312 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



would not seem to be much interfered with except in extreme protrusion of the 

 proboscis. All the oblique or transverse fibres are connected with the body-wall 

 and the inner muscular layer, as are also the longitudinal at the tip of the snout. 



This elaborate interlacement provides in the best possible manner for the 

 varied changes which this region undergoes during protrusion and retraction of 

 the proboscis, and the ordinary motions of a tactile and mobile, yet not too yield- 

 ing snout. The arrangement of the oblique and circular fibres around the longi- 

 tudinal layer of the central canal also must act the part of a constrictor, and 

 adapt the cavity to its ever-varying calibre. On the whole, the stroma in this 

 group, from the greater predominance of granular elements, is less dense than in 

 Borlasia, and the interlacing of the fibres, though not more complex, is more 

 beautiful, because possessing greater distinctness and regularity. 



The posterior end of the body has no such intricate arrangement, but the 

 muscular fibres blend together at the tip and close in the cavity, with the 

 exception of the small and sometimes indistinct opening of the great longitu- 

 dinal digestive chamber. The modes of fracture of these muscular coats in 

 some of the Ommatopleans in a sick and perishing condition are interesting, the 

 body being separated into a number of beads from the constriction and rupture 

 of the body- wall at somewhat regular intervals. 



My observations would lead me to follow a different arrangement in the 

 description of the cavities within the body-wall, from that pursued by MM. de 

 Quatrefages, Keferstein, and Van Bkneden, since there exist some differences 

 as regards interpretation of structures. Instead of speaking of a k ' general cavity of 

 the body," I shall first refer to that chamber in which the proboscis lies, and 

 which may be termed the cavity of the proboscidian sheath. 



Cavity of Proboscidian Sheath. — In Ommatoplea alba as well as in Tetrastemma , 

 this chamber commences just in front of the ganglionic commissures, and con- 

 tinues without interruption nearly to the posterior end of the worm. It is recog- 

 nised 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 commence- 

 ment of the chamber is shown in Plate VI. fig. 1, where a fold (a) from the tube 

 of the proboscis becomes attached to the parenchyma of the head, or where, in- 

 stead of a canal (ab) simply hollowed out in the tissues of the head, free and 

 dictinct walls to the proboscis become apparent. This reflection is the anterior 

 boundary of the proboscidian sheath under ordinary circumstances, and it is 

 against this obstruction 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, when 

 viewed from above, as a transparent object, is seen in Plate VI. figs. 3 and 8, but 

 it varies much according to the position, degree of extension or contraction of the 



