300 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



C. Annular ridges and Interannilar depressions 



The skin of the Enteropneusta is particularly characterised by the thickened 

 o-landular epidermal patches which are arranged in a more or less regularly zonary 

 manner so as to produce the appearance of raised glandular annulations separated 

 from one another by interannular non-glandular grooves. These annulations extend 

 from end to end of the trunk. 



In the hepatic region of the Ptychoderidae the apparently unimportant, but never- 

 theless ever-present, epidermal annular ridges are drawn into the service of the hepatic 

 diverticula of the gut, whose outer free edges they bound. The external liver-saccules 

 of the Ptychoderidae are, outwardly, nothing else than products of local hypertrophy 

 of the annular ridges, while the intervals between the successive saccules are the usual 

 non-glandular interannular tracts. 



The epidermal zonulation of the Enteropneusta is usually quite unjustly treated 

 as having no deep-lying significance at all. 



We have seen what can become of the annular ridges, but it is of more importance 

 for my present purpose to point out some of the potentialities of the interannular 

 grooves or tracts. 



The dermal pits of Spengelia are local intergonadial depressions of the interannular 

 tracts. 



In Sp. porosa the last gill-slits open at the base of the most anterior pits. The 

 succeeding pits, although they approach near to the wall of the gut, do not meet it. 

 If they did meet it they would probably fuse with it and form gill-slits. 



It would conceivably need but a comparatively slight functional stimulus to induce 

 either a pre-existing gut-sacculation to fuse with the epidermis or a pre-existing dermal 

 pit to fuse with the gut-wall. But when dermal depressions and gut-sacculations 

 coincide, then perforation is almost certain to follow sooner or later. It may readily 



Fig. 6. Diagram to illustrate the theory of the primary ixtergonadial position of the gill-clefts. 

 No insistence is placed upon the subdivision of the gonads into superposed follicles. They are thus repre- 

 sented in order to illustrate more clearly the principle of zonulation. 



be observed that the wall of the gut is thrown into transverse folds producing crests 

 and valleys. This is especially well seen in a large form like Pt. carnosa. The gut 

 of the Enteropneusta is intrinsically straight; at the same time it is necessary to have 

 as large a digestive surface as possible on account of the nature of its food. It 

 effects increase of surface by means of such transverse or circular or zonary plications 1 . 



1 Spengel points out that in Bal. koicaUv.ikii and Bal. kupfferi the intestine is considerably longer than 

 the body, and hence has a serpentine course (Scblangelung des Darmcanals). 



