258 ANNELIDA. 



classes have a nervous system, with numerous ganglia : all 

 the Entomoid classes have the ganglia fused into two or more 

 centres. 



The eyes are either simple or compound. In anneUda 

 and spiders they are simple ; in insects and Crustacea they 

 are compound. This division comprehends Six Classes : 



ANNELIDA. ARACHNIDA. 



CIRRHIPODA. MYBIAPODA. 



CETJSTACEA. INSECTA. 



CLASS I. ANNELIDA. 



Have an elongated vermiform body, divided into numerous 

 rings. They are for the most part naked, or are enclosed 

 in tubes ; sometimes membranous, and indurated with grains 

 of sand or other debris ; or calcareous, and adhering to shells 

 and other marine bodies. These tubes are almost the only 

 remains of the Annelida that lived in the ancient seas ; 

 although the foot-prints of some naked forms are impressed 

 on the ancient sedimentary strata. "We divide this class 

 into two orders. 



The ANNELIDA DORSIBRANCHIATA. Have a naked 

 body, composed of numerous rings ; the lower arc of each ring 

 carries bristles for locomotion, and the superior arcs of many 

 support the branchiaB for respiration. To this order belong 

 the remarkable imprints of Nerites Camlr lenses, impressed 

 on the Lampeter Cambrian rocks. 



The ANNELIDA TUBICOLA. Have the body enclosed in 

 membranous or calcareous tubes, generallyadherent to shells 

 or other marine bodies. These tubular sheaths are the only 

 remains we possess of this order. The genus Serpula extends 

 from the Devonian stage to the modern epoch : it attained 

 its greatest development in the oolitic seas. More than fifty 

 species are found in the rocks of this period. Terebella is 

 found in the oolites, Spirorlis in the chalk and tertiaries, and 

 Siliquaria in the tertiary strata of France and Italy. 



