904 



CHAPTER XVII 



POLYZOA AND TT'XICATA 



As in previous editions of this work the Author followed the once 

 prevalent habit of regarding the Polyzoa and Tunicata as structurally 

 allied, and as it would be necessary to entirely recast the work were 

 the two groups to be now otherwise dealt with, and as. finally, there 

 is no real inconvenience or impropriety in discussing them in one 

 chapter, it is proposed to continue, with this word of warning, the 

 original arrangement of the Author. Some members of both these 



o o 



Croups are found on almost every coast, and are most interesting 

 objects for anatomical examination, as well as for observation in the 

 living state. 1 



Polyzoa. The group which is known under this name to many 

 British naturalists (corresponding with that which by Continental 

 zoologists is designated 7>V//o~w) was formerly ranked as an order of 

 zoophytes, and it has been entirely by microscopic study that its com- 

 paratively high organisation has been ascertained. The animals of 

 the Polyzoa, in consequence of their universal tendency to multipli- 

 cation, by gemmation, are seldom or never found solitary, but form 

 clusters or colonies of various kinds ; and as each is inclosed in either 

 a horny or a calcareous sheath or 'cell.' a composite structure is 

 formed, closely corresponding with the ' polypidom ' of a zoophyte, 

 which has been appropriately designated the polyzoary. The indi- 

 vidual cells of the polyzoary are sometimes only connected with each 

 other by their common relation to a creeping stem or stolon, as in 

 /.ix/unci'la (fig. 687) ; but more frequently they bud forth directly, 

 one from another, and extend themselves in different directions over 

 plane surfaces, as is the case with Flustrce, Lepralice, &c. (fig. 688) ; 

 whilst not unfrequently the polyzoary develops itself into an arbores- 

 cent structure (fig. 689), which may even present somewhat of the 

 density and massiveness of the stony corals. Each individual is com- 

 posed externally of a sort of sac, of which the outer or tegumentary 

 laver is either simply membranous, or is horny, or in some instances 

 calcified, so as to form the cell ; this investing sac is lined by a more 

 delicate membrane, which closes its oritice. and which then becomes 

 continuous with the wall of the alimentary canal ; this lies freely in 

 the visceral sac. floating (as it were) in the liquid which it contains. 

 The principal features in the structure of this group will be best 

 understood from the examination of a characteristic example, such as 

 the LIII/H iirnlii /-I'/ir/tx. which is shown in the state of expansion at A. 

 fig. uX7. and in the state of contraction at I \ and < '. The mouth is 



i KHV .1 L I general account see IM-. Manner in vol. ii. of the ('amln-iili / \nlnral 



History, l*;ir,. 



