324 STRUCTURE AND REPRODUCTION OF HYDATINA. 



branches. The mouth is situated in the centre of the large 

 extremity of the body ; it leads, through a short oesophagus, to 

 the first stomach or gizzard, the jaws at whose entrance are pro- 

 vided with five or six pairs of teeth. Into this cavity a pair of 

 secreting caeca open ; and between the termination of these is 

 the commencement of the intestinal tube ; the upper part of 

 which, being very distensible, may perhaps be regarded as a 

 second stomach, since in this the digestive process is principally 

 performed. The intestine terminates, as in the Rotifer, near the 

 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. 



861 . 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 



