X X X l V INTRODUCTION. 



Hyatt supposes that the entire endocyst maybe pierced 

 to a greater or less degree by aquiferous pores, but admits 

 that he had sought for them in vain. Allman suggests 

 (as an alternative explanation) that the water may 

 transude through the walls of the alimentary canal into 

 the cavity of the cell*. It cannot be said that we have 

 any real knowledge on the subject. 



As to the second point, it appears all but certain that 

 the products of digestion must mingle to some extent 

 with the perigastric fluid, as there seems to be no other 

 way in which they can be utilized for the benefit of the 

 whole organism. And this fluid being in immediate con- 

 tact with so large a portion of the tissues, is clearly indi- 

 cated as the medium through which the nutritive material 

 is supplied to them. But if we inquire how the trans- 

 ference from the stomach to the outer sac is effected, we can 

 get no further at present than the hypothesis that the ali- 

 ment may transude through the walls of the digestive canal. 



The perigastric fluid is admitted freely to the interior 

 of the tentacular tubes, where, as in a system of gills, it is 

 aerated by the water, which is constantly renewed by the 

 ciliary currents. Amongst the freshwater Polyzoa the 

 surface of the endocyst is lined by a ciliated epithelium, 

 which keeps the fluid in constant movement and main- 

 tains a very regular circulation. But in the marine forms 

 this seems to be wanting, and its place is probably filled 

 and its function discharged by the general contractility 

 of the inner wall of the cell. 



Jolietf has investigated the corpuscles which are found 



* How, then, shall we account for the presence of a similar fluid in the 

 internodes of the stem of the Cteno&tomata, which (though homologous with 

 the zoo?cium) are destitute of a polypide ? 



t ' Bryozoaires des cotes de France,' pp. 39-41. 



