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



around the nuclei contained in this homogeneous intercellular substance, show a very- 

 delicate granulation, and it is ' often very difficult to decide whether a given one 

 belongs to the nervous network, or whether it is a more indifferent cell, appertaining 

 to the gelatinous ground-substance. In the former case the connection with the nerve- 

 plexus is of great advantage in the decision, and but for this such a decision would often 

 be wholly impossible. The fibres generally offer less difficulty, the delicate nervous fibres 

 being sufficiently distinct from the elastic or contractile fibrils. . The intermuscular 

 homogeneous tissue of Eupolia, and its inclusions in the region somewhat behind the head, 

 are figured in PL VII. figs. 4, 5. In its deepest part, immediately surrounding the 

 circular muscular layer, we find the nervous stratum, that will be more fully discussed 

 further on. We must mention this, because in certain of the Schizonemertea (PL XII. 

 fig. 10), to whose basement membrane and intermuscular tissue we have now to direct 

 our attention, glandular structures belonging to the integument reach as far down as this 

 layer ; a factor which we have to keep well in view when discussing the tissues to which 

 this paragraph is devoted. This is all the more necessary, because in that case the other 

 deeper cellular components of the integument are reduced in number, whereas the 

 outer longitudinal muscular layer having become more compact and dense, the inter- 

 vening region between these two, the region Bet of PL VII., has vanished from view. 

 The secondary basement membrane (B) is then the sole representative of such a 

 structure, and might easily, but as I hope I have demonstrated, injudiciously, be 

 looked upon as homologous with the basement membrane of Carinina, Carinella, &c. 

 (of. PL XL). 



An arrangement of the basement membrane, wholly comparable to what we have 

 described in Eupolia, is found in such Schizonemertea as Cerebratulus corrugatus 

 (PL XIII. fig. 6, B,b). In most of the others the strongly developed and massive 

 outer longitudinal muscular coat so much encroaches upon the deeper layers of the 

 integument in the way just noticed, that it is no longer possible clearly to distinguish 

 between the two integumentary muscular strata {Bern) and the subjacent one constituting 

 the body-wall (7). The extreme representatives of this development are figured on PL X. 

 fig. 7, and PL XII. fig. 10. 



We now resume our examination of the gelatinous tissue, which we have as yet 

 only examined as subintegumentary basement membrane {Carinina and Hoplonemertea), 

 or also as intermuscular substance {Eupolia and Schizonemertea) in its further partici- 

 pation in the muscular investment. In the circular and inner longitudinal layers gela- 

 tinous intermuscular tissue is unmistakably present, and its presence is revealed both by 

 the nuclei and by its peculiar homogeneous appearance, but at the same time, owing to 

 the far greater compactness of these last named muscular layers, when compared with the 

 outer longitudinal one of Eupolia, the position of the connective jelly is much more sub- 

 ordinate, and its presence less easily demonstrable. Still it may be observed in the larger 



