REPORT ON THE NEMERTEA. 71 



this muscular layer remains at the lowest level (PI. VII. figs. 2, 5), in Cerebratulus 

 corrugatus I found it very compact, and composed of very delicate fibres (PL 

 XIII. fig. 6). 



Another additional muscular layer, which is not found in all but only in certain 

 Nemertea, is marked S in the figures of PI. XI. It is an inner circular layer, and in the more 

 primitive types [Carinina, Carinella, and Carinoma) it may even become exceedingly 

 massive. It is directly applied against the longitudinal muscular layer a ; it forms at the 

 same time the dorsal wall of the proboscidian sheath, the exceedingly thin ventral wall of 

 which is in these species formed by fibres of the same inner circular layer that branch off, 

 and are directed inwards between the space for the proboscis and the oesophagus or intes- 

 tine, thus creating a floor to that proboscidian space. The inner circular layer is continued 

 ventrally, and embraces the other internal organs as well. In Carinoma, where the 

 layer has such a considerable thickness in the proboscidian and oesophageal region, and 

 where it has disappeared in the posterior region of the body, leaving only the longi- 

 tudinal and outer circular layers, the conclusion is of course tempting that the special 

 development of this layer is in a certain functional connection both with proboscis and 

 oesophagus. And if we then find that in the Schizonemertea this layer is absent, but 

 that, on the other hand, there is a circular muscular coat to the proboscidian sheath and 

 that this sheath has been raised to greater independence, and remains dorsally connected 

 with the rest of the muscular body-wall in exactly the way it would be if it were the 

 modified remnant of a restricted portion of the inner circular layer, we are led to the 

 further hypothesis that these two may indeed be homologous. Thus all the transverse 

 sections of the dorsal body- wall of Schizonemertea on PL XL, were they to be completed 

 by adding the circular muscular layer of the proboscis-sheath immediately applied against 

 them in the median line, would very strongly resemble the figures given of Carinoma 

 and Carinina. 



I will not at present venture to decide whether any of the muscular layers of 

 the oesophagus, noticed both in Eupolia and Cerebratulus (PL VI. fig. 9, oe.m ; PL XIII. 

 fig 6, mto), may also be looked upon as derivatives of this inner circular layer, but will 

 only add that in Hoplonemertea such a musculature is hardly developed ; whereas, on 

 the contrary, the circular muscles of the proboscidian sheath have attained a very high 

 importance, and are even more independent of the dorsal muscular body-wall than they 

 are in Schizonemertea (PL IX. figs. 1-9 ; PL X. fig. 1). 



Here, too, I would be tempted to hazard a comparison between the absent inner 

 circular layer and the musculature of the proboscidian sheath. 



The detailed histology of the Nemertean muscular system is hardly in its place here, 

 and may perhaps be more fitly reserved for the monograph that will shortly appear 

 in the Naples series. 



One point must, however, be mentioned, as its definite establishment seemed im- 



