338 THE PROGRESS OF ANIMAL LIFE. 



its trail si tions, and be in a better condition for applying: 

 our knowledge of physical power to the explanation of 

 the various geological phenomena. 



Among the earliest races we have those remarkable 

 forms, the trilobites, inhabiting the ancient ocean. 



These Crustacea bear some resemblance, although a 

 very remote one, to the common wood-louse, and, like 

 that animal, they had the power of rolling themselves 

 into a ba.ll when attacked by an enemy. The eye of the 

 trilobite is a most remarkable organ ; and in that of one 

 species, P/iacops caudatus, not less than two hundred and 

 fifty lenses have been discovered. This remarkable 

 optical instrument indicates that these creatures lived 

 under similar conditions to those which surround the 

 Crustacea of the present day. 



At the period of the trilobites of the Silurian rocks, 

 all the animals contemporaneous with them had the 

 organs necessary for the preservation of life in the 

 waters. 



Next in order of time to the trilobite, the most singular 

 animals inhabiting those ancient seas, whose remains 

 have been preserved, are the Cephalopoda, possessing 

 some traces of organs which belong to vertebrated 

 animals. There are numerous arms for locomotion and 

 prehension, arranged in a centre round the head, which 

 is furnished with a pair of sharp, horny mandibles, 

 embedded in powerful muscles. These prehensile arms 

 are provided with a double row of suckers, by which the 

 animal seized its prey. Of these cephalopodous animals 

 there are many varieties, but all of them appear to be 

 furnished with powers of rapid locomotion, and those 

 with shells had an hydraulic arrangement for sinking 

 themselves to any depth of the seas in which, without 

 doubt, they reigned the tyrants. 



Passing by without notice the numerous fishes, which 

 appear to have exhibited a similar order of progression 

 to the other animals, we must proceed to the more 





