STEUCTUKE OF EAETSWOEM ALLIED TO NEMEETODRILUS. 577 



cells ; these cells had the appearance rather of peritoneal cells 

 than of invaginated epidermic cells. 



The sac, where it lay beneath the nerve-cord, had a flattened 

 appearance, as if it had been squeezed between the nerve-cord 

 and the body-wall ; its cavity was chiefly developed in a hori- 

 zontal, not in a vertical, direction. It could be traced round 

 the nerve-cord for a short way on each side as a narrowish 

 tube running forwards and backwards in close contact to the 

 thinnish septum which divides the 13th from the 12th seg- 

 ment ; coming into close relation with the base of the ovary, 

 it ended in the neighbourhood of this organ. 



Traced in the other direction the sac penetrated the longi- 

 tudinal muscular coat for a short distance, and gave ofi' a 

 narrow tube, ending blindly some way below the circular mus- 

 cular layer : the space between the ends of this tube and the 

 epidermis was crowded with nuclei, but there was no break 

 that I could find in the circular layer of muscles, and no in- 

 vagination of the epiblast. The sac is thus entirely closed ; 

 it has no communication with the exterior, which might have 

 been expected. 



The opposite extremity of the sac does not form a closed 

 sac, as indicated in the figure ; it appears to do so in some 

 sections, such as the one illustrated in my drawing ; a complete 

 series of sections shows that this sac communicates freely 

 with the body-cavity. One wall is formed by the 

 septum which separates Segments 13/14, lettered spt ; 

 the opposite wall is thinner, and consists of a mem- 

 brane, which connects that septum with the one 

 behind ; the membrane in question is, however, not attached 

 to the intersegmental Septum 13/14 for the whole of its extent ; 

 it thus forms an aperture which leads into the subneural 

 pouch. 



In specimens rather more immature than the one just 

 described, the relations of the subneural pouch will be under- 

 stood by a reference to figs. 4 — 6, which represent three sec- 

 tions of a continuous series; those selected for illustration 

 represent the critical points in the series ; the first one (fig. 4) 



VOL. XXXII, PART IV. — NEW SEE. Q Q 



