274 J. T. CUNNINGHAM. 



into a sucking mouth. It was said that they had no limbs 

 because the skeleton of a limb was derived from an arch of 

 the branchial skeleton, and no true branchial arches were 

 present; the truth is that the limbs are not derived from 

 branchial arches, as is now generally acknowledged, and there 

 is a rudiment of the pelvic fin in Petromyzon, to be after- 

 wards described. 



The Origin of the Fins of Fishes. 



The true history of the origin of the limbs of fishes, paired 

 and unpaired fins, as Dohrn reads it, is set forth in the sixth 

 Study. In the original ancestral condition the Vertebrate body 

 was similar in most respects to that of an Annelid. The medul- 

 lary tube was an open plate, the intestine extended through 

 the whole length of the body to a terminal anus, and on each 

 segment were two pairs of appendages, processes of the body 

 wall provided with processes of the body musculature, in fact, 

 dorsal and ventral parapodia. The nerve plate was, of course, 

 ventral, when the animal was reversed in position and the plate 

 folded into a tube, the two series of ventral parapodia were 

 brought together in the median dorsal line and coalesced both 

 laterally and longitudinally, forming the dorsal fin, which was 

 originally continuous along the whole length of the body. 

 Another change which took place was that a new anus was 

 formed out of the fusion of two gill-slits, and in consequence 



one oigan, and the contrast between the functions of homologous organs in 

 two forms. An important function of the sucker-mouth of Petromyzon is to 

 adhere to stones in the bed of a river, and without this power the animal 

 would immediately lose control of its own movements, and be carried away at 

 the mercy of the currents in which it habitually lives. This function is 

 entirely wanting in Myxine, whose mouth is not truly a sucker at all, but a 

 boring apparatus. I have never seen a Myxine use its mouth to attach itself, 

 while Petromyzon never leaves its mouth attachment at one place, except to 

 immediately secure it again at another. Yet the mouth of Myxine can take 

 in food without boring, as is demonstrated every day in the North Sea when 

 the fisherman finds on his lines numbers of Myxine which have taken the 

 baited hook far down into the intestine without using their teeth upon the 

 bait at all. 



