LINNBAN SOCIETY OP LONDON. 29 



Cyclostoraes closely resemble such Ostracoderms, not, however, 

 as their descendants, but rather as their ancestors, although 

 modified and even somewhat degenerated. To appreciate this, 

 however, we must cease gazing at the Lamprey. The Ammoccetes 

 larva is the key. Of course, even this is not primitive enough for 

 the earliest Vertebrate, To reconstruct this we have to take 

 away its trunk, and such a creature may well be expected to have 

 lived in early ISilurian timeR. Although there is not yet known a 

 single fossil Cyclostome from the Silurian slates to recent river- 

 mud, such creatures may come to light and they would not be 

 more puzzling than Palteosponch/lns. 



Thus far it is plain sailing. The Vertebrate end of the broken 

 chain is clear enough. The attempts to bring Amphioxus into line 

 have not been successful, and the claims of the other " Chordata " 

 restrict themselves to a few features of doubtful value. JVowhere 

 could these comparisons be driven home, and what do these 

 attempts amount to against Dr. Gaskell's detailed, almost too 

 minute comparisons of a dozen of the most important organs ? 

 If his results were, every one, nothing but coincidences, analogies, 

 such a state of things would be much more astonishing and un- 

 likely than his whole hypothesis. His explanations of the huge 

 cavities in the brain, the peculiar structure of its roof, the ventral 

 and the neurenteric canal, are the only plausible ones ever offered. 

 It is a somewhat forgotten fact that in some Birds there is no 

 proper neurenteric canal, while in other species there are, not one, 

 but two and even three successively formed communications of 

 the central canal with the gut and passing right through the chorda. 

 An organ which, like the chorda of a bird, has passed its prime, is 

 liable in its degeneration to revert to primitive features, some\A hat 

 muddled. Here we have three neurenteric connections, their 

 respective funnels behaving as if the chorda were a negligible 

 quantity, or rather part of the gut. 



Gaskell's explanation of the chorda is by far the best we have. 

 If considered as a product of an endodermal gut, the chorda 

 cannot well have started as a supporting organ. It must have 

 started with gut-like function, but having lost this with its lumen, 

 its walls shrinking to rod-shape, may then well have formed a 

 useful axial support. Can it be upheld, that the chordoids of 

 Balanoglossus and Hhabdopleura ever had a gut function ? This 

 would mean that a glandular, secretive organ has lost its function 

 and yet waxed large. A proper chorda is not a glandular thing, 

 and even when it is a rod ten feet long and more than one inch 

 thick, it possesses neither nerves nor blood-vessels. 



Zoologists have allowed histology to slip out of their hands into 

 those of the physiologists, and it has done well there. Embryoloo-y 

 would likewise have fared better if the function of the aggregating 

 and growing masses of cells had been taken as the leading 

 principle, instead of the structures which they ultimately give 

 rise to. It is function which determines the organ, and the 



