444 



found a rod of hardened mucoid material. The organ has a uniform 

 structure throughout its length. 



It seems to me that these facts point to an actively secretory 

 function, and there is no indication that the organ is in any degree 

 vestigial. As already emphasised, it does not show any present or for- 

 mer modification into vacuolated tissue. In the higher Chordata, the 

 formation of hypoblastic chordoid tissue is usually regarded as a pro- 

 cess of histological degeneration, and in accordance with this, the loss 

 of passive supporting function is accompanied by extinction of the 

 organ (Vertebrata). There is, as far as I know, no instance of a 

 chordoid structure once having been acquired, becoming secondarily 

 redifferentiated into an actively secretory tissue. 



Apart from this , there can be little doubt that both in form and 

 function this organ differs from the 'Eicheldarm' of Balanoglossus and 

 the notochord of Eu-chorda. Mr. Harmer would account for this 

 by assuming a degeneration owing to the small size of the organism, and 

 although reduction in size is an undoubted element in the simplifi- 

 cation of structure, it would seem that we have to deal with an entirely 

 different kind of organ. We may also note that structures of a chor- 

 doid nature are present both in Actmotrocha and in young Phoronis, 

 organisms of smaller bulk than Cephalodiscus . 



b) Its relationship to other organs. 



Here Mr. Harmer difi"ers with me with regard to the facts and, 

 before proceeding further, an explanation is, I think, due. My paper, 

 'On the Diplochorda' was sent in for the Granville Prize of London 

 University ^ and in consequence it had to be finished by a certain date 

 (May 1st). Hence my revision of the plates was not as careful as I 

 would have desired. In my original drawing of PI. XXIV Fig. 14 

 the anterior body-cavity is completely continuous with the space that 

 Mr. Harmer would homologise with the 'heart'. It might perhaps 

 have been more clearly indicated in the figure alluded to, but as 

 Mr. Harmer has re-examined this part for himself he will not be 

 relying upon this evidence. 



Immediately after the first examination of my sections of Cpeha- 

 lodiscus I supposed that the organs brought to view had exactly the 

 structure and homologies since suggested by Mr. Harmer, but a clo- 

 ser inspection proved them to be untenable. 



My sub-neural blood-sinus is perfectly continuous with the dorsal 

 blood-vessel and is, in fact, merely the swollen termination of the latter. 



1 I ought in fairness to state that it was unsuccessful, as the examiners, Prof. 

 Lankester and M. Beddard ,did not consider it of sufficient merit'. 



