INTRODUCTION. XIU 



muscles, which plays an important part in the economy of 

 the Polyzoon, and which may be noticed here. It con- 

 sists of a number of short transverse fibres or bundles of 

 fibres, which pass from point to point on the surface of 

 the inner wall of the cell, and which by their contractions 

 are instrumental in compressing the membrane and so 

 reducing the dimensions of the perivisceral cavity. In 

 this way pressure is brought to bear on the contained 

 fluid, and a force is generated which discharges, as we 

 shall see hereafter, a most essential function. These 

 fasciculi are known as the parietal muscles. 



We are to conceive, then, of the zocecium as a chamber 

 inclosed by a more or less solid outer wall, and within 

 lined by a soft tapestry of living membrane, a well-built 

 and well-furnished dwelling, of which the polypide or 

 alimentary zooid is the tenant. 



The Polypide. 



The general plan of the structure has been already 

 sketched. In examining the details it Avill be necessary 

 to take the two great divisions of the Polyzoa (the Ecto- 

 procta and Entoprocta) separately. 



For a detailed account of the freshwater members of 

 the former group [Phylactolcemata), I may refer to the 

 writings of Allman*, Van Benedenf, Hancock J, Hyatt §, 



* 'Monograph of the Freshwater Polyzoa,' Eay Society, 1856. 



t " Recherches siir les Bryozoaires fluviatiles de Belgique," Mem. Ac. 

 Roy. Belg. xxi. 1847 ; (Dumortier and Van Beneden), ' Hist. Nat. des 

 Polypes composes d'eau douce, 2* partie : complement an tome xvi. d. M6m. 

 de I'Acad. Roy. Brus. 1848. 



I "On the Anatomy of tl;e Freshwater Bryozoa,"' Ann. & Mag. Nat. 

 Hist. March 1850. 



§ "Observations on Polyzoa, suborder Phylactolajmata,'' Proc. Essex 

 Inst. 1866-68. 



