576 



CHAPTEE XIII. 



POLYZOA AND 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 con- 

 formable in all essential particulars to that of the true Mollusks. 

 These animals are for the most part microscopic in their dimen- 

 sions ; 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. 



445. Polyzoa. — The group which is known under this name to 

 British naturalists, corresponds with that which by Continental 

 Zoologists is designated Bryozoa : the former name (though first 

 used in the singular instead of the plural number), as having been 

 introduced by Mr. J. V. Thompson in a memoir published in 1830, 

 seems to have precedence in point of time over the latter, which 

 was conferred by Prof. Ehrenbergin 1831 on a most heterogeneous 

 group, wherein the Bryozoa, as now limited, were combined with 

 the Foraminifera. It has been entirely by Microscopic research 

 that the Polyzoa have been 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 ; whilst the 

 Foraminifera have been remitted, by the more careful study of 

 their living forms, to the very lowest division of the Animal 

 Kingdom. The animals of the Polyzoa, in consequence of their 

 universal tendency to multiplication by gemmation, are seldom or 

 never found solitary, but form clusters or colonies of various 

 kinds ; and as each is enclosed in either a Horny or a Calcareous 

 sheath or 'Cell,' a composite structure is formed, closely corre- 

 sponding with the Polypidom of a Zoophyte, which has been appro- 

 priately designated the Polyzoary. The individual Cells of the 

 Polyzoary are sometimes only connected with each other by their 

 common relation to a creeping stem or Stolon, as in Laguncula 

 (Plate xxn.) ; but more frequently they bud-forth directly, one 

 from another, and extend themselves in different directions over 



