78 ORIGIN OF CHORDATES in. 10 



separately in larva and adult to become combined in a single stage. 

 This is indeed what has happened in the Appendicularia. 



Strangely enough, one of the chief difficulties of this theory is to 

 find the position of the enteropneusts. Since the larva is still in the 

 ciliated-band stage there should be no sign of organs characteristic of 

 the muscle-swimming, fish-like pro-chordate. Yet such signs are 

 present in the adult Balanoglossiis; there is a hollow nerve-cord and 

 some sign of a notochord. These features almost compel us to suppose 

 that the group has at one time possessed a free-swimming, fish-like 

 stage. The only escape from this conclusion would be by supposing 

 the hollow nerve-tube to be a case of convergence, for which a parallel 

 might be cited in the hollow nervous system of Polyzoa. But there is 

 no clear reason why the nerve-cord should become rolled up in the 

 collar, and it is easier to suppose it a vestige. This imposes two further 

 hypotheses on us. First that a fish-like stage once followed an advanced 

 ciliated-band stage in ontogeny, and secondly that this fish-like stage 

 later became adapted to a burrowing life, in fact that Balanoglossus is 

 a 'degenerate' chordate. Neither of these propositions is impossible, 

 but it must be admitted that the position of the enteropneusts is not 

 clear. Showing a combination of ciliated larva and chordate characters 

 they provide a valuable proof of the affinity of chordates and echino- 

 derm-like creatures, but these very chordate characters become an 

 embarrassment when we try to explain in detail how they have arisen! 



There is strong reason to suppose that what we may call the Bate- 

 son-Garstang theory of the origin of chordates is correct. There is little 

 doubt that chordates are related to the sessile lophophore-feeding 

 type of creature rather than to any annulate, and we can reconstruct 

 the course of events by which the lophophore-feeder may have come 

 to have a pharynx with gill-slits and its larva to have muscles, a noto- 

 chord, and a nerve-tube. Then by paedomorphosis the sessile stage 

 disappeared and the free chordates began their course of evolution. 

 There are some reasons for supposing that a type such as amphioxus 

 could have been derived from a creature not distantly related to the 

 simpler Appendicularia and this in turn from a neotenous doliolid or 

 some similar ancestral type. 



We need not, however, follow the theory into its details, which are 

 speculative. The whole treatment provides a conspicuous example 

 of close morphological reasoning, allied with proper consideration of 

 general biological principles, and establishes with some probability 

 the main outlines of the origin of our great phylum of active creatures 

 from such humble sedentary beginnings. 



