3l MANUAL OF ZOOLOGY. 



CHAPTER XLV. 



DISTRIBUTION OF MOLL US CO ID. 1 . 



DISTRIBUTION OF MOLLUSCOIDA IN SPACE. The Polyzoa, like 

 all the Molluscoida, are exclusively aquatic in their habits, but, 

 unlike the remaining two classes, they are not exclusively con- 

 fined to the sea. The marine Polyzoa are of almost universal 

 occurrence in all seas. The fresh-water Polyzoa, however, not 

 only differ materially from their marine brethren in structure, 

 but appear to have a much more limited range, being, as far as 

 is yet known, confined to the north temperate zone. Britain 

 can claim the great majority of the described species of fresh- 

 water Polyzoa, but this is probably due to the more careful 

 scrutiny to which this country has been subjected. 



The Tunicata are cosmopolitan in their distribution, and are 

 found in all seas, the Mediterranean appearing to be especially 

 rich in members of this class. Four genera are pelagic in their 

 habits, and several are found in the Arctic regions. 



The Brachiopoda, though of very partial occurrence, have a 

 wide range in space, being found both in tropical seas and in 

 the Arctic Ocean. Their bathymetrical range is also very wide, 

 extending from the littoral zone almost to the greatest depths 

 at which animal life has hitherto been detected. 



DISTRIBUTION OF MOLLUSCOIDA IN TIME. The Polyzoa 

 have left abundant traces of their past existence in the stratified 

 series, commencing in the Lower Silurian Rocks and extending 

 up to the present day. The Oldhamia of the Cambrian Rocks 

 of Ireland, and the Graptolites, have been supposed to belong 

 to the Polyzoa ; but the former is very possibly a plant, and the 

 latter should be referred to the Hydrozoa. Of undoubted Poly- 

 zoa, the marine orders of the Cheilostomata and C'yclostomafa 

 are alone known with certainty to be represented. Several 

 Palaeozoic genera such as Fenestella (the Lace-coral), Ptilo- 

 dictya, Ptilopora, &c. are exclusively confined to this epoch, 

 and do not extend into the Secondary Rocks. Amongst the 

 Mesozoic formations, the Chalk is especially rich in Polyzoa, 

 over two hundred species having been already described from 

 this horizon alone. In the Tertiary period, the Coralline Crag 

 (Pleiocene) is equally conspicuous for the great number of the 

 members of this class. 



The Tunicata, from the nature of their bodies, are not known 

 to occur in a fossil condition. 



The Brachiopoda are found from the Cambrian Rocks up to 



