XVlll INTRODUCTION. 



with tlie surrounding water. In the sheath muscular fibres, 

 both longitudinal and transverse, occur ; and the latter 

 form a broad sphincter at a short distance from the base 

 of the tentacles. 



By " the orifice " of the zooecium is meant the opening 

 through which the tentacular sheath and the corona are 

 protruded. Of course it is not an opening in the sense of 

 a passage giving access to the interior of the cell ; it is 

 the break in the more or less solid portion of the wall 

 through which the evaginable part of the zooecium is 

 pushed forth ; and by this and the tentacular corona it is 

 permanently closed. The orifice exhibits some remarkable 

 structural peculiarities, correlated with other important 

 characters ; and these have been happily employed by Mr. 

 Busk in the definition of some of the leading systematic 

 groups. They will be referred to more particularly in the 

 section on Classification. (See Woodcut, fig. 35, p. 562.) 



Alimentary Canal. — The plan of the digestive system 

 amongst the Polyzoa, though very constant in its leading 

 features, presents many variations in detail. I shall first 

 describe its essential parts, and then refer to some of 

 its more striking modifications. 



Three well-marked regions are distinguishable in the 

 alimentary canal — the oesophagus (including the pha- 

 ryngeal cavity, which is more or less strongly defined in 

 diff'erent species), the true stomach (or digestive sac), and 

 the intestine. 



The mouth, a simple orifice, pierced in the floor of the 

 tentacular crown, opens at once into the upper part of the 

 oesophagus, which is thickly clothed with vibratile cilia, by 

 whose action the food drawn in by the tentacular vortex 



