250 PEROT SLADEN TEUST EXPEDITION. 



larval is more primitive than its adult position. But I cannot see that this discovery 

 necessarily invalidates Willey's tlieory, as Goldschmidt thinks it does : a mouth migrating 

 fiom a dorsal position onto the side of the body, as this theory postulates, would 

 naturally lose all trace of its former symmetry and derive its musculature and iu ner- 

 vation from the side to \i hich it had passed. Goldschmidt's chief objection to Willey's 

 theory is, however, that neither the larval dispositions held thereby to be coenogenetic, 

 nor the factors conditioning them, can be regarded as adaptations to the larval manner 

 of life. No doubt this is a perfectly just criticism, and it would seem not unreasonable 

 to admit that the larval stage of Aiiq^Jrioxus is to a certain extent — more than Willey, 

 though considerably less than Goldschmidt supposed — a recapitulation of a more 

 primitive condition. Somewhat in this light it has been regarded l)y Van Wijhe (1906) 

 ia his latest exposition of the "Tremostoma" theory. He suggests that with the 

 gradual usurpation by the first left gill-slit of the functions of the ancestral mouth 

 (pre-oral organ), the left gill-slits acquired a predominance and the right became 

 partially rudiineuted : on the assumption of life in the sand and the formation of an 

 atrium, symmetry was re-established, and both series re-acquired an equal importance. 

 Attractive as tliis theory is, the evidence for it seems hardly sufficient as yet. Should 

 Van Wij lie's tlieory of the nature of the mouth have to be abandoned, his view of an 

 actual predominance, at one stage in phylogeny, of the left series of gill-slits, might with 

 advantage be incorporated into Willey's. 



Another probably genuinely primitive feature to which Goldschmidt has called 

 attention is the brauchial musculature. There can be little doubt that this was possessed 

 by the pelagic ancestors of Amphioa-us before they acquired an atrium as an adaptation 

 to littoral existence, and so ceased to need the muscles in question : probably it was also 

 possessed by the common ancestors of Ampliioxus and the craniates. We must assume, 

 contrary to Goldschmidt, that these ancestors had a double series of gill-slits, both sides 

 possessing the typical musculature. No doubt they also possessed symmetrical meta- 

 pleural folds, in which we may see the possible homologue of the hypothetical lateral 

 fin-folds of the craniates. Goldschmidt's view that the ati'ium is a structure peculiar to 

 Amphio.vus and a special adaptation to its mode of existence, finding no homologue in 

 the craniates, is one in which he will probably meet with general agreement, even 

 thouirh he differs therein from Boveri. 



Of his farther suggestions as to homologies between yLniphioxides and the craniates, 

 the greater part, perhaps, can no longer be maintained. Thus it is evident that the 

 paired gill-slits of Cyclostomes are homologous with the paired slits of the adult 

 yinipldoxus and not with the single series of the larva only : nor is it probable that the 

 endodermal folds of the gill-bars of the contracted Amphioxides have any connection 

 with the branchial lamellse of higher forms. But his suggestion as to the method by 

 which the altered relations of tlie branchial muscles of the Cyclostomes were derived 

 from those of an Amphioxkles-\\ke ancestor would seem to have considerable probability. 

 The same may be said for the belief that the paired aortae of Amphioxus and the 

 craniates were derived independently from a piiinitively single dorsal blood-vessel. 



