320 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



already brought together into real coral reefs ; some 

 special Hydroids, Stromatopores, and Graptolites ; 

 some Echinoderms, even then represented by five 

 types of this great group, Crinoids, Cystoidea, Blas- 

 toids, Asteroids, and Echinida ; some Bryozoaria, 

 and innumerable Brachiopods, followed by Molluscs 

 of every group, Lamellibranchs, Gastropods and 

 tetrabranchial Cephalopods or Nautilidae. These 

 last attain at this period their maximum of ex- 

 pansion. Even certain types of entirely soft 

 animals, such as the Medusae and marine worms, 

 have left traces of their presence, the first in the 

 form of natural casts of their general cavity, the 

 latter by their perforations in the sand, their 

 calcareous tubes, or the chitinous joints of their 

 jaws. 



The ramification of the articulated animals or 

 Arthropods is brilliantly represented both by 

 inferior types, such as Cirrhipedes, Ostracods, and 

 by the higher groups of Trilobites and Gigantos- 

 traca. These Trilobites, to which I shall several 

 times have to return, are curious natatory marine 

 Crustacea, of which the carapace, composed of a 

 head, a thorax with mobile segments, and a tail 

 or pygidium with rigid segments, is divided into 

 three lobes by two longitudinal lines. Their 

 limbs, rarely preserved, are slender, formed of two 

 articulated branches, and pretty uniform in the 

 different regions of the body. They are absolutely 

 peculiar to the Primary era, and their apogee 

 coincides exactly with the Silurian epoch, in which 

 they count about 75 genera and more than 900 

 species. The Trilobites represent a group apart. 



