204 TOWARDS THE HUMAN FORM 



finer divisions, or by contracting in such a manner as to fray 

 out from their surface lobes of varying depths, as in the case 

 of the Amoebae ; Infusoria of definite form, motile on account 

 of their one, two, or more long, fine flagella, or, thanks to the 

 waving of vibr utile cilia, arranged either as a uniform fleece or 

 along regular belts ; or Sporozoa, living a parasitic life in the 

 bodies of other organisms. Of all these generally microscopic 

 organisms only the Rhizopods, the most delicate, have left 

 behind any traces. Amongst them are trie Foraminifera, which 

 secrete a sort of calcareous covering that is often exceedingly 

 elegant ; and the Radiolaria, which secrete silica that is 

 distributed throughout their substance in delicate disjointed 

 bodies of varied form constant for each species, or is united 

 in a kind of skeleton which often has the appearance of 

 a ball of lace stuck all over with needles. Cayeux has 

 found spicules of Radiolarians in the phtanites of the 

 Algonkian strata of Lamballe (C6tes-du-Nord) . As beds of 

 lime are already found in these layers, one might imagine 

 that skeletons of Foraminifera had been incorporated in 

 them. But the earliest Foraminifera definitely identified 

 date back only to the Cambrian Period, and it is worthy 

 of note, as confirming the stability of the simplest living 

 forms, that these are Orbulince and Globigerince , whose ana- 

 logues still float on the surface of our seas to-day in consider- 

 able numbers, and when they sink to the bottom form a 

 Globigerina ooze very much like that which resulted in our 

 white chalk. 



Ramified animals, as we have seen, are divided into three 

 distinct series parallelly evolved, whose starting-point was 

 from certain initial forms that can be described schematically 

 as ovoid vase-shaped organisms fixed at the base and differing 

 among themselves in the make-up of their walls, 1 and which we 

 propose to call Spongomerids, Hydromerids, and Bryomerids. 



Embryogeny alone could have told us how a Spongemerid 

 becomes transformed into a Sponge ; but unfortunately the 

 Hexactinellidae, the sponges that go furthest back into remote 

 times, are not sufficiently well known from this point of 

 view, though their life-history would be particularly instructive. 

 The most beautiful of the Sponges, they have continued through 



1 Cf. p. 115. 



