MUSCLES OF THE BODY-WALL. 99 



inner thicker longitudinal (e"). The circular muscular fibres surrounding the digestive chamber, 

 however, are unusually powerful. In a curious specimen from Balta, with a bifid proboscis and 

 other peculiarities, the arrangement, which shows, perhaps, only an abnormality of this type, is 

 as follows : — Externally (Plate XXII, fig. 2, d')> beneath the basement-layer of the cutis (which 

 in the fragmentary specimen was almost absent), is a coat of circular fibres (e). Within the 

 latter is a very powerful layer of longitudinal fibres (<?), which (layer), however, is not continuous, 

 as in the ordinary form, but has at least one very distinct point of separation. On approaching 

 the middle line of the dorsum, this longitudinal coat becomes thinned, so as to end on each side 

 of the centre in a blunt point. In addition, a somewhat triangular portion (ea) is cut off by 

 interfascicular substance and fibres. The dorsal curve of the proboscidian sheath is closely 

 applied to this central point of separation, apparently receiving therefrom a few fibres, which 

 retain it in position, while other fibres pass downwards to join the circular layer (ja), which here 

 encloses the space for the digestive tract. The separation of the great longitudinal layer of the 

 body-wall is marked externally by a distinct median line, which is rendered more conspicuous by 

 the occurrence of the transverse striae of the dorsum on each side of it. There is also a slightly 

 marked median fissure in this muscular coat inferiorly. A pale, eyeless species (Carinella linearis), 

 from Lochmaddy, shows a similar arrangement, for the inner longitudinal layer in transverse section 

 is narrowed towards the centre of the dorsum, with traces of a separation by interfascicular sub- 

 stance. The fasciculi of the longitudinal muscular coat in this species and C. lineformis are 

 arranged in a linear manner, the rows passing from without inwards. 



b. Body-wall in the Cephalothricidae. — The dermal tissues of Cephalothrios are exceedingly 

 transparent, the pigment, when present, being only developed in the snout in front of the ganglia 

 as a rose-pink or reddish shading within the superficial portion of the cuticular layer. The 

 action of the cilia is most vigorous in the cephalic region. The cutis (d, Plate XXI, fig. 2), com- 

 posed of the usual granular cells and gelatinous matter in areolae, has on its inner margin a 

 trace of a translucent homogeneous basement-layer. A very thin band of circular fibres (e f ) comes 

 next, the exact structure of which is best demonstrated in fresh animals, after the addition of a 

 little dilute acetic acid. The fibres are also evident in fine longitudinal sections, but are not satisfac- 

 torily seen in transverse cuts on account of their tenuity. Beneath is a very powerful longitudinal 

 muscular coat (e"), the ends of the fibres having the usual fasciculated appearance, — the inner 

 being somewhat coarser than the outer. At each side a distinct increase occurs at the region of 

 the nerve, where the layer is separated into two portions by a septum of fibres from the circular 

 coat, the nerve lying in the line of demarcation. This arrangement is quite characteristic, and 

 the position of the nerve-trunk probably points to the compound nature of the great longitudinal 

 layer, viz., as analogous to the two longitudinal layers in Lineus, the circular muscular fibres 

 cutting off only the lateral portions (e), instead of dividing it completely. This genus shows the 

 mobility of the race even in a greater degree than the others. In progression the long yielding 

 snout is used as an exploratory or boring organ, which it stretches hither and thither with ceaseless 

 energy, and by its aid pushes aside its own mobile body in any direction ; while through a narrow 

 loop of mucus the latter is drawn like a thread of semi-fluid, yet coherent substance. The animal 

 also moves readily on the surface of the water. When tested with blue litmus-paper the skin 

 gives a most vivid red stain. 



Delle Chiaje mentions only two muscular coats in his Folia sipunculus, an outer of circular 

 muscular fibres, and an inner of longitudinal. He does not notice the external longitudinal layer. 



