40 GUSTAF EISEN, ON THE ARCTIC OLIGOOH^ETA. 



and size. Sometiraes we meet with two forms in one and the same species and in 

 others we seldom find two cells of exactly the same form. 



In the following species I have not found any perigastric cells: A. Levinseni, 

 profugus, afjinis, nervosus, N. Stuxbergi and hyalinus. 



In M. primcevus and mirabilis the cells are of variable form and alike in both 

 species. In M. falciformis they are also irregular, but opaque and characteristic of 

 the species. 



Large, round cells, elliptical when seen from the side, and furnished with a large 

 dark nucleus are found in A. tenellus, lampas, ochraceus, g emmatus and fenestratus, all 

 closely related species, except the last one, which belongs to another subgenus. In 

 _ZV. durus there exist two very different kinds of cells. One kind is round and filled 

 with dark granry contents, the other is of irregular shape, exceedingly thin, and desti- 

 tute of any cell-contents or nucleus whatever. The latter kind is the most common 

 and occurs in enormous masses in many specimens. 



In Archienchytraius callosus the perigastric cells are not free, but form a compact 

 layer or mäss lining the interiör surface of the body walL The outlines and nuclei 

 of the cells are plainly visible, yet I did very rarely succeed in separating the cells 

 from one another. In the cavity they are never found free. 



Also in A. Levinseni there exist no free perigastric cells, or rather no perigastric 

 cells at all, and it is very remarkable that in this species we meet with peculiar layers 

 of an intercellular substance, lining the perigastric walls, as in the former species. But 

 no cellmembranes are visible, nor any nuclei, nor any thing else that would indicate 

 its true nature. Only the interiör surface of the raass is full of small hexagonal im- 

 pressions or hollows not unlike the cells in a honeycomb. The want of a larger num- 

 ber of specimens for dissecting, has prevented me from a closer study of said tissues, 

 but from what I have seen I judge that the more differentiated stratum is homolo- 

 gous with the perigastric cell-stratum of N~. callosus. 



With the views of Ratzel that the perigastric cells originate from the epithels of 

 the alimentary canal I can not agree, since I have been able to study their develop- 

 ment in A. Dicksoni. In said species we find, dispersed in the perigastric cavity of 

 the body, a grainy substance, in which are seen numerous perigastric cells in all stages 

 of development (fig. 7, h). The smallest cells are translucent and destitute of nuclei 

 or cell-contents. In others we find nuclei, and in the most highly cleveloped also grainy 

 cell-contents. Befrween the largest perfectly developed cells and the smallest ones with- 

 out either nucleus or cell-contents, we find any number of intermediate forms, of all 

 sizes and in all stages of development, and it seems to me evident that, at least in 

 this species the perigastric cells originate directly from the lymphatic or perigastric 

 fluidum of the body. 



Phylogenetie development of the genera and species of Enchytra^us. 



A few remarks upon the phylogenetic development of the subgenera of Eneliy- 

 trwux, their species and their different organs may not be without some interest, how 



