ON THE ANOESTKAL FORM OF THE CHOEDATA. 361 



significance as a sustaining axis has been replaced by that 

 of the vertebral column. 



That in a far distant ancestor of the vertebrates it may simi- 

 larly have been subservient to the lodging of a retractile pro- 

 boscis, tactile in function, appears to me to follow from 

 a careful consideration of. the relations betvsreen the 

 hypophysis and the notochord, and between the 

 first-named rudimentary organ and the brain. 



An important fact, which in conclusion I must make men- 

 tion of, is a phenomenon wliich I have repeatedly observed in 

 the posterior portion of the proboscidian sheath of different 

 species of Cerebratulus, very long Nemertines, where the 

 sheath reaches down to the posterior extremity of the body. 

 Whereas in young specimens of this species the sheath was a 

 hollow tube down to the very end, in older and larger speci- 

 mens the aspect of things had changed. In the posterior ex- 

 tremity of the body the cavity was here nearly filled up by a 

 continuous cellular tissue with distinct nuclei (fig. 18), 

 sometimes even entirely obliterated. This cellular tissue is 

 sometimes apparently glandular, the arrangement in some cases 

 even such that it must be interpreted as a set of radial acini, by 

 which the surface is considerably enlarged. Future investiga- 

 tions will have to decide whether the cases of evident oblite- 

 ration may be interpreted as a step towards real solidification 

 of that part of the tube which is comparatively of the smallest 

 value for the general function in connection with the expul- 

 sion of the proboscis. This change of function and histolo- 

 gical appearance is only present in the more primitively organ- 

 ised groups, which rarely make use of their proboscis ; in the 

 more highly specialised Hoplonemertini, where the proboscis 

 is in constant play, and the development of the muscular ele- 

 ments in the proboscidian sheath vary considerably, it was 

 nowhere found. 



Apart from the argument which can be derived from the 

 nature of this cellular coating the significance of this pheno- 

 menon will have to be carefully inquired into. Along such a 

 line of development we might picture to ourselves the eventual 



