OBSERVATIONS ON POLYZOA. 195 



come adherent, the attached portion of the cell is short- 

 ened, and consequently the walls of the free portions of 

 the cells approximate. In some specimens of variety 6, 

 Plumatella Arethusa, this approximation takes place with- 

 out any sensible diminution of the lower portion of the 

 free part of the cell-walls ; but in variety b of P. diffusa, 

 and in the single branches of P. vesicularis, previously 

 mentioned, there is a very decided shortening of these 

 walls, and in P. vitrea only the orifices project above the 

 branch in some groups, while in others the free portions 

 of the cells are quite long. In Lophopus the cells have 

 similar characters, the whole lower portion of the divid- 

 ing walls becoming part of the common surface of the 

 lobe, and in Pectinatella and Cristatella the cells are en- 

 tirely obliterated, the dividing walls having disappeared. 

 Thus the entireness of the cell-walls is a diminishing 

 characteristic of generic value in Fredericella, of specific 

 and variable value in Plumatella, disappearing in Lopho- 

 pus and Pectinatella. The only reasonable cause for this 

 gradual obliteration of the cell is to be found in the agent 

 which crowds the branches and cells together, making 

 them adherent, which produces the lobes of Lophopus, 

 and consolidates them with the cosnoecial trunk in Crista- 

 tella, and which also completes the radiatory arrangement 

 of the polypides, namely, the increasing activity of the 

 reproductive function, as shown in the greater number of 

 buds produced from the mother cell. The cosnrecial 

 trunk of Fredericella and Plumatella is made by the 

 growth of the primary polypides. When several buds 

 develop simultaneously, this crencecial trunk is broad- 

 ened and becomes a common cavity containing the various 

 polypides, as in variety b of Plumatella vitrea. 



A farther development of the flabellate growth gives 

 the palmate aspect of the branches in Lophopus . A greater 

 increase in the number of buds growing from the mother 

 cell gives, as has been said in the article on reproduc- 

 tion, the radiatory character of Pectinatella, and also 

 broadens the coenoecial trunk at the expense of the 

 branches, which become short lobes. 



