INTRODUCTION. CV 



oral operculum. Rudiments of two more spines, placed 

 far forward, now make their appearance. Fourth stage. — 

 Spines lengthening, denticles on tlie low^er margin deve- 

 loped; mucro below orifice just beginning to rise. The 

 avicularia marked out on each side of the orifice, con- 

 sisting of raised hollow processes, subcircular, and covered 

 in by membrane, resembling in general structure the early 

 stage of the zooecium, 



Fiff. xliv. 



Young zooecium of Mucrouella coccinea. 



0. Orifice, m. Portion of primary (membranous) covering, cw. Calca- 

 reouB wall. «, h. Early stages of aTicularium. 



These four stages were exhibited by four neighbouring 

 cells on the growing edge of a colony. The front wall 

 in its earliest condition is of the most beautiful hyaline 

 textui'e, and has a line of circular foramina round the 

 margin. Further change in it consists chiefly of a gra- 

 dual thickening, which culminates in a strongly granulose 

 surface. (See Plate XXXIV.) 



