Ou the connective tissues and body cavities of the Neinerteans. 23 



In a single individual (out of 12 examined), about 15 mm in 

 length, I find in the posterior body region a noticeable body cavity 

 between the intestine and the body muscular wall (Fig. 25 B.C). 

 The cells found in this cavity are probably to be relegated to the 

 category of the connective tissue with intercellular substance, but re- 

 semble true mesenchym cells in: 1) their nuclei frec^ueutly showing 

 amitotic division stages ; and more especially 2) in the comparative 

 reduction of the intercellular substance, so that the fluid of the body 

 cavity surrounds them (Fig. 26). These cells are thus intermediate 

 (structurally) between the mesenchym and the tissue with intercellular 

 substance. In this tissue, more abundant above than below the 

 lateral blood vessels, are occasional large nuclei, which are about 

 three times the size of the average cells ; each such nucleus is en- 

 veloped in a more or less spherical mass of cytoplasm, and as a rule, 

 presents various mitotic arrangements of the chromatin, spirem stages 

 being especially abundant (Fig. 28). In following a continuous series 

 of sections, a few successive sections may contain 2 or 3 of these 

 cells apiece, and are followed by an equal number of sections where 

 they are absent ; this fact would indicate a metameric arrangement of 

 the cells in question. Now since the gonads are later also arranged 

 metamerically, and in the same position as these cells, we must re- 

 gard the latter as the primitive genital cells; and this conclusion 

 seems further justified, by the fact of the occurrence of mitotic 

 division stages in them — only amitotic stages occurring in the 

 connective tissue cells. In fact, in two other individuals, where a 

 few posterior gonads are present, showing early ovogenetic stages, 

 cells occur, in all respects similar to those just described. 



Thus in Lineus gesserensis the sexual cells are derivatives of the 

 connective tissue with intercellular substance, or of a tissue inter- 

 mediate between the latter and the true mesenchym tissue. 



4) The parenchym tissue is apparently absent in the head 

 and oesophageal regions. Behind the oesophagus it bounds the lateral 



around the posterior intestine, which is thickest ventrally, and ap- 

 parently absent dorsally. This layer consists of: 1) an inner, single 

 layer of circular fibres, directly beneath the basement membrane of 

 the intestine; and 2) an outer layer of longitudinal muscle fibres, 3 

 to 5 deep ventrally; these fine layers also envelop the intestinal di- 

 verticula. Neither Bükgeb ('90), nor any other author, has seen the 

 longitudinal muscle layer, nor found muscle fibres around the 

 diverticula. 



