GIBSON— CEPHALOCHOEDA: " AMPHIOXIDES." 249 



metrical relations with one another. TurtJier, this complicated evolution would surely 

 liave been unnecessary had the primitive gill-bars of Ampldoxidcs possessed the paired 

 structure assigned to them by Goldschmidt. Why should not the single series here also 

 (as he supposes it to liave done in the course of evolution leading from Amphioxides to 

 the Cyclostomes) simply have split into two ? 



Goldschmidt himself admits the prima -facie absurdity of supposing " dass durchaus 

 symmetrisclie Organe verschiedenerlei Ilerkunst sein sollen," but adds that it is no more 

 difficult to credit than that the apparently symmetrical mouth of Amphioxus should be 

 an organ purely of tlie left-hand side, as Van Wijhe has proved it to be. This com- 

 parison seems to me to be an unfortunate one. The symmetry of tlu; mouth is shown 

 to be more a[)parent than real, by the disposition of the coelomic cavities in relation to 

 it and by its innervation : the two rows of gill-slits, on the other hand, in addition to 

 their identical structure and symmetrical disposition, show perfectly symmetrical 

 relations with all the surrounding organs — metapleures, excretory canals, blood-vessels, 

 endostyle, peripharyngeal bands, and coelomic cavities. When to this we add that 

 closely similar symmetrical dispositions of gill-slits, endostyle, and peripharyngeal bands 

 cceur also in the Ascidiaus and in Ammocoeies, there can scarcely be any doubt that tlie 

 arrangement is a primitive one, inherited by each of the three from a common ancestor, 

 Goldschmidt himself suggests that the Ascidians are derived by degeneration from 

 Cephalochordates, but gives no grounds for the belief, to which there are many obvious 

 objections — the more complex structure of the brain and its sense-organs in the Ascidian 

 tadpole for one. 



This leads us to the last argument I shall advance against Goldschmidt's contentions, 

 one whicli has already been enforced by Van Wijhe. This is the well-known homology 

 drawn by Miiller and Dohrn between the endostyle of Amphioxus and the Ascidians and 

 the tliyroid gland of Anwiocoetes, supported by the further homology of the peri- 

 pharyngeal bands ; the truth of which is perhaps as certain as that of any homolon-v 

 could be. Van W^ijhe has iwintcd out the insufficiency of Goldschmidt's grounds for 

 doubting it, drawn from non-essential differences in the development in the three groups. 

 nis chief reason for doubting its validity between Amphioxus and Ammocoeies, however, 

 seems to be tlie impossibility of harmonising it with his views on the evolution of the 

 Cyclostomes from an Amphioxides-like ancestor. There can be little doubt that not 

 Dohru's well-founded homology, but Goldschmidt's assumption of the priraitiveness of 

 the asymmetrical endostyle and median gill-slits of Amphioxides, will have to be 

 abandoned. 



With tlu^se assumptions the theories of the ancestral history of Amj)hioxus and of the 

 craniates, which are founded upon them, must also fall to the ground. Nevertheless, 

 there are, I think, a considerable number of features in the structure of Amphioxides 

 which Goldschmidt was quite justified in claiming as primitive : and such points in the 

 two theories as are based on these will retain their value. 



Thus it cannot be doubted, in view of Van Wijhe's conclusive demonstration (1892, 

 1902), that the mouth of Amphioxus is essentially an organ of the left side, and that its 



