164 THE MECHANICAL FUNCTIONS. 



Corda respecting that of the Hydra fusca, a species 

 nearly alhed to the hydra viridis. By the applica- 

 tion of high magnifying powers he found the sub- 

 stance of the body of this animal to consist of an 

 assemblage of vesicles, disposed in layers: those 

 which form the outermost layer having a more com- 

 pact arrangement than the rest; and the inner- 

 most, or the surface of the stomach, being studded 

 with numerous vesicles projecting into its cavity. 

 Some of these have, at their apex, a minute oritice; 

 others are entirely closed. The tentacula, like the 

 rest of the body, is composed of cells; but are 

 hollow tubes, containing in their interior a spiral 

 fibre, which traverses their whole length, and pro- 

 bably serve as the agent of their rapid contractions. 

 Their surface is devoid of cilia; but, instead of 

 them, are thickly beset with short bristles. The 

 tentacula are also provided at particular parts, 

 which are raised like warts all round their circum- 

 ference, with firm and sharp spines, concealed, in 

 their ordinary state, in a cell, or sheath, beneath the 

 surface, but capable of being protruded by the ex- 

 pansion of a small vesicle which lies underneath 

 their root. The remarkable prehensile power of 

 tentacula thus armed on every side is, therefore, no 

 longer a matter of surprise ; and it affords a ready 

 explanation of the fatal injury sustained by all soft 

 animals that have once been encircled by these for- 

 midable weapons. From the suddenness of the 

 death of the victim, indeed, Corda conjectures that, 

 in addition to the mechanical injury inflicted by the 

 spines, they instil into the wounds some poisonous 

 liquid, like the fangs of venomous serpents. (Acta 

 Acad. Cses. Leop.-Carol. Naturae Curios, xviij.) 

 The Zoanthus (Fig. 58) belongs to a tribe of 



