330 GENERAL ZOOLOGY 



toidea, Crinoidea, Ophiuroidea, Echinoidea), the Bryozoa, 

 and certain molluscs. The trilobites attained their cli- 

 max here and declined during succeeding periods. The 

 cephalopods began to have coiled shells, which were less 

 cumbersome than the straight types characteristic of the 

 Cambrian. The graptolites, a peculiar group of ccelen- 

 terates, flourished in this time, but had a short racial exist- 

 ence for they were practically extinct at the end of the 

 next period. 



Silurian Period. The most notable event recorded in 

 Silurian rocks is the advent of fish-like animals. The 

 ostracoderms had no paired fins or true jaws and differed 

 from modern fishes in many other respects. Many of 

 them had the anterior end of the body covered with heavy 

 protective plates. There were also primitive sharks in the 

 Silurian seas. In this period also the corals developed suf- 

 ficient colonial life to begin to construct reefs; the brachio- 

 pods reached the height of their racial vigor; crinoids be- 

 came abundant; the trilobites persisted in considerable 

 numbers; the king-crabs had attained some specialization, 

 and primitive sea-scorpions were in existence. 



Devonian Period. Fish-like animals increased greatly 

 and many new types appeared. Though ostracoderms per- 

 sisted and the appearance of new forms indicates that they 

 attained considerable specialization, the whole group died 

 out at the end of the period. Cyclostomes and sharks 

 were present, and some of the latter attained a length of 

 five feet. Several types of true fishes were in evidence- 

 some were apparently ancestral to those living today and 

 some have become extinct. The arthrodirans had the 

 anterior part of the body covered with heavy exoskeletal 

 plates, and were unique in possessing a neck joint so that 

 they could nod the head. The trilobites were now declin- 

 ing and were replaced by crustaceans of more modern type. 

 Most of the cephalopods possessed coiled shells, which in 

 many species had a very complicated structure; clams be- 

 came specialized to such an extent that forms much like 



