OBSEEVATIONS ON POLYZOA. 209 



nerve-trunk, and the increase of the thickness and size of 

 the polypidal nerve-trunk in Plumatella, as compared with 

 the states of the same parts in Fredericella, are all evi- 

 dently due to the greater extent of the surfaces which 

 they have to supply in the former ; and this increased ex- 

 tent, as we have pointed out, is in its turn due to the 

 growth of the arms. Thus, every character in the Lopho- 

 phoric System appears to depend upon the increment of 

 the arms, and to owe its modifications to that, more than 

 anything else. The oesophageal, gastric, and intestinal 

 nerve-branches have been observed only in Fredericella. 



RECAPITULATION OF THE CHARACTERISTICS OF THE THREE 



SYSTEMS. 



Commencing with the coencscial system, we find that 

 in this system the increase in the radiatory character of 

 the coenoecium, the crowding and adherence of the 

 branches, the consolidation of the branches in the coence- 

 cial trunk, the adherence of the cells to each other, and 

 the obliteration of the free walls of the cells, were all 

 due to the increase in the number of the buds developed 

 simultaneously from the original, or parent cell of the 

 colony; and also, that these characteristics, jDrimarily ex- 

 jjressed in a slight degree either upon difi'erent branches 

 of the same colony, or in difi'erent varieties of the same 

 species, ultimately become of essential importance to the 

 organization of every species and of generic value. 



In the evagiuatory system we found that all the char- 

 acteristics, witli three exceptions, were due to the oblitera- 

 tion of the celMvalls, and, therefore, directly traceable 

 to the increase in the number of buds developing simul- 

 taneously from the parent cell ; that the governing char- 

 acteristic of the whole system, the extent of the evagina- 

 tion was a variable characteristic among the zooids of 

 the same colony in P. vitrea, although probably of fixed 

 and generic value in Cristatella. Thus most of the prin- 

 cipal changes in the complication of the coenojcial and 

 evagiuatory system are traceable to one incremental 

 characteristic, the gradual increase in the number of 



COMMUNICATIONS ESSEX INSTITUTE, VOL. V. 28 FEB., 1868. 



