CLASSIFICATION 



517 



of some genera (e.g. Cdlepora) possess a great variety of forms 

 of avicularia, the same pattern of avicularium may characterise 

 several widely different genera. Further, the fwsition of the avi- 

 cularium may be very different in species which are apparently 

 closely related. Well-developed vibracula, although constant in 

 their occurrence in such forms as Scntpocellaria (Fig. 254) and 

 Caber ea (Fig. 242), occur here and there in species of encrusting 

 forms which are ordinarily placed in very different families. 



Now although some of these discrepancies are perhaps due to 

 errors in classification, whereby species which are really allied have 



FIG. 254. A, Front view, and 

 B, back view of part of 

 a branch of Scrupocellarui 

 scabra, Van Ben., Durham 

 Coast, x 43 ; a, lateral 

 avicularium ; a', smaller 

 median avicularium ; ap, 

 membranous aperture ; /, 

 fornix ; r, rootlet ; , seta 

 of vibraculum ; vj;, vibra- 

 cular zooecium. 



been wrongly placed in distinct genera, this explanation would not 

 prove satisfactory in all cases. Thus in Bugula, a genus which is 

 specially characterised by the high development of its avicularia, 

 these organs are normally absent in B. neritina. The fact that 

 this species was rightly placed in the genus has been confirmed 

 by the discovery made by Waters * that avicularia occur in speci- 

 mens which are believed to be identical with that species. 



1. The Cyclostomata appear to fall naturally into two main 

 groups, (A) the AHTICULATA, including the Crisiidae (Fig. 237), 

 distinguished by their erect branches, divided at intervals by 

 chitinous joints ; and (B) the INAUTICULATA, which include the 

 remaining families, whether erect or encrusting, agreeing in the 

 negative character of being unjointed. 



1 A, r,i. Nat. Jlist. ser. 5, xx. 1887, p. 91. 



