HYDROIDS. 



73 



Class L — HYDROZOA. 

 Order I. — HYDROIDEA. 



In the year 1703, that charming old scientific gentleman, Anthony Van Leeuw- 

 hoek, of Delft, sent a very interesting paper to the Royal Society of London. In this 

 article he tells us that " the water of the river Maes is brought by means of a sluice dur- 

 ing the Summer flood, directly into our town, and it is as clear as if the river itself ran 

 through the town. With this water comes in also a green stuff of a vegetable nature, 

 of which, in a half hour's fishing, I got thirty pieces, and put them into an earthen pot 

 together with a large quantity of their own water. I took out several of these weeds 

 from the pot, one by one, with a needle very nicely, and put them into a glass tube of 

 a finger's breadth, filled with water, and also into a lesser tube, and caused the roots of 

 the weeds to subside leisurely ; then viewing them with my microscope, I observed a 

 great many and different kinds of animalcula. About the middle of the body of one of 

 these animalcula, which I conceived to be the lower part of its belly, there was another 

 of the same kind, but smaller, the tail of which seemed to be fastened to the other." 



Our author, in the latter part of his article, assures us that he saw the smaller ani- 

 malculum separate itself from the larger, and enter upon an independent existence ; 

 moreover, that he also determined by his microscope, the formation of a minute bud 

 upon one side of the anunalculum, which grew into an animal, perfect in shape, size, and 

 all particulars, and then detaching itself from its parents, floated free in the water. 

 That was the first discovery, so far as all the records give evidence, of the very wonder- 

 ful animal, which is now called Hydra, and which in many respects, both 

 in structure and in mode of life, is a very good type of its order, the 

 Hydroidea, and at the same time of the class of Hydrozoa. The body 

 of Hydra, which is entirely soft, having no skeleton without or within, 

 easily changes shape, and when entirely contracted, has the appearance 

 of a small dot or particle of gelatinous matter resting on the surface of 

 the aquatic plant, chip, stone, or whatever may be the object in the water 

 to which this small creature has attached itself. "Watching it slowly 

 expand in a dish of fresh water, it is seen to display a long, slender 

 cylindrical body, which, in Hydra viridis, is bright green, while in H. 

 fusca the color is light-brown. The base, or that end by which Hydra 

 fastens itself, is termed the disk or foot, and the external cells of this 

 part of the body secrete a gelatinous substance, which, hardening some- 

 what in the water, enables it to attach itself at will. Toward the 

 anterior or free end of the body, are a variable number of long, slender 

 processes, the tentacles, which are arranged in a single circle or wreath. 

 Within the ring formed by the bases of the tentacles, the body tapers 

 to a rounded elevation, where the mouth is found, and this tapering 

 portion of the body which extends beyond the retracted tentacles, is 

 known as the proboscis or hypostome. 



Within the body there is a cavity extending from one end to the other, from the 

 base to the mouth, and, as these processes are hollow in Hydra, to the tips of the 

 tentacles, Not only the body, but also the tentacles are very expansive and con- 

 tractile, and seldom retain the same shape and position for more than a few minutes. 



FiG.63.— JJydro 

 fusca ■with 

 youne bud- 

 ding from it. 



