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



In all Nemertea, to whatever order or genus they may belong, there is one muscular 

 layer that is always present. This layer may, indeed, be looked upon as identical through- 

 out the whole series, and consists of longitudinal muscle-fibres. It is the longitudinal 

 layer of Cephalothrix, in which genus definite or conspicuous circular layers sometimes 

 appear to be absent (PL XL ) ; it is the only longitudinal layer of the Carinellidse — the 

 longitudinal layer of muscle-fibres in the integument of Carinoma not being here taken 

 into account, as indeed belonging to another system — and it is the so-called inner longi- 

 tudinal layer of Polia, Valencinia, and of all Schizonemertea. In PL XI. this layer has 

 uniformly been lettered a. I hold it to be the most primitive of all the Nemertean 

 muscular layers, both on account of its constant presence and on account of the fact that 

 in the posterior region of the body, where growth in length of the animal takes place, it 

 often appears before the other layers that are present in addition to it in the anterior 

 region of the body. 



The layer second in importance to it (morphologically speaking) is a layer of circular 

 fibres marked ft, of very varying thickness, and which in the Carinellidse and the 

 Hoplonemertea is immediately subjacent to the basement membrane, and external 

 to the layer a. The very outermost fibres of this circular layer often take a differ- 

 ent course, making an angle of about 45° with the longitudinal body axis, instead of 

 being perpendicular to it. This, then, being the case in two directions, a decussation 

 of this exterior portion of the layer, especially in Hoplonemertea and Carinellidse, is 

 often noticed. 



Outside of the circular layer ft there is in the Schizonemertea and in Polia and 

 Valencinia the outer longitudinal layer y, sometimes exceeding in thickness the two 

 layers just noticed, and offering very varying conditions as to the compactness of its 

 bundles. In most cases it remains entirely distinct from the two thin epiblastic muscular 

 layers (see pp. 57 and 60) that make their first appearance in Carinoma, and are very 

 generally present in Polia, Valencinia, and the Schizonemertea (PL VII. figs. 5, 9, ef; 

 PL XIII. fig. 6, Ilcm). In some of the latter, however, a fusion occurs between the 

 outer bundles of the longitudinal muscular layer y, and those that are decidedly of 

 integumentary origin and significance, as was already noticed in discussing the integu- 

 ment. It needs no explanation that these latter species offer more difficulties in rightly 

 interpreting the relations between muscular system and integument than many others 

 (PL X. fig. 7 ; PL XII. fig. 10). 



The difference in compactness just alluded to is often dependent upon the degree of 

 development of the deep glandular layers of the integument. Sometimes these glands 

 penetrate the whole depth of the muscular layer, reaching as far down as the nervous 

 stratum (PL XII. figs. 2, 10); sometimes the muscles are kept further apart by the 

 gelatinous ground substance, as was more fully discussed in a preceding paragraph. 

 Eupolia may on the whole be cited as an example in which the degree of compactness of 



