166 



INTRODUCTION TO ZOOLOGY. 



Fig. 156 — Terebratvia 

 Psittacea. 



Fig. 157. — Valve op rnE shell op 

 Tekebratola. 



existing under the pressure of such a depth of sea-water 

 " are subjects suggestive of interesting reflections, and lead 

 one to contemplate with less surprise the great strength and 

 complexity of some of the minutest parts of the frames of 

 these diminutive creatures. In the unbroken stillness which 

 must pervade those abysses, their existence must depend upon 

 their power of exciting a perpetual current around them,^in 

 order to dissipate the water already laden with their eflete 

 particles, and to bring within the reach of their prehensile 

 organs the animalcules adapted for their sustenance."* Some 

 of these animals have been taken in deep water on the Irish 

 coast, at Cork, Youghal, Kinsale, and the entrance to Belfast 

 Bav.t 



LAMELLIBRANCHIATA. 



Fig. 158.— Mactra. 



The tliird and last class of those Mollusks which are headless 

 comprises those which have their gills iu the form of mem- 



* Lectures, p.ige 270. 



t W. Thompson, lieport on the Invertebrate Fauna of Ireland. 



