Gatty Marine Laboratory, St. Andrews. 19 



fibres which connected the inner ends of the vanishing 

 skeleton. Externally are circular fibres, which pass down- 

 ward to a firm connective-tissue area at each side of the 

 massive ventral hypoderm. This great muscular sheet is 

 most massive below, where it supports the origins of the 

 great nerve-trunks. At first no differentiation of the sheet 

 is observable; then pale connective-tissue fibres appear in 

 its middle opposite the upper end of the nerve-masses, and 

 in this an aperture appears, its cavity being surrounded by 

 stained granules, and now it is seen that there are two 

 longitudinal muscles, an upper and somewhat smaller 

 rounded muscle, which projects dorsally on each side of the 

 median groove, and a larger ovoid muscle at the outer side 

 of the nerve-trunk, the two being separated on each side by 

 an increasing coelomic area. The two dorsal muscles are 

 separated by a space, crossed by the circular fibres of the 

 body-wall, and others passing from the inner edge of the 

 muscle and from the six or more vertical bands from the 

 alimentary canal. The hypoderm covering the prominence 

 of these muscles dorsally is specially thickened. The second 

 or ventral pair of muscles are still lateral in position, have 

 the circular fibres, basement-tissue, and hypoderm externally, 

 the nerve-cords and neuroglia internally, and connective- 

 tissue bands and the hypoderm below. The dorsal muscles 

 remain more or less rounded in section (PL II. figs. 8 & 9, 

 dm.), but the ventral muscles become somewhat longer, 

 more oblique in position, and the nerve-cords now lie below 

 their inner edge inferiorly. Their elongation and obliquity 

 increase in the following sections, for they assume a spindle- 

 like outline, their limiting fibres fusing across the middle 

 line with each other and with those from the vertical bands 

 and those surrounding the gut, whilst the nerve-cords now 

 lie below this fibrous isthmus, with a small neural canal in 

 the neuroglia of their upper and inner border. The dorsal 

 muscles are still rounded or ovoid, separated by a consider- 

 able interval in the middle line and wholly dorsal in position, 

 but they by-and-by become pear-shaped in section, pointed 

 mid-dorsally, and thicker externally ; moreover, they slope 

 a little downward and laterally. The ventral muscles 

 stretch upward almost to the dorsal bristle-tuft, and are 

 thus longer than the former (PI. II. fig. 8, vm.) — indeed, 

 their mass exceeds that of the dorsal, a condition so different 

 from that in Pomatocerus. The dorsal muscles do not meet 

 in the middle line, though thinned like the ventral in 

 expansion of the body-cavity, and they are still less in bulk 



2* 



