lii INTRODUCTION. 



The Fall and Renewal or the Polypide. 



The ''Brown Body." 



Every student of the Polyzoa must have noticed that 

 the life of the individual polypides is comparatively ephe- 

 meral. In most cases a large proportion of the zooecia 

 have lost their original tenants^ while at the same time 

 the zoarium retains its vitality, and along the margin 

 fresh additions are continually being made to it by the 

 growth of new buds. Not uncommonly almost all the 

 cells covering the lower or older portions o£ a tuft of 

 Bugula or BiceUaria, for example, are thus emptied of 

 their polypides ; but towards the upper extremities of 

 the branches all are occupied by an active population, 

 and along the outer edge zooecia are to be seen in every 

 stage of development. The disappearance of the polypide, 

 however, is a comparatively unimportant incident in the 

 life of the colony ; and ample provision exists for securing 

 new tenants for the deserted dwellings. The zooecium 

 which has lost its polypide is in turn filled by another, and 

 may possibly be the home of a succession of occupants. 



If we examine a specimen of one o£ the erect plant-like 

 Polyzoa, we shall find, as I have stated, along the iipper 

 edge of the branches, zooecia in course of formation, and 

 exhibiting every degree of development; immediately 

 below these will be zooecia in which polypides are formed, 

 but immature ; below these again will extend a zone 

 inhabited by adult polypides in full vigour and activity ; 

 further down still we shall probably encounter dwellings 

 for the most part destitute of tenants. Of course these 



