

 toplasm ; mi. nucleus ; psd. pseudopods ; skel. 

 skeleton ; z. cells of Zoochlorella (After 

 Biitschli, from Parker's Biology.) 



