243 



true parenchym, describing also the interstitial tissue of the body epi- 

 thelium, the intracapsular tissue of the central nervous system, and, 

 very briefly, the coelomic endothelium. Having compared the various 

 connective tissue elements found in G. lacteus, and having controlled 

 these observations by studies of the same elements in Cannella, Linens, 

 Ampliiporus, Stichostemma and Tetrastemma, I propose the following 

 classification of these elements found in this species of Cerebratulus : 



I. Parenchym tissue, s. str. This consists of rounded, polygonal 

 cells, with thick cell walls, and a round or oval, deeply-staining, 

 peripherally situated nucleus, imbedded in a small mass of cytoplasm, 

 from Avhich a few fibres pass into the unstaining cell-fluid. A layer 

 of these cells surrounds each blood-vessel, — except the cephalic vas- 

 cular lacunae, they are also found externally from the proboscis sheath, 

 and along the lateral surfaces of the intestinal coeca. This tissue is 

 characterized especially by the presence of cell walls, and the ab- 

 sence of intercellular fibres or substances. 



II. Connective tissue with intercellular substance. This forms the 

 cutis, the interstitial layers in the ext. longitudinal, and the circular 

 muscle layers of the body wall, the external and internal neurilemma, 

 the intramuscular layers of the proboscis and its sheath, and the layers 

 beneath the endothelia of the blood-vessels, proboscis, and rhyncho- 

 coelom. It consists of multipolar, branched cells, with oval or spindle 

 shaped (sometimes »geschwänzt«, cf. Kürger, 1890) nuclei, which 

 stain deeply; between the cells and their branching fibres, a nearly 

 homogenous substance is found, which stains faintly with haematoxylin. 



III. Mesenchym tissue , s. str. This consists of usually bipolar, 

 but frequently multipolar, flattened cells, whose long, branching fibres 

 anastomose together, and with those of neighbouring cells ; the small 

 spherical or oval nucleus, surrounded by a mass of cytoplasm, stains 

 deeply. Between these cells, which are without membranes, there is 

 no intercellular substance. They are found in the coelom (perivisceral 

 cavity), passing from the body-wall to the bloodvessels, proboscis 

 sheath, and intestine ; they are especially numerous on the dorsal side 

 of the oesophagus, and are not found externally from the int. longi- 

 tudinal muscle layer of the body wall. Between the cells and their 

 fibres is situated the unstaining, structureless coelomic fluid, in which 

 floating cells occur, the latter being detached mesenchym cells. The 

 cells also produce a single- layered, though not continuous pseudo- 

 epithelium on the inner surface of the int. long, muscle layer, and 

 around the intestine, thus bounding the coelom. The latter layer 

 corresponds with Salensky's (1. c.j splanchnopleura ; but the outer 

 cannot be homologized with his somatopleura, since it does not enve- 



15* 



