Gatty Marine Laboratory ^ St. Andrews. 2-45 



wliicli the two cavities communicate superiorly under the 

 transversely enlarged canal. 



The anterior end of the stomach is a narrow tube as seen 

 in PI. XI. fig. 23, and in the various transverse sections. It 

 further presents a bifid border ventral ly, a narrow process of 

 the cavity ending in a dilated rim on each side below, the 

 ventral blood-vessel and the mesentery occupying the gap, 

 whilst a spacious sinus surrounds the stomach. This bitid 

 condition gradually disappears, the organ assuming the 

 outline shown in PI. X. fig. 30. 



A mesentery with the dorsal blood-vessel in the centre 

 passes from the upper arch of the gut to the dorsal Avail, 

 and another mesentery, with the ventral l)lood-vessel, goes 

 to the mid-ventral tissues, the coeloui being thus divided 

 into halves. Then a process from the wall of the stomach 

 above the rugose and somewhat triangular ventral arch 

 appears, and a little behind is tacked to the ventral portion, 

 and thus cuts it oft" as a separate canal with folded mucous 

 membrane internally, the longer upper chamber having its 

 inner surface smooth and symmetrically folded. The in- 

 ferior and somewhat pear-shaped chamber (PI. VIII, ficr^ 8, 

 St., stomach) is surrounded by l)lood-vessels, which form a 

 vascular plexus around it on their wjiy to the branchial 

 region, and from its apex interiorly a mesentery passes 

 to join a mid-ventral homogeneous (pale tough) area arching 

 •over a special muscular region which terminates on each 

 side over the outer edge of the nerve-cord, now approaching 

 that of the opposite side. 



At this level the body-wall has thin hypoderm in the 

 mid-dorsal line, then it increases in depth laterally, again 

 becomes thinner, and then swells out ventrally at the nerve- 

 cords. Within are the basement-tissue and circular fibres, 

 then the dorsal longitudinal muscles {dm.), which end below 

 the attachment of the upper canal on each side, and the 

 ventral longitudinal (v7n.), which are more massive, and 

 have the differentiated region with the arched fibres in the 

 middle line, such, indeed, forming the only separation 

 between them. This differentiated region is prol)ably in 

 connection with the movements of the alimentary canal. 

 The nerve-cords in section show a granular and fibrillar 

 aspect, and they are much better differentiated than in front. 

 The occurrence of bristle-tufts makes the separation between 

 the dorsal and the ventral longitudinal muscles more pro- 

 nounced, and below the tuft is a well-defined pore of the 

 mucous gland with large nuclei in its cellular wall (PI. VIII. 

 fig. 9, mp.), one side abutting on the hypoderm, the other 



