POLYZOA. 413 



time, the Cuttle-fishes (Dibranchiate Cephalopods), the cham- 

 bered Cephalopods, and the Polyzoa ; in Kainozoic time, the 

 Lamellibranchs and Gasteropods. The Polyzoa are compara- 

 tively poorly represented in Palaeozoic rocks, and attain their 

 maximum (as fossils) towards the close of the Mesozoic period. 

 The Brachiopods are vastly more abundant in Palaeozoic 

 deposits than in Mesozoic, and have gradually declined to 

 the present day. The Lamellibranchiata seem to have been 

 gradually increasing in importance since their first appearance 

 in the Upper Cambrian seas, and they have attained their 

 maximum at the present day. The Gasteropods, upon the 

 whole, like the Bivalves, seem to have reached their culmin- 

 ating point in recent seas ; whilst the Pteropods seem to have 

 been as abundant in Silurian seas as they are at present. 

 The history of the Cephalopoda is a remarkable one. The 

 Tetrdbrancliiate forms, with chambered shells, attained their 

 maximum in the earlier portion of the Silurian period, as 

 regards their simpler types ; but the more complex types of 

 the group swarmed in the seas of the Secondary period, and 

 finally disappeared at the close of this epoch. This group 

 at the present day is represented solely by the Pearly 

 Nautilus. The Dibranchiate Cephalopoda, on the other hand, 

 represented at the present day by the Cuttle-fishes, did not 

 make their appearance till the commencement of the Secon- 

 dary period, and seem to have reached their maximum in 



existing seas. 



The sub-kingdom Mollusca is divided into two great divi- 

 sions, termed respectively the Molluscoida and the Mollusca 

 Proper. The division Molluscoida comprises the three classes 

 of the Polyzoa, Tunicata, and frachiopoda, characterised by 

 having a nervous system consisting of a single ganglion or prin- 

 cipal pair of ganglia, whilst there is either no distinct organ of 

 the circulation or an imperfect heart. In the division of the 

 Mollusca Proper are comprised the classes of the Lamelli- 

 Iranchiata (Bivalves), Gasteropoda (Univalves), Pteropoda, and 

 Cephalopoda. All these classes are distinguished by having 

 a nervous system composed of three principal pairs of ganglia ; 

 whilst there is a well-developed heart, consisting of at least two 

 chambers. 



