214 Prof. M'Intosli's Xotesfrom the 



ending at the junction of the ventral transverse band, and, 

 lastly, of the median (venti'al) longitudinal fibres {vm.) 

 beneath the latter (PL YIII. fig. 7). The nerve-cords at 

 this part are separated by fully a sixth of the circumference 

 of the body-wall. The transverse septal plate above the 

 ventral longitudinal muscles has a central structureless part 

 — apparently of a homogeneous nature (pale and elastic), 

 the muscular fasciculi fraying out especially at the dorsal 

 surface and ends (PI. VIII. fig. 7 a). In its progress back- 

 ward a change in tlie diminishing area between the nerve- 

 trunks is inaugurated, the homogeneous central region of 

 the transverse band, the anterior part of which is indicated 

 in PI. VIII. fig. 7, being shortened transversely and increased 

 vertically, so that it pushes as a lozenge-shaped and then 

 ■wedge-shaped area into the centre of the ventral muscular 

 mass, whilst the upper muscular fibres externally become 

 defined as more or less independent masses, bounded exter- 

 nally by sloping muscular fibres which simulate the oblique 

 in certain sections — at least, at their insertion. Tiie lozenge- 

 shaped area of the homogeneous (for it can scarcely be called 

 " tendinous ") tissue thins off on each side to a plate, to 

 the upper edge of which fasciculi of muscular fibres are 

 attached, whilst ventrally processes pass into the median 

 ventral longitudinal muscles. Tlie whole thus forms a com- 

 plex muscuiur apparatus attached to the central tough tissue, 

 which gradually in its progress backward shrinks, leaving 

 the fused muscular fasciculi to form the massive ventral 

 longitudinal muscles as shown in PI. VIII. fig. 8, vm. By 

 the gradual diminution of the tough central area of the 

 before-mentioned transverse band to which the median 

 mesentery from the gut is attached, and by the grouping of 

 the several longitudinal ventral muscles into a mass on each 

 side, the ty[)ical ventral longitudinal muscles are formed, 

 and at this part they exceed in bulk the dorsal muscles. 

 This evolution of these continuous ventral fasciculi out of 

 the elements in front is probably connc(!t(;d with a change 

 in the function of the contents as well as in the body-wall 

 itself. 



The disappearance of the longitudinal muscular fibres and 

 the radiating strands from the dorsal wall of the gut leaves 

 the two halves of the upper division of the coelomic space for 

 ccelomic fluid only, and it is separated from the two much 

 larger spaces iuferiorly by strong muscular bands at each 

 side of the transversely enlarged alimentary canal ; yet the 

 appearance of the canal beneath — to which the median 

 mesentery is attached ventrally — apparently leaves a gap by 



