748 MICROSCOPIC FORMS OF ANIMAL LIFE PROTOZOA 



precipitate, consisting of sulphate of lime, slowly deposited in water 

 to which strong spirit has been added. Whatever be their nature, 1 

 coccoliths and coccospheres are bodies of great interest ; since their 

 occurrence in chalk and in very early limestones is an additional link 

 in the evidence of the similarity of the conditions under which they 

 were formed to those at present prevailing on the sea-bed of the 

 Atlantic and other oceans. Two distinct types are recognisable among 

 the coccoliths, which Professor Huxley has designated respectively 

 discoliths and cyatholiths. The former are round or oval discs, having 

 a thick strongly refracting rim and a thinner internal portion, the 

 greater part of w^hich is occupied by a slightly opaque, cloud-like 

 patch lying round a central corpuscle (fig. 518, 5). In general, the 

 ' discoliths ' are slightly convex 011 one side, slightly concave 011 the 

 other, and the rim is raised into a prominent ridge 011 the more 



FIG. 581. Coccoliths and Coccospheres : 1, 2, 7, cyatholiths seen 

 obliquely ; 8, coccosphere with imbedded cyatholiths ; 4, coccoliths im- 

 bedded in supposed protoplasmic expansion ; 5, discolith seen in front 

 view; 6, cyatholith seen in front view, showing (1) central corpuscle, (2) 

 granular zone, (8) transparent outer zone ; 8, 9, discoliths seen edgewise. 



convex side ; so that when viewed edgewise they present the appear- 

 ances shown in figs. <9, 9. Their length is ordinarily between 



of an inch ; but it ranges from ^Vo-th to 



T t 



Th 



largest are commonly free, but the smallest are generally found im- 

 bedded among heaps of granular particles, of which some are probably 

 discoliths in an early stage of development. The ' cyatholiths,' also, 

 which have the general appearance of a cup and saucer, have, when 

 full grown, an oval contour, though they are oftrtl circular when 

 immature. They are convex 011 one face and flat or concave on the 

 other ; and when left to themselves they lie on one or other of these 

 two faces. In either of these aspects they seem to be composed of 

 two concentric zones (fig. 6, 2, 3) surrounding an oval thick- walled 

 central corpuscle (^), in the centre of which is a clear space some- 



1 Messrs. Murray and Blackmail have, in a preliminary notice (Pror. Hoy. Soc. 

 London, Ixiii. 1898, p. 269), suggested that the Coccospheraceae are unicellular Algtv. 



