BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 337 



of communication with the posterior chamber is somewhat short and wide, and in 

 marked contrast with the same part in the latter form. The long posterior 

 chamber has its inner surface thrown into more prominent rugae than in most 

 species, so that they sometimes appear like large papillae covered with the glands- 

 proper of the cavity. These plaits are not mere wrinkles and folds caused by the 

 contraction of the elongated organ, but are present under severe pressure ; 

 indeed, they are characteristic and original processes of the chamber (Plate IX. 

 fig. 16). The granules of the peculiar fluid therein are also very distinct. It 

 may be mentioned here, that after prolonged confinement the integrity of the 

 proboscis in this and other species is affected, the stylets degenerating, and even 

 disappearing altogether, both from the central and lateral structures. Not only is 

 this the case in the adults themselves, but under the same circumstances the 

 more advanced young in the interior of Prosorhochmus Claparedii undergo a like 

 degeneration. In a specimen of the former species where this had occurred, the 

 wave of granular fluid driven forward by the contraction of the reservoir distended 

 the muscular cavity in front of the granular basal sac of the central apparatus 

 (which in this instance was devoid of a stylet), and as the aperture into the 

 anterior chamber permitted only a limited discharge at a time, the fluid rushed 

 into the centre of the granular sac, and distended it and its wedge-shaped setting 

 with every impulse. The absence of proper nutriment and free aeration — for the 

 salt water was but rarely changed during the year — are sufficient causes for the 

 above-mentioned degeneration. 



In 0. pulchra (Johnst.) the anterior region of the proboscis has a decidedly 

 pinkish hue, and numerous small clear globules at its commencement, as well as 

 over the reservoir. The large glandular papillae in the anterior chamber have 

 their marginal globules less distinctly marked than in 0. alba or Tetrastemma, and 

 hence the structure has a smoother or finer appearance. The lateral stylet-sacs 

 ( Plate VII. fig. 3, v) are very large, and each contains, in well-developed specimens, 

 from five to nine stylets, a large circular globule, and a granular orange pigment- 

 mass, besides a fluid rich in moving granules, similar to the secretion from the 

 long posterior chamber. It is, however, in the apparatus of the central stylet 

 that the greatest deviation from the typical structure occurs. The basal sac of 

 the stylet (Plate VI. fig. 11, A) is small, elliptical rather than ovoid, and its 

 granules are very minute. In addition to the ordinary stylet (a) fixed to its 

 anterior end, another stylet (b) projects into its posterior portion, enclosed in a 

 kind of sheath, and whose point extends forwards almost to the butt of the an- 

 terior stylet. This reserve-stylet is not in all cases fully formed, but apparently 

 awaits the rejection of its progenitor for complete development. The head of the 

 reserve-stylet projects into a large cavity formed by a peculiar disposition of the 

 fibres composing the setting of the basal sac and the region behind. Instead of 

 the usual wedge-shaped structure, radiating fibres pass outwards from the sides 



VOL. XXV. PART II. 4 R 



