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



communicating by tortuous tubes with the exterior, and being separated from the outer- 

 most cell-layers with unicellular glands by a special secondary and continuous, though 

 thin basement layer (PI. XIII. fig. 6). Below the latter, longitudinal ahd circular fibres 

 proper to the integument are also present, corresponding, even in their more massive 

 develoi3ment, to those of Carinoma. 



There apjDcars to me to be no doubt that this same arrangement holds good for the 

 great majority of Linei and Cey'ebratuli, and the only reason why the separation of 

 the parts is often less marked is the stronger development of the outer longitudinal 

 muscles of the body- wall, concordant with the disappearance of the connective tissue 

 separating the integument and body musculature, and also the fusion to a smaller or 

 larger extent of the longitudinal muscle-fibres proper to the integument with those of the 

 body-wall. From this it inevitably results that the line of separation between the 

 body-wall and the integument seems to be formed by the external, secondary basement 

 membrane, the deeper glands having the appearance of being imbedded within the longi- 

 tudinal muscles of the body (PL XI. figs. 10, 11 ; PL XII. figs. 2 and 10). That this 

 is a secondary arrangement, and that the real and original line of separation was another 

 one, has been demonstrated in the foregoing pages. 



It is not necessary, after the detailed description of Eupolia given above, once more fully 

 to discuss the same details for those Schizonemertea that wholly correspond to the same 

 type of integument. In those species in which the more developed longitudinal muscular 

 layer more or less effaces the boundary line between integument and muscles (Cerehratulus 

 macroren, Cerehratulus medullatus, &c.), the characteristic and sometimes massive layer 

 of vacuolated cells surrounding the deeper glands is considerably reduced. The other con- 

 stituent parts have retained their original character, with the exception of the thin muscu- 

 lar strata of the integument, which are no longer separately recognisable (PL XII. fig. 10). 



The integument is generally very completely preserved in the cephalic fissures; it 

 may here be noted that there, too, the deeper gland-structures of the integument may be 

 noticed, although they are much more sparingly set. In a few cases it would appear as 

 if they are wholly absent, and as if only the outer integumentary cell-layer is preserved in 

 the cephalic fissures ; others, again (PL XIV. fig. 11), offer special differentiations in the 

 region of the cephalic fissures of the glands, which may there be united in paired accumu- 

 lations. I must also mention a somewhat aberrant ty^Q of integument, as we find it 

 represented in a Cerehratulus?,^. inc. {medullatus f), from Kerguelen Island. The integu- 

 mentary layers offer more general resemblance to what obtains in the more primitive 

 Palaeonemertea {Carinina, Cephalothrix, &c.) than to Eupolia. Eventually it might be 

 said to retain a more primitive embryonic condition. I have at least described a develop- 

 mental phase of the integument very similar to what I am now about to describe for 

 adult forms, as occurring in the ontogeny of Linens ohscurus (XIV). The integument 

 m c[uestion may, however, also be looked upon in another light, i.e., as in no way 



