BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 325 



happened, was thus emptied, while the other retained its three stylets. The 

 loose stylets were very slowly moved forwards, scarcely any progress being made 

 during an hour's observation. At this time the sac from which they had been 

 liberated contained numerous granules, but no circular or ovoid vesicle. Twenty- 

 four hours after, the stylets had disappeared. The sac is now observed to be 

 much less than its fellow of the opposite side, and somewhat shrivelled and 

 undefined ; but it contains a small ovoid vesicle, which is traversed by a minute 

 slender spike, whose long diameter exceeds that of the globule, and therefore it 

 cannot be supposed to be within it. In addition, there is a free spike about a 

 third the length of the larger one. The former has assumed the shape of a stylet 

 without a head ; the latter is as yet nearly cylindrical (Plate VI. fig. 4). What- 

 ever the function of these stylets in the lateral sacs may be, there can be no doubt 

 they have nothing to do with the supply of the central apparatus, for that fur- 

 nishes its own stylet. 



The middle or stylet-region is likewise the seat of other structures of import- 

 ance, viz., the central stylet and its basal sac, the ejaculatory duct or canal of com- 

 munication with the reservoir, and the circlet of granular glands. It is of the 

 same vitreous translucency posteriorly as the succeeding region, while both the 

 anterior chamber and the posterior region are of an opaque white in the fresh 

 specimen. Externally there is the investing layer (Plate IX. fig. 3, g), continued 

 from the anterior chamber, and which passes backwards to the next region. 

 Beneath this lies a series of very powerful and conspicuous longitudinal muscular 

 fibres (/, same plate), apparently to some extent continuous with the more bulky 

 longitudinal layer of the preceding region, but few of which pass on to the 

 next. Internally oblique and radiating fibres occur, the former slanting forwards 

 and outwards from the setting of the central stylet, and forming a kind of mus- 

 cular sling, well marked in 0. melanocephala (Plate VI. fig. 7). This layer is dis- 

 tinctly separated at its posterior border from the succeeding region or reservoir 

 by a pale boundary-line under pressure, so that the parts have a somewhat 

 jointed appearance. In transverse section, the complicated structure of this 

 part is well observed (Plate V. fig. 5). The longitudinal fibres form a thick belt 

 exteriorly, and send gradually diminishing bundles inwards towards the central 

 point. This peculiar appearance in transverse section must be due to some 

 difference in the arrangement of the ultimate fibres, as such sections of other 

 muscles usually show a much coarser, more fasciculated, and less granular aspect. 

 There can be no mistake as to the true structure and arrangement of these fibres, 

 since I have cut them both obliquely and transversely in the same specimen. 

 The last-mentioned transverse section also shows a complicated arrangement 

 round the central stylet-apparatus ; exteriorly there is a firm setting, next a layer 

 which seems to be closely united with the coat of the ejaculatory duct in front, 

 and other two more immediately connected with the granular sac itself. Some of 



VOL. XXV. PART II. 4 



