BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 357 



cellular, being composed of minutely granular nerve-cells, and is connected with 

 its fellow of the opposite side by the long or superior commissure (Plate V. fig. 1,/), 

 which passes over the proboscis. In ordinary circumstances, this commissure is 

 less than half as broad as the inferior, but it is considerably longer. It is a simple 

 ribbon of transverse fibres. As observed in the living animal, these fibres pass 

 on to the superior lobe, where they diverge, some turning slightly forwards, but 

 the majority passing obliquely backwards to the pale central part of the lobe. 

 The only remark made by M. de Quatrefages with regard to the physiology of 

 this band is, that it removes the somewhat surprising state of matters of having 

 a brain composed of two lateral masses, and only one (" sub- oesophageal") com- 

 missure. To me, however, this band seems of more importance, since, during the 

 enormous distention which takes place in the extrusion of the proboscis, it is the 

 superior commissure which is stretched to an extreme degree of tenuity. The 

 proboscis, as mentioned, passes through a complete ring of nervous texture, and, 

 during extrusion, forces this outwards in all directions, but chiefly superiorly, 

 the inferior commissure, indeed, being little altered. Nearly half the circum- 

 ference of the proboscis projects above the level of the ganglion (Plate IV. fig. 5), 

 and the superior commissure must be correspondingly elongated ; hence, if this 

 is purely a nervous band, we have a very interesting example of the elasticity of 

 such texture. It may possess elastic as well as nervous fibres, but such are not 

 distinguishable. The inferior commissure consists of a thick mass of nerve-fibres, 

 the majority of which sweep backwards to form the lateral nerve-trunks ; thus 

 it becomes an isthmus between these cords. A few of the anterior fibres are 

 connected with the central region of the former division of the ganglion. 



In long species, such as 0. gracilis and 0. purpurea, the ganglia are not cor- 

 respondingly lengthened, but are rather rounded. In Tetrastemma the arrange- 

 ment of these organs is very similar to that in 0. alba, so that a special descrip- 

 tion need not at present be given, further than by referring to Plate VIII. fig. 7, 

 which represents the ganglia in a small specimen of T. varicolor, where the inferior 

 commissure is shorter and broader, and the lobes more elongated. This is also 

 the case in ProsorJiochmus. In the aberrant form, Polia involuta, Van Bened., 

 the ganglia are strictly Ommatoplean in shape, and the lateral nerves, which are 

 not shown by the discoverer of the species, comparatively large. M. Beneden's 

 figure of the anterior branches of the ganglia is erroneous. The lateral nerves 

 lie quite within the longitudinal muscular coat. 



Carefully made transverse sections show how incomplete is the impression 

 conveyed by the examination of the parts in a compressed, though living animal. 

 Instead of forming a flattened organ, whose greatest transverse diameter is across 

 the plane of the body, each ganglion has its longest (transverse) diameter nearly 

 perpendicular to the latter (Plate IV. fig 5, and Plate V. fig. 1). The nerve-cells 

 do not appear to be confined to the superior portion, but occur in the inferior also 



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