EVOLUTION OF ORGANS IN THE CHORDATA. 281 



given why the spiracular endoderm sac should open into the 

 thyroid endoderm sac, since these were presumably originally 

 separate; the spiracle being anterior to the hyomandibular, 

 the thyroid between hyomandibular and hyoid. Dohrn does 

 not mention this question, being satisfied so far to show that 

 the condition of the ciliated grooves in Tunicates is directly 

 derivable from the condition in Ammocoetes. The derivation 

 of the arrangement in the latter from that in Selachians is not 

 discussed. 



Rudiments of Paired Fins in Petromyzon. 



In the ninth Study Dohrn returns again to the question of 

 the fins. How, he demands, could an animal of the size and 

 complication of the Cyclostomata obtain for itself organs of 

 such fundamental efi'ect on the whole organisation as pectoral 

 and pelvic fins ? The question is perhaps not so convincing as he 

 thinks ; for, on his own hypothesis, the neural and ventral para- 

 podia must at one time have arisen, and the theory of the evo- 

 lution of organs is not at present in such a state as to make it 

 any more easy to understand how these organs arose than how 

 limbs could arise in the Cyclostome, unless, indeed, it were 

 postulated that the segmented vertebrate ancestor, with its 

 dorsal and ventral parapodia, was a creation into whose pre- 

 vious origin it were impious to inquire. But what is more to 

 the point is that, although Gegenbaur believed no rudiment of 

 fins could be discovered in the Cyclostomata, Dohrn has dis- 

 covered in Ammocoetes rudiments of muscle-buds similar to 

 those which in other fishes form the muscles of the unpaired 

 fins. These buds, however, remain as indifferent cells during 

 the Ammocoetes stage, and are only differentiated into the fin 

 muscles when the metamorphosis into Petromyzon takes 

 place. The buds are given off ventrally as well as dorsally, 

 and as the dorsal series forms the muscles of the dorsal fin, 

 the prseanal ventral ones must at one time have formed mus- 

 cles of then existing paired fins. Moreover, there is, accord- 

 ing to Dohrn, a rudiment of the pelvic fins in Petromyzon 

 namely, the longitudinal folds bordering the anus. Below 



