Gait]) Marine Laboratory, St. Andrews. 15 



The great cords after the disappearance of the eyes pass 

 downward with their cellular sheath to the sides of the 

 cesophao;iis (PI. 111. fig. 15), having beneath them only the 

 dense nui>;s of the ventral glandular hypoderm, the oesophagus 

 being surrounded by the tissues of the region before this 

 takes place, and, as those arouud the organ are chiefly 

 muscular, firm constriction of this part can readily occur, 

 the distinction between this region, imbedded as the gullet 

 is in firm contractile tissues (PI. II. figs. 8 & 9), and that 

 ■which follows — in which the canal is more or less free — is 

 therefore marked. Proceeding backward the oesophagus is 

 fixed by a median mesentery ventrally and by various strands 

 dorsally to a transverse siieet above it and the nerve-cords, a 

 space, divided into two by a median muscle, occurring above 

 — that is, below the dorsal longitudinal muscles (PI. II. fig. 8) . 

 The nerve-cords with their investment then pass below the 

 level of the alimentary canal and lie at some distance fi'oiu 

 each other at the inner border of the ventral longitudinal 

 muscles, the ventral blood-vessel being between them and 

 the massive ventral hypoderm externally. A small neural 

 canal is now visible at their upper and inner border, no trace 

 of this having been observed previously, as the great cords 

 lay at the sides of the gullet. Passing gradually downward 

 the cords are enclosed by fibres from the circular coat 

 crossing above and below them (PI. II. fig. 8), the small 

 neural canal,sometimes two, being visible — for in stance, at the 

 ganglia in the nerve-sheath at the upper and inner angle of 

 each. The nerve-cells are confined for the most part to the 

 exterior investment of the ganglia and the trunks, though 

 some are in the substance of both. The transverse (circular) 

 fibres above the cords increase in strength, and are further 

 stifl^'ened by the fusion of strong muscular fibres from the 

 sheath of the alimentary canal in the middle line. Other 

 fil)res pass outside the cords, and even between them in the 

 intervals between the ganglia, so that in this region they are 

 well supported and they are nearer each other than in 

 front. The transverse (circular) fibres above the cords 

 remain after the muscular band from the gut disappears and 

 a median mesentery takes its place, whilst the small neural 

 canal shows little change. Proceeding backward, the ventral 

 blood-vessel is surrounded by a thick ring of muscular and 

 connective-tissue fibres fixed ventrally between the neural 

 canals and beyond them. The neural canals are now con- 

 siderably larger, and the gut and the ventral ves^sel are 

 connected with the slender transverse fibres bv a thin 

 mesentery ; but this only lasts for a short distance, when the 

 thick investment of the trunk again appears in the progress 



