No.1.] MIDGUT OF THE TERRESTRIAL JSOPODS. 89 
It would seem, then, that the fibres were partly chitinous and 
partly of a substance similar to that which forms the basement 
membrane. This double structure will not seem anomalous 
when we consider that the epithelial cells manufacture on their 
two surfaces substances which are chemically distinct; on their 
outer surface the basement membrane, and on their inner sur- 
face the layer of chitin. That the basement membrane is formed 
by the epithelial cells and not by the mesoderm is indicated, it 
seems to me, on the one hand by the perfect continuity of the 
basement membrane and by the manner in which it moulds 
itself over the bases of the cells and frequently is thickened at 
the point of contact of two or more cells, and on the other hand 
by the absence of a definite layer of mesodermal cells on the 
outer surface of the gut.1 We need not be surprised, then, to 
find that the fibres, in those portions of their extent which trav- 
erse the regions of the cytoplasm which have manufactured 
the basement membrane, are of the same material as that struc- 
ture, while those portions of them which traverse the chi- 
tinogenous regions of the cytoplasm are chitinous in character. 
The occurrence, however, of what, for convenience, may be 
termed supporting fibres traversing the cytoplasm is somewhat 
unusual, chitin and the basement membrane substance being 
usually found in layers resting upon or underlying epithelia. A 
certain peculiarity of structure found in the Crustacea is, how- 
ever, of some interest in this connection, namely, the insertion 
of the muscle tendons into the exoskeleton. This is accom- 
plished by means of tendons which, according to the observa- 
tions of Reichenbach ('86), have an ectodermal origin in Astacus. 
It will be conceded that the peculiar mode of insertion of the 
extrinsic muscles of the cesophagus and rectum of the Isopods, 
described by Ide (93), in all probability has a similar origin. 
1] have in another place (7Zext-book of Invertebrate Morphology, New York, 
1894) insisted upon the homology of the basement membrane, which exists between 
the ectoderm and mesoderm of the Turbellaria, and the mesogloea of the Ccelen- 
tera, which, as is well known, is primarily a formation of the epithelial cells, — 
the mesoglceal cells, when they occur, only secondarily migrating into it. In the 
Isopod “ midgut ” we have to deal with an arrangement similar to that found in 
the Turbellaria, since the gut is of ectodermal origin. The occurrence of coelomic 
cells in the thickenings of the basement membrane of the Isopod “ midgut ” has 
additional interest in this connection. 
