xvii PKOTOCHOEDATA 607 



pathological. That a bilaterally symmetrical embryo should pass 

 through an asymmetrical pelagic larval stage, in order to attain the 

 form of a practically symmetrical burrowing adult, seems to be in 

 complete contrariety to what we should expect. 



But a very plausible solution has been suggested by Stafford 

 (quoted in MacBride, 1909). He points out that the ancestor of 

 AmpMoxus could not have suddenly jumped from a pelagic to a 

 burrowing existence. Some intermediate condition of life must have 

 existed. What was the intermediate state of affairs ? In the case of 

 other animals which have deserted a pelagic for a more humble form 

 of existence, we have reason to believe that a habit of gliding over 

 the surface of the mud preceded a burrowing life. But a vertically 

 flattened animal like AmpJiioxus could not do this and keep on an 

 even keel. The moment it ceased swimming it must have tended to 

 fall on one side. This circumstance can actually be observed in the 

 older larvae of Amphioscus. Let us assume, therefore, that the ancestors 

 of Amphioxus, after deserting their free-swimming life, passed through 

 a stage when they lay on their left sides on the bottom. Under these 

 circumstances it would be an advantage to twist the mouth round to 

 the left side, so as to bring it near the detritus which served as food, 

 and to twist both sets of gill-slits round to the upper right side, so 

 that the waste water might be ejected without disturbing the sub- 

 stratum on which the animal was feeding. Similar conditions of life 

 have led the modern flat-fish to become asymmetrical, they have 

 twisted the mouth down and the two eyes round to the upper side of 

 the body. 



But if this solution sounds plausible two further questions may 

 be asked, viz. () Why does only one row of gill-slits develop ? and 

 (&) Why does the asymmetry appear at that stage of its development 

 when, to judge from its environment, the larva ought to be bilaterally 

 symmetrical ? 



The answer to the first question has an important bearing on the 

 way in which ancestral adult organs are represented in the larva. 

 The larva in almost every case (see Chap. I.) has been much reduced 

 in size in comparison with the ancestor which it represents. If larval 

 organs were reduced 011 the same scale as the whole animal they 

 might be reduced to dimensions which would render their functions 

 difficult, if not impossible of performance. Too narrow gill-slits, for 

 instance, might become useless owing to the viscosity of water in 

 passing through small apertures. Hence arises a tendency, which we 

 have noted elsewhere, to sacrifice number to size; thus, of the two series 

 of gill-slits which the ancestor undoubtedly possessed, only one is 

 developed in the 'larva in order that the individual slits may have 

 room to spread. The missing gill-slits are held over, so to speak, till 

 adult habits are about to be taken up. 



That a left-sided mouth and asymmetrical gill-slits should appear 

 in a free-swimming larva is only another example of an exceedingly 

 widespread phenomenon, viz. the reflection of features belonging to 



