PROTOZOA : SPONGIDA. 93 



Mesozoic, and Kainozoic epochs. The keratose or horny 

 sponges are obviously incapable of leaving any evidence of 

 their existence, otherwise than by the preservation of the 

 spicula with which the skeleton is sometimes furnished ; and 

 such are occasionally found, though they are of rare occurrence. 

 The calcareous sponges are found from the Silurian rocks 

 upwards, though great obscurity still rests upon the true nature 

 and affinities of many of the fossils which have been referred 

 to this group. 



The siliceous sponges are now known to possess a very high antiquity, 

 both the sections of the Hexactinellidce and Lithistidce being developed as 

 early as the Silurian period. Of the fossil Hexactinettida, the best-known 

 group is that of the Ventriculitidcz, comprising a large number of beautiful 

 Secondary sponges. Of the fossil Lithistids, the best-known genus is the 

 widely distributed Siphonia of the Cretaceous period. 



AFFINITIES AND SYSTEMATIC POSITION OF THE SPONGES. 

 Great doubts still exist as to the real relations and zoological 

 place of the sponges ; and though placed here with the Rhizo- 

 poda, there are many considerations which render this col- 

 location objectionable. The opinions of naturalists in this 

 matter are still unsettled, and the whole subject is a very com- 

 plex one, so that it will be sufficient to simply allude to one 

 or two of the more important points affecting this question. 

 We have seen that each sponge may be regarded as an aggre- 

 gate of protoplasmic masses (sarcoids), each of which is mor- 

 phologically a single "cell." This fact in itself, as pointed 

 out by Hseckel, is an important one, for the typical Rhizopod, 

 such as Awceba, is to be looked upon as morphologically a 

 single cell leading an independent life, and singly discharging 

 all the functions of vitality. Upon this ground, as well as 

 upon weighty developmental grounds, Haeckel would remove 

 the sponges altogether from the Protozoa, and would place them 

 among the Ccdenterata. On the other hand, the individual 

 " sarcoid " of a sponge, if non-flagellate, presents a resemblance 

 to an Amoeba, which is far too striking to be overlooked ; 

 whilst the flagellate sarcoids present an equally conspicuous 

 similarity to the flagellate Infusoria. Indeed it is very diffi- 

 cult to see upon what classificatory principle those sponges 

 (such as Halisarca) which have no skeleton can be separated 

 from such compound flagellate Infusoria as Phalansterium, 

 except that the latter is without the canal-system which tra- 

 verses the protoplasm of the former. There are also many 

 points of affinity between the sponges and the Radiolaria. 

 These considerations appear to be sufficient, in the meanwhile, 

 and in a work of this nature, to justify the retention of the Spon- 



