INTRODUCTION. XV 



of its inner layer, and which shuts clown over the polypite 

 in a slanting position when it withdraws itself. When the 

 polypite emerges, it slowly pushes the valve back and keeps 

 it erect so long as it is exserted : on its retreat, which is 

 as quick as light, the lid flies back to its place. (Woodcut, 

 fig. iv. a, the operculum closed ; b, ditto open.) 



Dr. Wright has described a similar structure as occurring 

 in the beautiful Protozoan Vaginicola valvata* ; and 

 amongst the terrestrial mollusca it has its analogue in the 

 clausium of the genus Clausilia. In some species the 

 operculum seems to be a simple piece attached to the 

 margin at one side by a kind of hinge, which falls down 

 over the orifice like the lid of a box. 



The life of the polypites is by no means commensurate 

 with that of the zoophyte. They frequently perish from 

 various causes falling, in some cases, like leaves, in others 

 being absorbed into the substance from which they sprung ; 

 while the ccenosarc retains its full vitality, and, in time, 

 will bud forth a fresh crop. When the polypite is under- 

 going the process of absorption, an extraordinary ferment 

 is visible in its digestive sac; the granules contained in 

 the nutrient stream are seen to be as actively at work as 

 when a new portion is being added to the organism, 

 moving restlessly about within the cavity, then hurrying 

 from it, and soon reentering it, until the materials of the 

 body have been, as it were, broken up and borne away to 

 be wrought into fresh structures. 



Dr. Wright has observed, in the case of Hydractinia, 

 that in the winter the coenosarc often exists in a high 

 state of development, while the polypites are few in number 



* " Description of New Protozoa," Edinb. N. P. Jouru. N. S. for April 

 1858. 



