THE EARLY DEVELOPMENT OF AMPHIOXUS. 607 



ence being tliat the pouch corresponding to the trunk coelom 

 of Balanoglossus becomes segmented. 



3. Hatschek's nephridium is the persistent connection of 

 the left of the pair of collar-pouches with the gut. 



4. The metapleural 'Mymph canals" found in the atrial 

 folds arethe persistent ventro-lateral extensions of the ''collar- 

 pouches.^^ 



The general conclusions which can^ I think, be fairly de- 

 duced from the foregoing study are^ in the first place, that all 

 attempts to explain the formation of the nervous system of 

 Vertebrates by the coalescence of the two halves of a nervous 

 ring lying in the lips of a long slit-like blastopore must be 

 given up, any appearances interpreted thus being due to 

 secondary disturbances introduced by increasing food-yolk ; 

 for of any such process no trace is observable in the simple 

 development of Amphioxus. 



Secondly, that the theory of the descent of the Vertebrates 

 from a form somewhat like Balanoglossus receives strong 

 support from the early developmental history of Amphioxus. 

 I say from a form somewhat like Balanoglossus advisedly, for 

 it may not be superfluous to lay stress on this point, that we 

 can no more suppose Vertebrates to be descended from Balano- 

 glossus than from Amphioxus. The main stem of the Verte- 

 brate phylum most probably continued throughout its whole 

 history to lead an active predatory existence, and Balano- 

 glossus and Amphioxus are to be regarded as degenerate ofl- 

 shoots from different levels of this stem. It is much more 

 probable that the Tornaria larva of Balanoglossus gives the 

 best idea of the remote ancestor of the Vertebrates, and in 

 this respect the condition of the nervous system in the larval 

 Amphioxus is of great interest. So far as we know central 

 nervous systems are generally developed in close connection 

 with prominent sense-organs. Now the Tornaria (in its later 

 stages) has two main nervous centres — (a) at the apex of the 

 prseoral lobe, a sensory nervous plate with two eye-spots ; (b) 

 a short nervous tube in the collar region. Of these, the 

 second has probably been developed in connection with a 



