490 W. H. Coe — A7iato'}ny of Cerebratulus lacteus. 



it, through the outer longitudinal layer, above and below, and unite 

 with the thin, outer circular layer (cm') just internal to the outer 

 endothelium. Dorsally, these fibres leave the inner circular layer to 

 the right and to the left of the middle line and pass outwards in such 

 a manner that the fibres from the right side cross those from the left, 

 in the midst of the outer longitudinal layer, in the middle line. Yen- 

 trally, the fibres from each side leave the circular layer in the 

 median line. These crosses (co,co') are wanting only at the anterior 

 and posterior ends of the proboscis. The fibres making such crosses 

 do not occur in a continuous sheet but pass out in bundles between 

 the bundles of longitudinal muscles. 



For a considerable distance back from the anterior end, the internal 

 longitudinal layer {ln%) is much more massive than the external, but 

 the muscle-bundles are much more widely separated by the connec- 

 tive tissue framework and gelatinous tissue. Towards the middle 

 region of the proboscis this layer becomes much reduced, and near 

 the posterior extremity it disappears altogether. The inner circular 

 layer (cm) continues nearly the whole length of the proboscis with- 

 out noticeable change of character. The external longitudinal\2ij^x 

 (Im') maintains nearly the same thickness throughout the entire 

 length of the proboscis and continues back to the very extremity, 

 after both the other principal layers have disappeared. The 

 external circular layer [cm') is made up of a few fibres lying imme- 

 diately beneath the outer endothelium. 



The proboscis is innervated by means of a pair of nerves (Plate 

 XII, fig. 1 1, pn) which arise from the ventral brain-lobes and 

 pass with the muscles into the proboscis, where they at first occupy a 

 lateral position (similar to that of the lateral cords of the body) in 

 the midst of the muscle-bundles. Farther back they expand dorsally 

 and ventraliy, inside the forming circular muscular layer, until each 

 meets the other above and below to form a cylindrical nerve-i^lexus 

 (fig. 12, 0120) analogous to that in the body-wall. Towards the pos- 

 terior end this layer becomes much reduced and at length disappears. 



The outer epithelium, or endothelium, (en) rests upon a thin base- 

 ment membrane and is made up of a single layer of flattened endo- 

 thelial cells quite similar to those which clothe the proboscis-sheath 

 (fig. 7). Each cell rests upon the basement membrane below, with its 

 free surface bathed in the rhynchocoelomic fluid. As in the endothe- 

 lial cells of the proboscis-sheath, the oell-boundaries are not to be 

 readily distinguished. The anterior end of the proboscis forms a 

 circular line of attachment to the cephalic tissues. This attachment 



