Vf. C. M'INTOSH ON VALENCINIA ARMANDI. 77 



generally gets thicker, the median raphe superiorly and inferiorly more distinct, and the 

 fasciculi more regularly arranged. Then the nerve presses more and more on the circular 

 coat, penetrates it, indents the longitudinal layer deeply, and finally is included in its 

 outline (fig. 6, n), though not at first surrounded by its fasciculi. The next step (which 

 occurs a little further backward) is the increase in the relative thickness of this coat 

 on the disappearance of the great internal circular layer.. The dorsal region dimi- 

 nishes (in transverse section) to a blunt point at the raphe (fig. 7, r), forms a consider- 

 able enlargement at the side of the proboscidian sheath, becomes broad and prominent 

 internally near the lateral nerve-trunk (which it now encloses), contracts a little again 

 below the nerve, and increases largely towards the inner edge of the vascular canal (this 

 being its thickest part), and, lastly, diminishes rapidly towards the ventral raphe. The 

 arrangement of the fasciculi has now become very regular, viz. in the form of long 

 rows running, chiefly in pairs, from without inward (fig. 9, d), and which fairly indicate 

 a stage in the penniform condition of the muscles, observed, for instance (first by M. 

 Marion), in AmpAiporus spectabilis, De Quatref. {Drepanophorus, Hubrecht). 



In Valencinia Mneiformis the arrangement of the muscular fasciculi in the same region 

 (from the great vascular trunk downward) takes the form of long narrow rows 

 (fig. 10, d), quite different in appearance from those in the present form. Dorsally the 

 rows in V. Armandl are for the most part directed downward and outward on each side 

 (fig. 7), then downward and inward, nearly horizontal above the nerve-cord, irregularly 

 arranged round the latter, and with the chief mass external to the trunk, then upward 

 and inward below the nerve, and upward and outward from near the vascular canal to the 

 ventral raphe. The whole field, therefore, has a striking appearance ; and the arrange- 

 ment continues, with little change, to the posterior region of the body. At the tail, 

 again, the layer becomes somewhat thinner, and the diminution of the proboscidian 

 sheath causes a greater proportional increase near the dorsal median line, while the nerve- 

 trunks have the fasciculi only on their inner side. 



To comprehend the next muscular layer (the inner circular layer, fig. 6, o s), it is 

 necessary to examine the commencement of the proboscidian sheath. Just in front of 

 the mouth the sheath (fig. 4, a 6) is complete — that is, has externally a strong layer of 

 circular fibres, and internally a coat of more slenderly developed longitudinal fibres. The 

 inferior part of the sheath rests (over the mouth) on a series of longitudinal muscular 

 fibres (z). Behind the mouth a very noteworthy change rapidly takes place, viz. the 

 gradual absoption of the contiguous portion of the proboscidian sheath into the upper 

 wall of the digestive chamber, and the envelopment of both by the strong inner circular 

 layer. The latter is first formed by a splitting of the outer (circular) fibres of the pro- 

 boscidian sheath (fig. 5, os) ; and while the coat remaining between the sheath and the 

 digestive canal gradually diminishes, this new circular belt soon envelops the united 

 area in a dense muscular mass. Indeed so little of the thin partition between the 

 sheath and the digestive wall is visible, that at first sight it seems as if both chambers 

 floated in a common cavity. After a considerable course this special inner circular layer 

 diminishes, the proboscidian sheath gets thicker, and its proper layers more evident, and 

 finally the former disappears, while the latter (fig. 7, a o) continues to the posterior end 



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