Gatty Marine Laboratory, St. Andrews. 21 



still continues. The gut in the middle of the body is capable 

 of great dilatation, and there is a slight separation of the 

 dorsal longitudinal fibres in the mid-dorsal line, but the 

 fasciculi are similar to those in front, and the muscles are 

 broader — that is, stretch further downward. On the other 

 hand, the ventral muscles are more compact, and the hypo- 

 derm in the mid-ventral arc has diminished and shows a 

 furrow (" copragogue ") in the centre, and the sides project 

 a little. The area of the nerve-cords in section is smaller, 

 and the neural canals are proportionally larger. The same 

 alternation of the muscular arches and tunnels with the free 

 mesentery and its vessel occurs, but the ventral longitudinal 

 muscles are thicker, their transverse diameter less, and 

 their inner ends rise much above the nerve-cords, though 

 these ends are thinner than the outer in section. The dorsal 

 longitudinal muscles have attained great preponderance in 

 bulk. In this region muscular fibres pass downward by the 

 side of the gut and from the inner border of the lower mass 

 of the dorsal longitudinal muscles, and cause, by passing 

 through the fasciculi "of the ventral sheet, a differentiation 

 into an inner and outer belt at intervals. 



Posteriorly the chief changes are the diminution and 

 flattening of the body-wall, the great lateral expansion of 

 the dorsal longitudinal muscles, so that each has a clavate 

 outline in section, and a median hiatus, to which the 

 mesentery goes, is present. The ventral muscles have pro- 

 portionally increased in bulk and each is also clavate in 

 section, the broad end being exterior, but they do not pro- 

 ject above the great nerve-cords as in the middle region of 

 the body. One of the most evident changes is the appear- 

 ance of vertical bands of muscles which connect the dorsal 

 with the ventral longitudinal muscles on each side of the 

 alimentary canal, and they penetrate the fasciculi in both 

 to the basement-membrane. The nerves and the neural 

 canals are likewise diminished. Toward the tip of the tail 

 an increase in the hypoderm takes place all round, the 

 shrunken muscles rendering this more conspicuous, the 

 dorsal longitudinal thinning off in the middle line much 

 more than the ventral, so that the gut occupies the dorsal 

 arch, whilst a thick mass of hypoderm occurs ventrally. 



The muscles of the spines and bristles follow the same 

 plan throughout, forming a fan- like or radiating series in 

 each case. 



Bristles. — When the setigerous process in the middle of 

 the body is cut at right angles to its long axis two groups 

 of bristles are found, a more compact series arranged in a 



