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



few fibres. So it is in Carinina (PI. II. figs. 3, 6, 7, 9, 10 ; PI. IV. fig. 6, Ps). In Eupolia 

 the proboscis is longer, but the sheath is still most insignificant, as may be gathered from 

 the figures (PI. VI. figs. 9, 10 ; PI. VII. fig. 10). It is a space having internally a cellular 

 coating very similar to that of the blood-spaces, the cells of this internal epithelial covering 

 often more or less projecting into the lumen of the sheath. Outside of these cells a few 

 circular fibres are seen to have developed ; outside of these there is again the body-paren- 

 chyma, with the enclosed blood-lacunas. There is no doubt that from sections of Eupolia 

 alone nobody would be inclined to look upon the cavity of the proboscidian sheath as a 

 very independent cavity, nor is it possible to affirm that the mode of the protrusion of 

 the proboscis, as it was sketched above, is indeed fully developed in Eupolia and 

 Carinella. There is no doubt that of all Nemertea observed alive, these two were 

 never seen to protrude their proboscis spontaneously, and very often even preserved 

 them in death, when the Hoplonemertea always forcibly expel and even spontaneously 

 detach their proboscis. 



There is, on the other. hand, no evidence at all which would justify us in regardiug 

 the arrangement of these Palseonemertea as secondary or degenerated from a higher 

 differentiated stage. The participation of the body musculature in bringing about 

 the movements of the proboscis in these lower forms renders this more intelligible. 

 Only in the more highly differentiated Schizonemertea, and especially in the Hoplo- 

 nemertea, the muscular walls of the proboscidian sheath undergo a very rapid increase 

 in bulk, and at the same time become more and more, and in the last-named group 

 even wholly independent of the body musculature. This increase of an organ so 

 eminently mesoblastic as the proboscidian sheath, by gradual addition of new fibres that 

 are even arranged in multiple layers, can thus be traced in all its various stages in the 

 different genera of Nemertea. Salensky would probably not have made his startling 

 hypothesis above alluded to, 1 based on ontogenetical observations of a scission in the 

 proboscidian wall, by which (l) a muscular proboscidian sheath surrounding the proboscis 

 becomes separated from, and independent of, the musculature of the proboscis itself, and 

 (2) an isolated ccelome — the proboscidian cavity — is originated, if he had been as well 

 acquainted with the comparative anatomy of the animals about which he writes as he is 

 with certain details of their ontogeny. 



Granting even that the development may, in the species observed by him, 

 follow the paths he has sketched (my own observations on the ontogeny of Lineus 

 obscurus (XIV) have led to wholly different results on this head), it is not yet per- 

 missible to base upon those two ontogenetic observations phylogenetic speculations 

 wholly at variance with all the facts that are furnished by a comparison of the different 

 living genera. The woodcuts given by Salensky, in which a Rhabdoccele proboscis and a 

 Nemertean one are put side by side, look very tempting, but cannot be accepted by me. 



1 Archives de Biologie, vol. v. p. 561 ; Zeitschr.f. wiss. Zool, Bd. xliii. p. 508. 



