THE LOWER VERTEBRATA. 193 



here there is a median ventral aperture called the atriopore 

 (or atrial pore), which cannot be paralleled with any 

 structure in our other types. In front of this pore the 

 ventral surface no longer comes to a sharp edge, but is 

 flattened (compare figs. 105 and 104), and instead of a median 

 fin, a pair of metapleural folds are found. There is no 

 trace of paired fins (limbs), but it may be that these meta- 

 pleural folds represent in a continuous form the paired fins 

 of the true fishes, just as the continuous median fin re- 

 presents the separate median fins of the dogfish : this is 

 uncertain, however. 



Through the transparent skin of this flattened ventral 

 surface a series (twenty-six pairs) of closely-set squarish 

 bodies can be seen, one to each inyomere from the tenth to 

 the thirty-fifth. These are the gonads whether ovaries 

 or testes can only be told by microscopic examination, and 

 there is no other means of distinguishing the sexes. This 

 metameric repetition of the gonads is a feature quite new 

 to us, and one of considerable interest. Accompanying it 

 is a peculiar modification of bilateral symmetry it is easily 

 seen that the gonads of one side alternate with those of the 

 other. This peculiarity is not confined to the gonads ; care- 

 fid inspection shows that the myomeres also alternate in the 

 same way, and so do all the metameric structures in the body. 



The metapleural folds extend forward to the mouth 

 region, where they form a prae-oral hood and run in front 

 into the median fin again. This prse-oral hood encloses a 

 space (prae-orai chamber) at the posterior end of which 

 appears the true mouth a circular aperture devoid of jaws. 

 From the sides of the hood come off a large number of 

 finger-like processes the cirri. 



2. Mode of Life. Such are the general external char- 

 acters of Amphioxus : we must now say a few words about 

 its mode of life. It may be wondered how it can ever 

 obtain food without eyes, ears or nose to perceive it 

 or jaws to seize it. The explanation leads us to one 

 of the most interesting facts of. marine life. Vegetable 

 organisms, as we have seen, are in the long run the 

 source of all animals' food ; but such plants as afford food 

 ZOOL. 13 



