PROBOSCIDIAN CHAMBER. 47 



instancing Nemertes balmea, our N gracilis), as consisting of " external longitudinal and internal 

 transverse" fibres. In the Enopla, as just mentioned, it is exactly the reverse, the circular fibres 

 being external, and the longitudinal internal. He also represented another layer, within the 

 internal, as forming an aponeurosis, apparently referring to the fibrous prolongations from the 

 internal or longitudinal coat. Physiologically, it is certainly a better arrangement for an animal 

 possessing only two muscular coats, to have the longitudinal fibres internal, for, on the occur- 

 rence of rupture, they, as well as the other tissues, are clasped by the circular; whereas, 

 in the supposed arrangement of M. de Quatrefages, the longitudinal are beyond the reach of the 

 constricting belt. Other organs also in the same animal, such as the proboscidian sheath and long 

 posterior gland, have their circular fibres exterior to the longitudinal. 



The actions of this muscular system are very varied, and include swimming or floating 

 on the surface of the water, an action performed, as in the Nudibranchiate mollusks, by aid of the 

 mucous exudation, and not, as stated by M. de Quatrefages, chiefly by the cilia. 



3. Proboscidian Sheath or Chamber, 



In the Enopla this chamber originates just in front of the ganglionic commissures, and 

 continues without interruption nearly to the posterior end of the worm. It is recognised in the 

 living animal under the lens, or even with the naked eye, as that forming a pale dorsal streak, 

 and containing a transparent fluid. The commencement of the chamber is constituted by a fold 

 (Plate XV, fig. 4, a) from the tube of the proboscis becoming attached to the parenchyma of the 

 head, or, in other words, instead of a canal (ab) simply hollowed out in the cephalic tissues, free 

 and distinct walls to the proboscis become apparent at this point. The reflection is the anterior 

 boundary of the proboscidian chamber under ordinary circumstances, and it is against the obstruction 

 so formed that the wave of proboscidian fluid first impinges in the evolution of the proboscis. 

 The cavity gradually increases in diameter, and again diminishes towards the posterior end, where 

 it terminates in a distinct cul-de-sac, a short distance in front of the tail. Its general appearance 

 varies much (Plate XV, fig. 3, and Plate X, fig. 10), according to the position, degree of extension 

 or contraction of the animal, sometimes almost clasping the elongated proboscis, at others havin fl- 

 its attenuated walls stretched over the doubled organ. 



The various transverse sections of the worms also render the relations of the cavity more 

 apparent. Like the proboscis, its anterior end passes through the ring formed by the arch of 

 the superior commissure, the inferior commissure and the sides of the ganglia. The nervous 

 matter must thus occasionally be very much stretched, or else the proboscis is rarely 

 launched out. This will be more particularly noticed in the description of the ganglia 3 and a 

 reference to Plate X, fig. 8, will suffice for the present. The inferior commissure separates 

 it entirely from the ciliated oesophagus (Plate XI, fig. 1). In structure the wall of the 

 chamber is distinctly muscular, as evinced by its ever-varying condition. The fibres, however, 

 are much less powerful at its commencement (Plate XI, fig. 1, o) than they afterwards become 

 (Plate XI, fig. 2, o). Externally it is furnished with a layer of circular, and internally with a series 

 of longitudinal muscular fibres, both again becoming thinner posteriorly. The comparatively large 

 size of the chamber during life has probably caused several observers to err, by confounding it 

 with the supposed general cavity of the body. This may readily be understood by examining a 



