STRUCTURE AND REPRODUCTION OF HYDATINA. 325 



posterior part of the body ; and the oviducts open into the dis- 

 tended extremity of its tube. The nervous system is here easy 

 to be distinguished. It consists of a kind of circle surrounding 

 the oesophagus, on which three pairs of ganglia may be observed. 

 From the lower pair there proceeds a double cord, which passes 

 along the ventral surface of the body to its opposite extremity. 



933. The reproductive powers of the Hydatina are very remark- 

 able. The number of eggs contained in the ovarium at once is 

 never large, seldom exceeding three or four ; but they are frequently 

 deposited and renewed, and themselves soon arrive at maturity. 

 The following experiment is related by Ehrenberg : " On Nov. 

 21, I placed in a jar a young Hydatina containing an egg nearly 

 mature. I added for its food a drop of liquor containing Monads. 

 On the morning of the 22nd, the egg had been deposited. On 

 the 23rd I met with four individuals, of which two were fully 

 developed. On the morning of the 24th, there were twenty. 

 The observation ceased at this point ; as it became too difficult 

 to count the numbers which thus rapidly increased. In a space 

 of 72 hours, twenty individuals had been formed one only 

 having been employed as the stock ; and at this rate of increase, 

 the numbers would be, at the end of ten days, 1,048,576 ; and this 

 number would be quadrupled in another day. Even if only two 

 instead of four were produced daily by each individual, a million 

 would be called into existence in twenty days ; and on the 

 twenty-fourth day, we should have 16,777,216 animalcules." 

 When we consider, in connection with this rapid increase in 

 number, the curious power of revivification possessed by these 

 beings, it is obvious that we need not have recourse to the 

 idea of spontaneous generation,* to account for their sudder* 

 appearance in various situations, and for their speedy multiplica* 

 tion, wherever the conditions, in regard to food, temperature, &c., 

 are favourable. 



* This term refers to an idea, which has been entertained at various times, 

 that animals of low organization may of themselves originate, by the accidental 

 meeting of particles adapted to form their structure. 



VOL. II. A A 



