HYD 



HYD 



the arms; and in conveying it to the 

 mouth, it frequently twists the arm into 

 a spiral, like a cork-screw, by which 

 means the insect is brought to the mouth 

 in a much shorter time than otherwise it 

 would be ; and so soon are the insects on 

 which the polypes feed killed by them, 

 that M. Fontana thinks they must con- 

 tain the most powerful kind of poison ; 

 for the lips scarcely touch the animal 

 when it expires, though there cannot be 

 any wound perceived on it when dead. 

 The worm, when swallowed, appears 

 sometimes single, sometimes double, ac- 

 cording to circumstances. When full, 

 the polype contracts itself, hangs down 

 as in a kind of stupor, but extends again 

 in proportion as the food is digested, and 

 the excrementitious part is discharged. 

 The manner in which the polypes gene- 

 rate is most perceptible in the grisca and 

 fusca, as being considerably larger than 

 the viridis. If we examine one of them 

 in summer, when the animals are most 

 active, and prepared for propagation, 

 some small tubercles will be found pro- 

 ceeding from its sides, which constantly 

 increase in bulk, until at last in two or 

 three days they assume the figure of 

 small polypes. When they first begin to 

 shoot, the excrescence becomes pointed, 

 assuming a conical figure, and deeper 

 colour than the rest of the body. In a 

 short time it becomes truncated, and then 

 cylindrical, after which the arms begin to 

 shoot from the anterior end. The tail 

 adheres to the body of the parent animal, 

 but gradually grows smaller, until at last 

 it adheres only by a point, and is then 

 ready to be separated. When this is the 

 case, both the mother and young ones fix 

 themselves to the sides of the glass, and 

 are separated from each other by a sud- 

 den jerk. The time requisite for the 

 formation of the young ones is very dif- 

 ferent, according to the warmth of the 

 weather, and the nature of the food eaten 

 by the mother. Sometimes they are fully 

 formed, and ready to drop off, in twenty- 

 four hours; in other cases, when the 

 weather is cold, fifteen days have been 

 requisite for bringing them to perfection. 

 The polypes produce young ones indis- 

 criminately from all parts of their bodies, 

 and five or six young ones have frequent- 

 ly been produced at once ; nay, M. 

 Trembley has observed nine or ten pro- 

 duced at the same time. Nothing like 

 copulation among these creatures was 

 ever observed by M. Trembley, though 

 for two years he had thousands of them 

 under his inspection. 

 When a polype is cut transversely, or 



longitudinally, into two or three parts* 

 each part in a short time becomes a per- 

 fect animal ; and so great is this prolific 

 power, that a new animal will be pro- 

 duced even from a small portion of the 

 skin of the old one. If the young ones 

 be mutilated while they grow upon the 

 parent, the parts so cut oft' will be repro- 

 duced ; and the same property belongs 

 to the parent. A truncated portion will 

 send forth young ones before it has ac- 

 quired a new head and tail of its own, 

 and sometimes the head of the young one 

 supplies the place of that which should 

 have grown out of the old one. If we 

 slit a polype longitudinally through the 

 head to the middle of the body, we shall 

 have one formed with two heads ; and by 

 again slitting these in the same manner, 

 we may form one with as many heads as 

 we please. A still more surprizing pro- 

 perty of these animals is, that they may 

 be grafted together. If the truncated por- 

 tions of a polype be placed end to end, 

 and gently pushed together, they will 

 unite into a single one. The two portions 

 are first joined together by a slender 

 neck, which gradually fills up and disap- 

 pears, the food passing from one part 

 into the other : and thus we may form 

 polypes, not only from different portions 

 of the same animal, but from those of dif- 

 ferent animals. We may fix the head of 

 one to the body of another, and the com- 

 pound animal will grow, eat and multiply, 

 as if it had never been divided. By push- 

 ing the body of one into the month of an- 

 other, so far that their heads may be 

 brought into contact, and kept in that 

 situation for some time, they will at last 

 unite into one animal, only having dou- 

 ble the usual number of arms. The hy- 

 dra fusca may be turned inside out like a 

 glove, at the same time that it continues 

 to eat and live as before. The lining of 

 the stomach now forms the outer skin, 

 and the former epidermis constitutes the 

 lining of the stomach. See Adams on the 

 Microscope. 



HYDRACHNA, a genus of insects of 

 the order Aptera Head, thorax, and 

 abdomen united ; two feelers, jointed ; 

 from two to six eyes ; eight legs, ciliate, 

 and formed for swimming. The insects 

 of this genus are inhabitants of the water, 

 and swim with considerable swiftness : 

 they prey on the larva of Tipulae and Mo- 

 noculi : the eggs are red and at first sphe- 

 rical, but afterwards become semi-lunar ; 

 larva six-footed, and furnished with a sin- 

 gular proboscis. There are about fifty 

 species. H. geographica, so called from 

 the fancied map.like distribution of its va- 



