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



This nerve is furnished with fibres directly passing downwards out of the medullary 

 nerve (PI. XII. fig. 9 ; PI. XV. fig. l); it is situated below this, and is entirely parallel to it. 



In one case of a very large specimen of Cerebratulus it appeared in its turn to 

 be splitting up into two parallel nerve-stems. This proboscidian sheath-nerve more 

 especially deserves its name because of its situation immediately above the muscular wall 

 of the proboscidian sheath, into which it may be seen to give off fibres. It is not noticed 

 in Carinina, Carinella, or the Hoplonemertea, but it is in Carinoma, Eupolia, and all the 

 Schizonemertea. Its absence in the two first-named genera would appear unaccount- 

 able if we did not remember that in both of them the proboscidian sheath is of hardly 

 any importance, being extremely thin- walled (cf. PL II. figs. 4-7). And in this case it 

 is all the more natural that in the oesophageal region of Carinoma it has become specially 

 developed, being here even thicker than the medullary nerve, and about as thick as 

 the lateral nerve-trunks of this species (PI. XL fig. 6). This is another example 

 of sudden increase of a portion of the nervous system, and at the same time of the 

 existence of a very marked degree of supremacy to which certain apparently subordinate 

 parts of the organism may all at once attain. This unexpected change of size of the 

 proboscidian sheath-nerve in one species is certainly a valuable fact for a hypothesis that 

 will in a further chapter be enunciated (p. 133), according to which the possibility of a 

 decrease in size of the lateral nerve-trunks is supposed to have been accompanied by an 

 increase in significance of the medullary nerve. 



The fact that in this region of Carinoma the proboscidian sheath-nerve comes into 

 the foreground so strongly that it might easily be mistaken for the medulla, may probably 

 be ascribed to the massive development of the inner circular muscular layer S, which in 

 Carinina, Carinella and Carinoma acts at the same time as part of the wall of the 

 proboscidian sheath. The fact was already noticed as a peculiar feature of the species 

 by M'Intosh (XXIV), when he first described Carinoma (under the name of Valencinia 

 armandi). 



That a proboscidian sheath-nerve is wholly absent in the Hoplonemertea is still more 

 easily accounted for. From the moment the brain and longitudinal trunks of the ances- 

 tral Hoplonemertea were no longer lodged in the midst of the muscular tissue of the 

 body-wall, but have come to be situated within the gelatinous tissue that fills up the space 

 inside this muscular body-wall, not only has the plexus disappeared and been replaced 

 by the remaining metameric nerves described above, but at the same time the innervation 

 of the proboscidian sheath has altered. This innervation is now brought about by the 

 peripheral and metameric nerves, which, in favourable cases (Pelagonemertes, &c), may 

 be seen to send fine twigs into the muscular tissue of that wall. With this freer develop- 

 ment of the peripheral nerve-system, the special arrangement by which the innervation 

 of the proboscidian sheath is brought about, as long as the nerve-sheath is the source 

 from which all peripheral nerve-fibres take their origin, has at the same time disappeared. 



