520 POLYZOA 



for Polyzoa to have free larvae, the period during which these 

 larvae are free-swimming is, so far as is known, a short one in 

 most cases. Cyphonautes is a common pelagic form (see p. 

 510), and probably remains for a considerable period in the 

 larval condition. Other Polyzoon-larvae appear to fix themselves 

 very soon after their birth ; and this would not appear to give 

 much time for them to be carried to great distances by ocean- 

 currents. It may, however, be suggested that it does not follow 

 that because we know that a larva may, under favourable con- 

 ditions fix itself a few minutes after it becomes free, we should 

 be justified in assuming that that larva would riot retain for a 

 long period the power of undergoing a normal metamorphosis 

 should it be drifted away from suitable fixing-grounds. 



Palaeontology. 1 The number of fossil Polyzoa is enormous. 

 D'Orbigny devoted two hundred plates and more than a thousand 

 octavo pages 2 to a Monograph on the Cretaceous Polyzoa of 

 France. Many of the fossil forms are extraordinarily well pre- 

 served, and there is often no difficulty in recognising the identity 

 between certain fossil species belonging to the more recent forma- 

 tions and living forms. It thus becomes necessary to consult 

 Palaeontological memoirs in working at recent Polyzoa. 



While the great majority of fossil Polyzoa do not differ in 

 any essential particular from recent species, this is not altogether 

 the case with the Palaeozoic forms. Leaving out of account the 

 Stromatoporoids, which have been variously referred to the 

 Sponges, Hydrozoa, and Foraminifera, as well as to the Polyzoa, 

 the Palaeozoic strata contain large numbers of peculiar Cyclo- 

 stomata, together with members of the Trepostomata, a fourth 

 Sub-order of Gymnolaemata, allied to the Cyclostomata. The 

 Trepostomata are for the most part Palaeozoic, but a few survived 

 as late as the Jurassic period. 3 These, with the other Polyzoa 

 from the same formations, are considered by Dr. Gregory in his 

 recently published Catalogue of the Fossil Bryozoa in the British 

 Museum (1896). 



The number of Polyzoa recorded from the earlier secondary 

 strata is small. The Jurassic period saw the appearance of 



1 Zittel, Handbuch d. Palaeontologie, I. Bd. I. Abth. 1876-1880, p. 575. 



2 Pdleontolofjie Franfaisc. Terrains Cretacts, tome v., Bryozoaires, 8vo. Paris, 

 1850-1851. This great work refers, however, to recent as well as to fossil species. 



3 Hctcropora, of Avhich recent species .exist, is placed by Dr. -Gregory in the 

 Trcpostoiiiiita. 



