BRYOZOA. 345 



sclerenchyma. In the remaining sections the zoaria are ramose, 

 with the zocecia arising from a real or imaginary axis and 

 opening on all sides of the cylindrical stems. Usually the zoaria 

 are continuous; but in some of the bifoliate and ramose forms 

 they are composed of a greater or smaller number of segments. 

 The variations in the form of the primitive cell suggest a rough 

 division of the suborder into three sections. In the first, con- 

 taining only the CYSTODICTYONID.E, the cell is semi-cordate or 

 obovate-acuminate in outline; in the second, comprising the 

 PTILODICTYONID .E. STICTOPORHXE, FENESTELLIIXE and ACANTHOCLA- 

 DIIIKE, it is oblong quadrate, hexagonal or rhomboidal; the 

 third section, to which the PHYLLOPORINID.E, ARTHROSTYLID.E and 

 RHABDOMESONTID.E belong, is distinguished by the tubular form 

 of the primar}* cell. In this respect several members of the third 

 section make a close approach to the TREPOSTOMATA. 



All positive evidence goes to show that the primitive cell of 

 the CRYPTOSTOMATA corresponds to the entire zooecium of the 

 ordinary CHILOSTOMATA. The only ground upon which the 

 new suborder can be distinguished from Busk's well estabh'shed 

 division is the production of the primary orifice into a tubular 

 shaft ("vestibule"). Whether this will be regarded as a suffi- 

 cient reason for the separation of these Bryozoa into two equal 

 divisions, may be well questioned, since several genera now 

 classed with the CHILOSTOMATA (e. g. Steganopora and Distega- 

 nopora of d' Orbigny. and SteganoporeUa of Sinitt) exhibit practi- 

 cally the same peculiarity. Provisionally at least, it seems 

 desirable to hold them as distinct, because the feature pertains 

 to such a large number of Palaeozoic Bryozoa, which have re- 

 ceived through it a decidedly different aspect from that ex- 

 pected of CHILOSTOMATA. The fact that the peculiarity is so 

 rare in secondary and more recent ages, in which the true CHI- 

 LOSTOMATA are so abundant, also argues strongly in favor of the 

 separation by allowing us to assume that such genera asStegano- 

 pora are only the remnants of an extinct Palaeozoic type of 

 structure. 



In a large proportion of the CRYPTOSTOMATA the primitive 



aperture is marked by the development of an incomplete plate 



which extends downward and forward from the posterior side 



of the base of the vestibule into the primitive or true cell. This 



-43 



