PALAEOZOIC AGE 



CAMBRIAN PERIOD 



THE seas in Cambrian times, no doubt, spread far more 

 extensively than now ; but probably only the shallows 

 and moderate depths were as yet the haunts of marine 

 life. 



Tiny one-celled plants and animals must have abounded in 

 the waters. As to this, however, it is not likely that proof, 

 except of a meagre description, will ever be forthcoming. 

 PROTOZOANS The evidence as to seaweeds is extremely doubtful ; but 

 remains of some minute one-celled animals, that had for 

 purposes of protection secreted particles of hard substance, 

 have been brought to light. Some of these microscopic 

 creatures were foraminifers or " aperture-bearers " the 

 name having reference to perforations in the body-covering, 

 affording outlet for fine threads used in locomotion, and as 

 food-collectors (Cristellaria). Others sphere-shaped crea- 

 tures with minute flinty skeletons were probably radio- 

 larians (Sphceroidea). 



SPONGES Sponges were in the seas, and in some variety. Certain 

 transformed cells, it is clear, had long been at work in the 

 flabby objects for the purpose of providing hard internal 

 supports. Flinty material had been collected, and worked up 

 into needles or spicules from the various centres of secretion. 

 These structures, according to the canal system of the sponge, 

 took various shapes. Originally they developed, no doubt, as 

 tiny rods ; but, in course of time, by throwing out cross- 

 rays, they became four-rayed and six-rayed. Sponges, thus 

 fortified, were certainly living in the Cambrian (Protospongia, 

 Archaoscyphia). Two well-known existing groups of Sponges 

 were, therefore, already in being (Hexactinellids, Tetracti- 



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