82 ANATOMY OF THE ENOPLA. 



large or superior commissure (Plate XI, 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 forms a simple ribbon of transverse fibres, some of which, after diverging, turn 

 slightly forward, but the majority pass obliquely backwards to the pale central part of the lobe. 

 The only remark made by M. de Quatrefages with regard to its physiology is that it removes 

 the somewhat surprising condition of having a brain composed of two lateral masses, and 

 only one commissure. This band, however, seems of more interest, since during the enormous 

 distension which takes place in the extrusion of the proboscis, it is the superior commissure 

 which is stretched to an extreme degree of tenuity. The organ, passing through a complete 

 ring of nervous texture, must force this outwards in every direction during extrusion, and espe- 

 cially superiorly, the inferior commissure, indeed, being only slightly affected. Nearly half the 

 circumference of the proboscis projects above the level of the ganglion (Plate X, fig. 8), and the 

 superior commissure must be correspondingly elongated ; hence we have an interesting example 

 of the elasticity of a nervous band. The inferior commissure consists of a thick mass of 

 fibres, the majority of which sweep backwards to form the lateral nerve-trunks ; thus it becomes 

 a commissure between these cords. A few of the anterior fibres are connected with the central 

 region of the former division of the ganglion. 



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 diameter is across the plane of the body, each ganglion has its long 

 diameter nearly perpendicular to the latter (Plate X, fig. 8, and Plate XI, fig. 1). The nerve- 

 cells do not appear to be confined to the superior portion, but occur in the inferior also (Plate XV, 

 fig. 4), on each side of the origin of the great nerve-trunks. In the fresh specimen the sheath of 

 the ganglion is moderately resistant; for under pressure the cells from the interior do not 

 pass readily through, but escape by travelling along a portion of the great lateral trunk, and out 

 at its torn end, or by other branches, such as the superior and inferior commissures and 

 the anterior nerves, or through accidental punctures. The nerve-cells have a yellowish tinge, 

 are minutely granular (Plate XV, fig. 6), and rapidly alter their appearance after escape into the 

 water. Many contain a large reddish granule or granules, to which the colour of the organ is 

 partly due ; but I cannot say all the numerous pigment-granules are so located, though they 

 may have been. In the fresh, as well as in the prepared condition (Plate X, fig. 8), the entire 

 ganglion is dotted w r ith minute pigment-specks and granules, which are also continued along the 

 great nerve-trunk for a considerable distance. The superior commissure is faintly tinged with 

 colouring matter, the inferior more so ; both are paler than the masses of the ganglia. The 

 hue of the ganglia is not destroyed by sulphuric ether, but is rendered pale by acetic acid. 

 The distinction between the superior and inferior lobes is sometimes clearly shown in oblique 

 sections, which give, on the same side, a portion of the superior lobe, as well as the origin of the 

 nerve-trunk (Plate XV, fig. 1). 



In the elongated species, such as N. gracilis (Plate V, fig. 4) and N. Neesii (Plate XV, 

 fig. 5), the ganglia are not correspondingly lengthened, but are somewhat rounded. They are 

 also rounded in A. pulcher (Plate XV, fig. 2) and A. spectabilis. In Tetrastemma melanocephala 

 (Plate XIV, fig. 2) the arrangement is very similar to that in A. lactiforeus. In T.Jlavida, how- 

 ever, the inferior commissure is rather shorter and broader (Plate XIV, fig. 14), and the lobes 

 more elongated. This is also the case in Frosorkoc/imus. 



