358 DR W. CARMICHAEL M ; INTOSH ON THE STRUCTURE OF THE 



(Plate VI. fig. 1), where they are seen 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 nerve-cells from the softer interior do not pass through this, 

 but escape by travelling along a portion of the great lateral trunk, and rushing out 

 at its torn end, or pass along other branches, such as the superior and inferior com- 

 missures, and the anterior nerves, or through accidental punctures. The nerve- 

 cells are of a yellowish tinge, and minutely granular ( Plate VII. fig, 11), and rapidly 

 alter their appearance after escape into the water. Many contain a larger reddish 

 granule or granules, to which the colour of the organ is partly due ; but I cannot 

 say I saw all the numerous larger pigment-granules so located, although they 

 might have been. In the fresh as well as in the prepared condition (Plate IV. 

 fig. 5), the entire ganglion is dotted with 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, but the inferior 

 more so ; both are paler than the masses of the ganglia. The colour of the ganglion 

 is not destroyed by sulphuric ether, but is rendered paler by acetic acid. 



M. de Quatrefages mentions that in a large Borlasia (anglice f) he found the 

 cephalic ganglia surrounded by a sheath forming a sort of dura mater, but he 

 could see none in the smaller species. In the Ommatopleans, the muscular and 

 other structures of the head form a somewhat condensed capsule round the ganglia, 

 independently of the delicate sheath-proper of the nervous matter. The longi- 

 tudinal fibres of the former, indeed, form powerful bands between the ganglia 

 and the inner muscular layer of the body-wall. M. de Quatrefages mentions 

 the occurrence of ventricles in the interior of these organs (ganglia), and figures 

 them in Polia berea; such have never appeared in any British form, though, 

 under pressure, collections of oil closely resemble the drawing given by this 

 author I have also never been able to see so many branches proceeding from 

 the ganglia (as he shows)* to the eyes, cephalic fossoe, "mouth," and other 

 tissues from the anterior borders, in addition to the great trunks and other twigs 

 posteriorly. The arrangement in the British Ommatopleans is represented in 

 Plate VI. fig. 1, and consists of the following, viz., three very distinct branches 

 on each side of the superior lobe anteriorly ; two about equal in size ; and a third 

 much smaller, to the outer side. Traces of a fourth branch are also present. The 

 outline of the ganglion throughout the rest of its extent is quite smooth. Various 

 branches from these trunks proceed in the direction of the eyes ; but the nature 

 of the cephalic tissues renders it very difficult to trace such an object as a pale 

 nerve-branch with certainty. Dr M. Schultze| gives a tolerably correct view of 

 the ganglia and nerve-trunks in Tetrastemma obscurumj no branches, however, 

 occur on the trunks in his figure. This author, in a later publication,]; founded 



* Op. cit. e.g. pi. xv. fig. 14 ; pi. xviii. fig. 1 ; pi. xix. fig. 1 ; and the whole of pi. xxiv. 

 t Beitrage zur Naturges. Turk, 1851. \ Zeitsch. fur wiss. Zool. iv. 1852. 



