BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 331 



specimens and in regenerating organs, again, it assumes a nearly globular form in 

 contraction. Externally, it is covered by a very delicate investing layer. Within 

 this lies a series of powerful circular muscular fibres, which towards the taper-, 

 ing posterior end become indistinct, and finally disappear altogether, after the 

 csecal tip is reached (Plate VII. fig. 4). The next coat is formed of an equally 

 strong series of longitudinal fibres, the anterior or primary ones being continuous 

 with the longitudinal layer of the reservoir, as previously mentioned. These run 

 throughout the entire length of the posterior chamber, becoming proportionally 

 more developed as the central cavity diminishes towards the csecal end, and 

 finally merging into the muscular ribands which terminate the organ. The 

 mucous layer with its glands lies within the latter, though in several views, both 

 in the living animal and in transverse sections, I fancied some sub-mucous circular 

 fibres were present ; they are at any rate insignificant, and the two chief layers 

 explain all the motions which ensue in this division. This mucous layer in con- 

 traction of the organ forms many rounded folds, which are especially distinct in 

 0. gracilis (Plate IX. fig. 16). A transverse section of the chamber is repre- 

 sented in Plate IX. fig. 14, and the great increase of the glandular mucous layer 

 in contraction is conspicuous. The two muscular coats are about equal in thick- 

 ness. From the commencement of the region behind the translucent reservoir 

 almost, but not quite, to its csecal tip, its entire inner surface is covered with a 

 series of glandular papillae, which differ materially in structure from those of the 

 previous regions. Viewed as a transparent object under moderate pressure 

 (Plate V. fig. 9), the field is found to be covered with globular glands containing 

 clear rounded vesicles in their interior. In contraction, and when the wall is less 

 compressed, the glands have an enlarged and coarse appearance, only the 

 external wall of each being visible. When the pressure has been increased, these 

 glands, especially towards the posterior end (where, from their lessened numbers, 

 a clearer view can be obtained), alter their shape apparently by bursting (fig. 10, 

 same plate), and seem like a double ring of a minutely hirsute aspect, while the 

 contained globules are scattered over the membrane. If the organ has been rup- 

 tured and partly inverted, the free edge of the laceration and the shrivelled 

 glands have the appearance shown in fig. 8, same plate. The globules from the 

 glandular papillae (fig. 3) and glands whose contents have been evacuated (and 

 which are minutely hirsute) readily pass forwards to the reservoir, and roll through 

 the ejaculatory duct under pressure. The function of the vast array of glands in 

 this chamber would seem to be the formation and elaboration of the remarkable 

 fluid with the dancing granules previously alluded to. This secretion is produced 

 in considerable quantities, and towards the posterior portion frequently distends 

 the organ into a translucent pouch (Plate VII. fig. 4, a), wherein the moving 

 granules are in full action, and even the experienced are apt to err in regard to 

 the nature of the movements, so like are they to those caused by ciliary currents. 



