PROBOSCIDIAN APERTURE AND SHEATH. 101 



by other than the animal itself. The extreme stretching which the body undergoes before it 

 snaps — as in attempting to secure a specimen in an intricate and inaccessible pool — and the 

 extraordinary shortening on immersion in spirit, are only well-marked conditions into which the 

 animal throws its yielding textures at will. Borlasia Elizabeths contracts itself so firmly during 

 life as to form a hard flattened mass, which somewhat resembles the siphonal process of a Mya 

 (Plate VII, fig. 2). Micrura fusca, again, swims freely on its edge like a freshwater Nephelis, 

 or its own ally A. pulcher, lashing the water with alternate strokes of its muscular and flattened 

 posterior extremity. Sir J. G. Dalyell likewise noticed this edge-motion in his great " Gordius " 

 fragilis {Cerehratulus angulatus), but he was not sure whether it was a natural condition, or caused 

 by the confined vessel. Carinella annulata secretes in captivity a beautiful silky sheath, within 

 which it lies in comparative security, until, tempted perhaps by love of change, it searches for a 

 fresh site, whereon to manufacture a new chamber for its protection. In unhealthy and slowly 

 dying animals the skin becomes raised into pale • bullae, not only from corrugation, but from de- 

 generation of the dermal textures. 



3. Proboscidian Aperture. 



A channel, ciliated for some distance, leads inwards from the terminal pore of the 

 snout to the reflection of the proboscis just in front of the commissures. This channel, 

 shortly after its commencement (Plate XVIII, fig. 7, a), is surrounded by an elaborate 

 series of muscular loops (indicated at 2), which, while keeping it closed under ordinary 

 circumstances, permit of rapid and easy dilatation. Immediately below is a series of longi- 

 tudinal muscular fibres, which attain a more distinct development somewhat posterior to this 

 point (a, Plate XVIII, fig. 8). A very beautiful group of circular and diverging fibres 

 lies outside the first series (2, in the last-named figure), crossing each other in a striking 

 manner superiorly and inferiorly, as well as less distinctly at intermediate points, and forming 

 with the longitudinal and other fibres the intricate stroma of the snout. The terminal pore is 

 furnished with a prominent papilla, covered with a fan-shaped brush of cilia, the whole being 

 only occasionally extruded, and no doubt assisting the papillae previously mentioned in the tactile 

 functions of the snout. This central papilla is sometimes bilobed, each division being sup- 

 plied with cilia. In spirit-preparations of large examples of Lineus marinus the proboscidian 

 aperture is distinguished by a slight slit on the inferior surface immediately behind the tip of the 

 snout, the minute anatomy and relations of which agree very closely with the same parts in 

 L. gesserensis. 



4. Proboscidian Sheath and Chamber. 



The proboscidian sheath forms a shut sac, as in the Enopla, from the bridge of the ganglionic 

 commissures to the posterior end of the worm. The long proboscis glides smoothly in this 

 chamber, the walls being united with it and other tissues just in front of the commissures. The 

 other contents are the clear proboscidian fluid and its discs. The latter are circular granular 

 bodies, similar to, though smaller than, those of the Enopla, and when seen on edge present a 



