Tasch: Growth and Form of Fossil Protists 441 



The form of radiolarian species seems to be adaptive to environmental 

 conditions'*'^ although experimental study of factors influencing shell mor- 

 phology are wanting.'^^ Thompson/'* as noted previously, provided some useful 

 insights into radiolarian morphology. 



Free floating radiolarians, among both fossil and living assemblages, tend to 

 be spherical and elliptical, with a foamy or spongy appearance. Such forms 

 occur in the Spumellina, Nasselina, and Acantharina. The shells are delicate, 

 small, and bear various structures such as, numerous slender apophyses, large 

 pores, thin bars between pores, and varied spinose development. Inhabitants 

 of deeper layers (Phaeodarina and some Nassellina) are heavier, more massive, 

 and tend to bilateral symmetry. They are infrequently burrlike. Structures 

 found in such forms include: short apophyses and small pores with thick tra- 

 beculae.^'*'^^ . 



In some forms (Semantidae, etc.)'*^ one can observe configurations not too 

 different from those of the silicoflagellates.^® The shell in the Challengeridae 

 bears a fine hexagonal mesh resembhng similar structures of the diatom frus- 

 tule.^^ Some configurations of radiolarians are analogous to those of armored 

 dinoflagellates, for example, Coelodecas?^ Hexaspyris papilio^^ is reminiscent 

 of the bizarre spinose development in the dinoflagellate Ceratium. 



It is generally agreed that variations in scleratoma configuration and in 

 skeletal structures found in radiolarians reflect adaptations to retard sinking 

 below certain depth levels of the sea. Within a given species of course, varia- 

 tions of shape and structure are merely those of a normal population spread. 



The gelatinous or pseudochitinous cuplike or elongate lorica of tintinnids is 

 frequently agglutinated. Foreign particles encrusted or included in the deli- 

 cate membranous wall may consist of fine mineral grains, coccoliths, diatoms, 

 and organic debris.^^'^'^'^^ Shape of skeleton in both fossil and living tintinnids 

 is extremely diverse. ^^ Surface markings of the lorica include: ribs, ridges, 

 plications, flutings, shelves, reticulations, fenestrae, and lacunae. Among 

 aboral structures are apophyses, pedicel, knob, lance, and skirt. ^'"^ 



Because tintinnids move like squids with oral end directed backwards, 

 streamline configuration of the aboral tip would offer less frictional resistance 

 to forward movement. It is also likely that the lorica may aid flotation.^^ 

 The total effect of such configurations is to check descent below optimal levels. 



Modification of Shape and Form in Foraminifera 



Work on living foraminiferal distribution and ecology has clearly established 

 characteristic faunal suites in distinct brackish and marine environments. '"'"■*'' 

 Although the majority of foraminifers are vagrant benthos, planktonic forms 

 that float at or near the surface such as the Hantkennidae, Orbulinidae, and 

 Globorotalidae have been more closely studied in the past decade.'*" '^^ 



Bandy^* found a striking correlation in form, structure, and environment in 

 benthonic foraminiferal assemblages in modern seas. Among the variations 

 he observed are overall size, shape, and size of chambers, chamberlets, coiled 

 and uncoiled forms, spinosity, surface sculpture of the test (costa, striae). 

 These were found to vary with bathymetry (bay, shelf, and bathyal zones). 

 Phleger'*" believes that the influence of temperature may have been overstated 



