ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 389 



tissue of the skeleton of the Lancelet is based on a law of 

 organisation which is not usually recognised in questions like 

 the present, viz. — that adult organs representing embryonic 

 organs, are altered so as to be tit for the performance of their 

 functions, but never so far as to depart, either in tissue or 

 form, from the type of their corresponding embryonic organs. 

 The arch-shaped fin rays, described by Mr. Couch, are merely 

 the dissepiments between the cylindrical germs of the fin 

 bones. 



The leading peculiarity of the Lancelet, considered as a 

 representative of an embryonic form in the adult series is the 

 want of true gills or branchial arches — the deficiency of 

 branchial clefts. Retzius, indeed, describes an opening at the 

 posterior part of the branchial cavity, which he compares to 

 the abdominal openings in the Myxine ; but as I have been 

 unable to discover this opening in my specimens, I agree with 

 M idler in considering its existence as highly problematical, and 

 I shall proceed to demonstrate that, in accordance with the 

 plan on which the other organic systems of this animal are 

 formed, such an opening into the branchial chamber could not 

 exist. The abdominal openings in the Myxine are the result 

 of the closure of its numerous branchial clefts by the integu- 

 ments. They are analogous, in fact, to the branchial orifices 

 of the tadpole, immediately before cessation of the aquatic 

 respiration. The respiratory apparatus of the Myxine, then, 

 although inferior in functional activity to that of other fishes, 

 is actually referable to a more elevated type. The Myxine 

 possesses a brain in which the central masses are considerably 

 evolved, and a nervus vagus of sufficient development. The 

 brain of the Lancelet, again, is reduced to a mere filament, 

 and the existence of a nervus vagus appears to be highly 

 problematical. These considerations, and the fact that 

 branchial upcnings have not been detected by Yarrcll, Couch, 

 M iillei, or myself, must, lead to the conclusion that this fish 

 has cither never bad branchial clefts at an\ period of its 



