INTRODUCTION. Ixix 



operculum is combined with the slightest change in the 

 character of the cell itself (Woodcut, fig. xxx.). In this 

 species a number of zooecia are scattered over the colony, 

 which, whilst retaining in great measure the usual form, 

 are distinguished by a remarkable elongation of the oral 

 valve. This structure, which is normally semicircular 

 in shape, is here much produced and somewhat elevated 

 above, and stands out conspicuously on the surface of the 

 zoarium. It is fully four times the size of the ordinary 

 operculum, and of course increases very considerably the 

 length of the whole zocecium, which in other respects de- 

 parts very slightly from the normal condition. The aper- 

 ture is somewhat reduced, and the spines are aborted ; but 

 in general appearance these abnormal cells very closely re- 

 semble the other members of the colony. The polypide 

 in such cases is probably suppressed. We have here, it 

 would seem, one of the earliest and simplest departures 

 from the normal type of the zorecium in the direction of 

 the avicularium. In Flustra the change is much more 

 marked, as the cell is merely rudimentary, and the 

 movable operculum constitutes the essential feature. 



These slightly specialized forms, which fill the place of 

 a zoo2cium in the colony, may be distinguished as the 

 primary avicularia. They occur under various modifica- 

 tions : in Schizotheca fissa the avicularium has an area of 

 the same size and form as that of the zooecia, the beak 

 and mandible occupying much the same position as the 

 oral opening (see Plate XLI. fig. 3) . Nowhere is its mor- 

 phological significance more apparent. A striking case 

 of the same kind is presented by Cellaria tenuirostris, 

 Smitt (Woodcut, fig. xxxii.). In Schizoporella venusta, on 

 the other hand, the area (which replaces a cell) is reduced 



