76 Evolution and Distribution of Fishes 



the two evidently took place embryologically, when the 

 sheath became — as suggested by Hubrecht and fully ac- 

 cepted by the author — by degrees modified into the noto- 

 chord. We have already pointed out that this circular 

 zone corresponds to the velar rim in vertebrates. 



Structurally the proboscis-sheath consists of an external 

 circular and an internal longitudinal zone of thick mus- 

 cular fibres, which posteriorly tend to intermix. Filling 

 the cavity of it and surrounding the proboscis, is a corpus- 

 culated fluid, except in the marine metanemertean genus 

 Cerebratulus, where the corpuscles* or loose cells seem by 

 multiplication largely to fill up the posterior part of the 

 cavity with a loose nucleated cellular mass. So, as Hu- 

 brecht remarks, the cavity may become "sometimes even 

 entirely obliterated." He adds (^7:361-63): "we might 

 picture to ourselves the eventual conversion of a hollow 

 proboscidian sheath into a solid notochord, the more so 

 as functionally the proboscidian sheath in nemerteans may 

 already be looked upon as an axis, around which the other 

 organs symmetrically arrange themselves as they do around 

 the notochord in vertebrates. It must at the same time be 

 borne in mind that the muscular coating in this posterior 

 portion is found to be considerably reduced and replaced by 

 a more or less homogenous and comparatively thin sheath." 

 But the writer would emphasize that in the last detail the 

 resemblance becomes the more marked to the vertebrate 

 notochord. 



Now, if in a remote geologic past, namely In Ordo- 

 vician or in lower Silurian time, organisms like metane- 

 merteans existed rather abundantly, a stage may well have 

 been reached when the proboscis-sheath became, from en- 

 vironal action and proenvironal response, more and more 

 a mechanical and strengthening cylinder, rather than as at 

 first a protective sheath for the highly sensitive proboscis. 

 The latter then might have its structure and function more 

 and more condensed into the head region, while the muscu- 

 lar substance of the sheath might by degrees become 

 changed Into "a more or less homogenous sheath of fibrous 

 tissue." As condensation and morphological conversion of 

 the proboscis took place, separation of the proboscis sheath 



