XX. INTRODUCTION. 



Buds or newly-formed Poljpites not being 

 detached. The connective parts are tubular, 

 and allow the circulation of nutriment to all 

 members of the colony. 



In nearly all Hydroid colonies there is an 

 outer horny cuticle (the Polypary), which in 

 some cases does not clothe the Polypite itself, 

 but only the connecting parts (the Coenosarc). 

 These naked Polypites form the division 

 ATHECATA. Thosc in which the cuticle is 

 prolonged to form protective cups or calycles 

 constitute the division thecaphora. 



The forms of the calycles vary, the rim 

 being either plain, dentate, or with denticles 

 which meet above the polypite and form a lid, 

 or operculum. 



The arrangement of the calycles on the 

 stem and branches also differs. They may be 

 either terminal (ex. Campanidaria), biserial — 

 opposite or alternate — (ex. Sertulaiia), or uni- 

 serial (exs. S. Plumularia, and Antennularia). 



In one family of thecaphora there 

 are found certain Amoebuid bodies called 

 Nematoi)hores, more or less closely associated 

 with the Polyi)ites, and provided with darts 

 and cuticular receptacles. The functions of 

 these bodies are not known. It is possible 

 that they are to be regarded as modified 

 Polypites. 



The principle of Alternation of Generation 

 makes its first appearance in the Animal 

 kingdom in this group. The Polypite is not 



