Dreyer has shown that in many cases it may be explained by geometrical considerations. V. Häcker has written a most valuable account of the Biological relations of the skeleton of Radiolaria in Jen. Zeitschr. xxxix. 1904, p. 297.
Zool. Jahrb. Anat. xiv. 1900, p. 203.
Porta has described reproduction by spores and by budding in Acantharia, Rend. R. Ist. Lomb. xxxiv. 1901 (ex Journ. R. Micr. Soc. 1903, p. 45). In Thalassophysa and its allies zoospore reproduction appears to be replaced by a process in which the central capsule loses its membrane, elongates, becomes multinuclear, and ultimately breaks up into the nucleate portions, each annexing an envelope of ectoplasm to become a new individual (see Arch. Prot. vol. i. 1902).
Brandt, "Die Koloniebildenden Radiolarien," in Fauna u. Flora des Golfes v. Neapel, xiii. 1885, gives a full account of the Zooxanthellae and Diatoms, and notes the parasitism of Hyperia.
See Köppen in Zool. Anz. xvii. 1894, p. 417. For Sticholonche, see R. Hertwig in Jena. Zeitsch. xi. 1877, p. 324; and Korotneff in Zeitsch. wiss. Zool. li. 1891, p. 613. Borgert's paper on Dictyochidae is in the same volume, p. 629.