446 SCIENCE PROGRESS. 



ventral and one on each side. These ridges divide the 

 circular outline of a section through the body of a Round 

 Worm into equal quadrants. Each ridge pushes between 

 the muscle-cells which line the inner surface of the rest of 

 the epidermal tissue and freely projects into the body 

 cavity (Fig. II. b). The dorsal and ventral ridges sur- 

 round the longitudinal nerves, the lateral ridges embrace 

 the excretory canals. 



There seems no reason to doubt that these ridgfes are 

 epidermal in structure. On each side of them lie the meso- 

 blastic musculo-epithelial cells, so characteristic of the 

 Nematodes (Fig. II. c), which, together with the above- 

 mentioned ridges, bound a spacious cavity — the so-called 

 body-cavity — on the outside. This cavity is traversed by 

 the alimentary canal, which for the greater part of its 

 length consists of a single layer of cells, there being ap- 

 parently, in these animals no mesoblastic perivisceral layer 

 such as is ordinarily found in the Tripoblastica or animals 

 with the three well developed embryonic layers. 



Thus in the Nematoda the body-cavity has very pecu- 

 liar and, in fact, unique relations. On the outer side it is 

 bounded for the most part by mesoblast, but at the four 

 axes by epiblast, and on the inner side it is limited by 

 hypoblast. The relationship of this space to the embryonic 

 layers is closely paralleled by that of the blood-containing 

 spaces of such Ccelomata as the Vertebrata, which many 

 years ago were shown, both on theoretical grounds l and 

 by actual observation on the developing Lamprey, 2 to be 

 primarily spaces between the epiblast and the hypoblast, 

 or in other words, to be part of the segmentation cavity. 

 This space in the higher forms subsequently becomes sur- 

 rounded by mesoblast and forms the cavity of the blood 

 vascular system. 



Thus it is obvious that the space between the body-wall 

 and the alimentary canal of Nematodes is not homologous 

 with the ccelom of the Chsetopoda, Mollusca, Arthropoda, 

 Vertebrata, etc. The fact that it is not is further supported 



1 Biitschli, Morphol. Jahrb., Bd. viii., 1883, p. 474. 



2 Shipley, Quart. Jour. Micr. Set;, vol. xxvii., 1887, p. 325. 



