BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 323 



(v, Plate IX. fig. 3)— occupy the exterior portion (covered only by the elastic coat 

 and external longitudinal fibres) of the somewhat solid wall of the section imme- 

 diately succeeding the foregoing cavity, and in some views cause a distinct bulg- 

 ing. They are conspicuous by their aqueous translucency, as well as by the 

 nail-shaped stylets in their interior, though the exact position of their long axes 

 is rather difficult to determine. In ordinary views, when the animal is examined 

 as a transparent object under pressure, their long diameter is antero -posterior, or 

 slightly oblique ; but when the worm has been killed and hardened in alcohol, 

 their long diameter is often found to be transverse (Plate V. fig. 5). Each sac is 

 somewhat ovoid in outline, has a thin, transparent, contractile investment (suffi- 

 ciently tough to prevent the points of the stylets piercing it during the motion of 

 the worm), which lies immediately under the superficial layers of the section, and 

 a duct passing from its central region to communicate with the pit of the anterior 

 chamber of the proboscis. The direction of this duct under ordinary circumstances 

 (i.e. when the animal is viewed from above as a transparent object) is forwards and 

 inwards, but, like other structures pertaining to this mobile organ, it is liable to 

 many alterations, and is occasionally much stretched and attenuated. It is also 

 slightly narrowed on approaching the sac, and has at its junction therewith a 

 series of protecting fibres (Plate VI. fig. 9, a). MM. de Quatrefages and Max 

 Schultze do not notice the duct at all, and M. Claparede's figure* shows it dis- 

 torted from pressure in Tetrastemma, but M. Keferstein's representation is more 

 accurate. Each sac contains a variable number of the characteristic nail-shaped 

 stylets (/3), from three to five, more or less— in different stages of development, as 

 well as certain clear fluid vesicles (£), globules and granules, and is quite filled by 

 a transparent fluid. The relations of the sac and its contents are shown in the 

 various figures. In Tetrastemma algw I have seen, besides the ordinary stylets, 

 a group of minute crystalline spinets, which had no connection with the clear 

 vesicle of the sac. The stylets very much resemble a lath-nail, and are formed 

 of a translucent calcareous secretion ; indeed, they appear like spikes of the purest 

 glass. The head is bulged, rounded at the edges, and somewhat flattened on the 

 top, from which an elongated conical spike proceeds to a sharp apex. The per- 

 fect spike or spikes in these sacs are usually about the size of the central stylet, 

 and there are often three or four that can scarcely be distinguished from each 

 other. Besides the perfect spikes, there are some with heads not fully developed, 

 but complete in other respects ; others again present the form of simple spikes of 

 various lengths devoid of any head. In some instances the centre both of the 

 head and point of the stylet is granular, while the superficial portion is of the 

 usual homogeneous aspect. These stylets are secreted by the sac, yet I do not 

 think they are always developed originally in one of the contained globules, as 

 Dr Schultze says; and this would not signify much, since the entire cavity must 



* Recherches Anat. sur les Annelides, &c. plate v. fig. 6. 



