CHAPTER XIII. 



POLYZOA, AND COMPOUND TUNICATA. 



AT the lower extremity of the great series of Molluscous 

 animals we find two very remarkable groups, whose mode of 

 life has much in common with Zoophytes, whilst their type of 

 structure is conformable in all essential particulars to that of the 

 true Mollusks. These animals are for the most part microscopic 

 in their dimensions ; and as some members of both these groups 

 are found on almost every coast, and are most interesting 

 objects for anatomical examination, as well as for observation 

 in the living state, a brief general account of them will be here 

 appropriate. 



325. Polyzoa. The group which is known under this name 

 to British naturalists, corresponds with that which by Continental 



zoologists is designated 



FIG. 245. Bryozoa: the former name 



(though first used in the 

 singular instead of the plu- 

 ral number), having been 

 introduced by Mr. J. Y. 

 Thompson in a memoir 

 published in 1830, seems 

 to have precedence in 

 point of time over the 

 latter, which was conferred 

 by Prof. Ehrenberg in 1831 

 on a most heterogeneous 

 group, wherein the Bryo- 

 zoa, as now limited, were 

 combined with the Fora- 

 minifera. As the history 

 of the researches by which 

 the Polyzoa have been 



c, L. vermcosa. raised from the class of 



Zoophytes (in which they 



were formerly ranked, for the most part in apposition with the 

 Hydrozoa), to the Molluscan sub-kingdom, has already been 

 sketched (p. 49), we may now proceed, without further preface, 

 to a survey of the leading features of their organization. The 



