ENDOTHELIUM OF THE EARTHWORM. 159 



peritonseum, described by Klein in this Journal (Vol. XII, 

 p. 43.) 



Claparede draws attention to the peculiar swollen character 

 which the cells of the peritoneal endothelium (or epithelium) of 

 the Earthworm usually present, and remarks that whilst it may 

 be disputed as to whether they should be considered strictly 

 as an epithelium or as belonging to that " globular-celled form 

 of connective tissue " made known in Mollusca by the researches 

 of Leydig — yet, as a matter of fact, the distinction is not im- 

 portant; for such globular-celled connective tissue actually 

 passes by gradual transition into a more flat-celled epithelium. 



At the present day no one would venture to draw a distinc- 

 tion in kind between the cells bounding the surfaces of con- 

 nective tissue-membranes and those more deeply placed. In 

 the mollusca (for instance in the Lamellibranch^s gill) we find 

 the swollen globular cells, which form so much of the connective 

 tissue in some parts, passing over into branched corpuscles form- 

 ing trabeculse about the blood-passages of the gill (the " lacunar 

 tissue" described by Peck, this Journal 1877, p. 45), and also 

 proliferating and giving rise to amceboid floating corpuscles. 

 So, too, the connective-tissue cells of the Earthworm present 

 us with a wide range of form, exhibiting in parts, particularly 

 on the intersegmental septa, the character of globular cells 

 (Schleimzellen of molluscan histology), and again where the 

 membrane is subject to tension, as on the surface of the en- 

 larged testicular sac becoming thin, flat, and in fact pavement- 

 like ; whilst yet again at other points it seems that these endo- 

 thelial cells proliferate and give rise to the amoebiform corpuscles 

 which float in the perivisceral fluid. 



The very remarkable and special characters assumed by this 

 layer of cells on the surface of the intestine and of the large 

 vessels (forming what has been called, on account of its bile-like 

 colour, the " hepatic '■' tunic), I am not now prepared to discuss, 

 but I may point out that the yellow granules which here so 

 deeply colour the cells are similar to pigment granules developed 

 in tracts of connective tissue in the Lamellibranchs (see Koll- 

 man, ** Bindesubstanz der Acephalen," * Archiv fur Mikr. 

 Anat.,' vol. xiii.) 



The observations of Claparede were made without the use 

 of silver nitrate ; it is possible without this reagent by means 

 of haematoxylin or carmine to bring into view the outlines of the 

 more swollen globular epithelial cells, but the flatter and more 

 delicate tracts of epithelium are missed. 



The most striking result of my observations is the very close 

 resemblance which appears to obtain between the epithelium 

 clothing the body-cavity of the Earthworm and that occupying 



