Gatty Marine Laboratory, St. Andrews. 23 



on the sides of the dorsal furrows have the same structure 

 and all are modifications of the wall of the hody. 



Alimentary Canal. — The aperture of the mouth, fed by 

 the grooves from the branchial fan, besides those elsewhere 

 described, and with its dorsal transverse fissure and the two 

 lateral folds or lappets on each side below, soon assumes in 

 section the form of a transverse slit, the dorsal epithelial 

 wall of which is boldly scalloped or crenate, with two pro- 

 jections in the middle line, whilst the ventral is two-lobed — 

 two prominent lobes or projections occurring ou each side 

 of the central fissure. Then, passing backward, the canal 

 forms a long transverse or slightly fusiform slit, its epithelial 

 surface becoming at the same time less dense, whilst various 

 mesenterial strands are attached to its outer wall ; but soon 

 the epithelial lining diminishes in depth and the canal be- 

 comes more capacious — shorter in transverse and longer in 

 vertical diameter ; its walls increasing in thickness, and its 

 muscular and mesenterial strands more numerous. There- 

 after its inner lining is thrown into narrow longitudinal 

 ridges, and strong muscular fibres are attached to its outer 

 surface. The great increase of the mucous lining and the 

 diminution of the diameter of the canal cause the organ in 

 section to be ovoid or even rounded, the entire area being 

 occupied by the folds of the inner lining and the basement- 

 tissue — circular and radiating fibres externally giving firm- 

 ness to the rounded canal (PI. I. fig. 1, a 1 .). Then the 

 mucous folds change their character, and the inner lining 

 is thrown into slightly arborescent ridges in transverse 

 section, somewhat after the fashion of the gizzard of certain 

 Orthoptera, but it is not chitinous. Behind this, though 

 still in the anterior or " thoracic " region, the canal retains 

 the bold longitudinal ridges of the tuueous surface, though 

 they are less arborescent; the suspensory mesentery from 

 the mid-dorsal arch is short and strong, and the walls 

 of the gut are massive, since, besides the coats formerly 

 mentioned, a reticulated connective-tissue layer with vascular 

 spaces, as well as a chlorogogenous coat, surround it. 

 Besides, it is further clasped by powerful vertical bands 

 passing on each side from the dorsal longitudinal muscles to 

 the area of the nerve-cords (PI. II. fig. 8). The ventral 

 blood-vessel lies in the thick investment immediately beneath 

 it, and a complicated plexus of muscular and connective- 

 tissue fibres takes place beneath the canal and above the 

 ventral vessel in various sections at intervals. Posteriorly, 

 the canal considerably diminishes and its internal surface is 



