Ixx INTRODUCTION. 



to a very diminutive size^ and has a minute rounded 

 mandible (Plate XXX. figs. 6 a, 7) *. 



The next marked stage in the developmental series is 

 charaeterized by the contraction of the area combined with 

 the assumption of a more or less peduncular character by 

 the hollow portion of the structure. The external re- 

 semblance to the ordinary zocEcia has disappeared ; the 

 cell is commonly represented by a subconical elevation, 

 on the summit o£ which are placed the beak and man- 

 dible. At the same time the avicularium is now, for the 

 most part, a secondary growth, and is developed, not on 

 the original plane of the colony, but on the zooecia them- 

 selves. There has been a large reduction in the size of 

 the chamber, no longer required for the accommodation 

 of the polypide, and a growing specialization of the man- 

 dible and its adjuncts. To a great extent the avicula- 

 rium has lost its apparent status as a distinct zooid in the 

 colony, and become an appendage of the zocecium. The 

 bosses or mounds, so often forming part of it and sup- 

 porting the mandibular apparatus, are to be regarded as 

 the homologue of the chamber in the normal zocecium. 

 Such forms as I have now described, and others allied to 

 them, may be classed as secondary or transitional avicu- 

 laria. We must not suppose, however, that they con- 

 stitute a clearly defined section ; they are connected at 

 all points by intermediate forms with the primary group. 

 Nor are these divisions coincident with any particular 

 genera or families ; the various modifications of the avi- 

 cularium are distributed sporadically over the whole Sub- 



* In this species the avicularia alternate with the ordinary cells 

 throughout the colony. In the specimen represented in Plate XXX. 

 fig. 7 there is a group of variously modified cells which strikingly illus- 

 trates the relationship between the primary avicularium and the zocecium. 



