IOI4 PRINCIPLES OF STRATIGRAPHY 



zoa. After attachment this develops into the polyp, which early 

 begins to secrete its horny or calcareous corallum. 



IV. MOLLUSCOIDEA. 



Bryozoa. The Bryozoa or Polyzoa are marine or fresh water, 

 chiefly colonial, benthonic animals. A few occur parasitic on a liv- 

 ing substratum, but the majority of species are epiphytically attached 

 to algae, to hydroids, etc. (epizoon), or to inorganic objects (epilith), 

 either basally or in an encrusting manner. The majority of species 

 are marine; and their bathymetric distribution ranges from the 

 shore zone, where they are exposed at low tide, to the abyssal 

 depths, a species of Bifaxia having been obtained below 3,000- 

 fathoms. The majority of species, however, live in moderate 

 depths. While the Bryozoa normally lead a strictly sedentary ben- 

 thonic life, a few species may drift about with the sea-weed to which 

 they are attached, thus assuming an epi-planktonic habit. Holo- 

 planktonic forms are, however, unknown. 



Many of the Palaeozoic species resembled and had a habitat 

 similar to that of certain corals, often forming extensive beds or 

 even reefs composed of few species but of an enormous number of 

 individuals. 



The egg of the bryozoan develops into a meroplanktonic cili- 

 ated larva, which later on settles down, becomes attached, and 

 develops into a full-grown individual which, by budding, produces 

 the colony. 



Brachiopoda. The brachiopods are marine benthonic organ- 

 isms, of exceptional stratigraphic importance, since they are to a 

 high degree dependent on the facies of the sea bottom. Some 

 species of Terebratula and Lingula can withstand a considerable 

 exposure, the former having been noted out of water for hours 

 together at low tide. Lingula is buried, by means of its long 

 fleshy peduncle, in the sand near shore ; Crania is attached to rocks 

 and shells by one of its valves; but the majority of brachiopods are 

 attached by their fleshy pedicles to rocks, shells, corals, or to one 

 another. They seldom live on muddy or sandy bottoms, but are 

 readily embedded in these, by becoming detached, after death, 

 from the rocks or other objects to which they adhered. 



The bathymetric distribution of the Brachiopoda ranges from 

 shallow water to 2,900 fathoms (in one case) ; the majority of 

 species occurring, however, above the bund red- fathom line, while 

 a goodly number have been obtained in depths of ten fathoms or 



