106 THE VOYAGE OF H.M.S. CHALLENGER. 



In many Schizonemertea the proboscidian sheath is thus constituted ; in others I find 

 that a phenomenon, which receives its more final expression in certain Hoplonemertea, 

 is not wholly absent — I mean the presence of varicosities, in which the inner space of 

 the sheath is bulged out, without the musculature following. In this way more or less 

 irregular appendages are brought about, generally along the side or the lower corners, 

 having the aspect of accessory reservoirs. In Cerebratulus sp. inc. (PL XV. fig. fi), 

 from Japan, it seems as if these appendages in the posterior region of the body 

 even surpass in size the sheath itself, which is not a very significant organ in that 

 region of the body, and, moreover, as if these two caeca are filled with a sub- 

 stance of the nature of which the available spirit specimens do not enable me to 

 judge. 



I do not wish to discuss here the significance of these facts, not having for the present 

 sufficient material to study them more fully ; it is only my purpose to call the attention 

 of future investigators to the phenomenon, which may be so significant for a correct inter- 

 pretation of the posterior, often semi-rudimentary portion of the proboscidian sheath. 



It has been observed that in certain Hoplonemertea the phenomenon just noticed 

 finds a more definite and more regular expression. The first observation of this is due to 

 M'Intosh, who detected in the proboscidian sheath of Drepanophorus regular metameri- 

 cally placed openings, by which the space inside the muscular sheath communicated with 

 other cavities outside of it, that had no muscular walls (XX). 



While M'Intosh supposed these accessory cavities to establish a communication 

 between the cavity of the proboscidian sheath and the blood-vascular system, I have 

 since demonstrated (VII) that no such communication exists, but that Dre-pano-pliorus 

 possesses closed membranous sacs communicating with the proboscidian sheath, and 

 probably serving as reserve spaces for the fluid contents of the proboscidian sheath 

 during the very powerful contractions and distentions which the organ may undergo. In 

 the Challenger specimens the same phenomenon was observed, and I have even ventured to 

 assign all those specimens in which these regular paired appendages of the proboscidian 

 sheath were found to the genus Drepanophorus, even when I have not succeeded in deter- 

 mining the armature of the proboscis so characteristic of the genus. 



Two sections through the proboscidian sheath of Challenger Drcpanopliori are figured 

 on PI. X. figs. 4, 5. 



The curious arrangement of circular and longitudinal fibres, having the appearance of 

 basket-work in the transverse section, may be understood from these figures, even without 

 any further description. 



Between the musculature and the inner epithelial layer there is again a homogeneous 

 membrane, with longitudinal folds indicative of the contracted state in which the sheath 

 here figured was at the moment of its preservation. In neither of the two was the section 

 quite vertical, thus only one of the lateral diverticula is cut, instead of the pair that are 



