THE NOTOCHORD AND ALIMENTARY CANAL 44 1 



notochord blind, just as it had already left the end of the infundi- 

 bulum blind. 



The whole evidence points to the derivation of the notochord 

 from a ventral groove on the surface of the animal, which closed 

 to form a tube capable of acting as an accessory gat at the critical 

 period before the new gut was fully formed. The essentials of a gut 

 tube are absorption and digestion of food ; is it likely that a tube 

 formed as I have suggested would be efficient for such purposes ? 



As far as absorption is concerned, no difficulty would arise. 

 The gut of the arthropod is lined with a thin layer of chitin, 

 which is traversed, like all other chitinous surfaces, by fine canali- 

 culi. Through these canaliculi, absorption of fluid material takes 

 place, from the gut to the body. Similar canaliculi occur in the 

 chitin covering the animal externally, so that, if such external 

 surface formed a tube, and food in the right condition for absorption 

 passed along it, absorption could easily take place through the 

 chitinous surface. The evidence of Apus proves that food does 

 pass along such a tube in the open condition, and in the trilobites 

 the chitinous surface lining a similar groove was apparently very thin, 

 a condition still more favourable to such an absorption process. 



At first sight the second essential of a gut -tube — the power 

 of digestion — appears to present an insuperable difficulty to this 

 method of forming an accessory gut-tube, for it necessitates the for- 

 mation of a secretion capable of digesting proteid material by the 

 external cells of the body, whereas until recently it was supposed 

 that such a function was confined to cells belonging to the so-called 

 hypoblastic layer. Experiments were made now years ago of 

 turning a Hydra inside out so that its internal layer should become 

 external, and vice versa, and they were said to have been successful. 

 Such an animal could go on living and absorbing and digesting food, 

 although its epiblastic surface was now its digestive internal surface. 

 More recent observations have shown that these experiments were 

 fallacious. At night-time, when the observer was not looking, the 

 hydra rein verted itself, so that again its original digestive surface 

 was inside and it lived and prospered as before. 



Another piece of evidence of somewhat similar kind, which has 

 not as yet been discredited, is seen in the Tunicata. In many of 

 these, new individuals are formed from the parent by a process of 

 budding, and it has been proved that frequently the gut of the new 



