100 THE LIVING WORLD. 



we reach the Mesozoic (5, 6, and 7) rocks that their remains become 

 abundant. 



The other Protozoa, having no hard parts, could not, of course, 

 have been preserved as fossils. There can be no doubt now of their 

 existence through all of the geological ages, since, as we have seen, 

 the Foraminifera and Radiolaria, which are the highest of the Protozoa, 

 have lived during all these periods, and the lower orders of any group 

 always appear before the higher ones. In general, then, we may 

 conclude with little chance of error that the Protozoa were in exist- 

 ence at the beginning of the Silurian (2), in practically the same 

 condition that they are now, and that the long ages since have seen 

 very little modification in their structure. They have been practically 

 stationary, and their development was almost wholly pre-Silurian. 



CCELENTERA. 



Four classes of animals are to-day included under the head of 

 Ccelentera. 



Port/era (sponges). These are the lowest existing multicellular 

 animals. As would be expected, therefore, they are very ancient in 

 origin. They are found in the lowest Silurian (2) rocks and have 

 been found in every age since that time. Here also the probability 

 seems to be that the sponges of early times were much like those of 

 to-day, so that there has been, very little real advance in structure. 

 The variety of type was however much smaller in early times than 

 to-day. 



Hydrozoa. This class includes the hydroids (see Fig. 7) and the 

 well-known jelly-fishes. The jelly-fishes being so soft, it is, of 

 course, impossible to say when they really appeared, their absence 

 as fossils proving nothing. The first traces we have of them are 

 in the Jurassic (7). Many of the hydroids, however, have a shell, 

 either of lime or of some horny material, and they have been found 

 in all ages. The Silurian (2) rocks contain them in abundance. But 

 the hydroids of this period were quite unlike those found at the 

 present day. One entire class found in the greatest abundance at 

 that time has since entirely disappeared (Graptolitoided), the last traces 

 being found in the Silurian (2). Several smaller groups have also 

 disappeared. Of our modern forms, only one class (Thecophora) was 

 in existence, and this was quite different from its representatives 

 to-day. This class continued with little change during the whole of 



