THE ATTACHMENT OF MUSCLES TO THE EXOSKELE- 



TON IN THE CRAYFISH, AND THE STRUCTURE 



OF THE CRAYFISH EPIDERM 



HAL DOWNEY 



From the Histological Laboratory, Department of Animal Iiiology, University of 



Minnesota 



FIVE FIGURES 



According to Leydig, Janet, Hecht and Holmgren the striated 

 muscles of Arthropods may be inserted without an intervening 

 tendon. In such cases the muscle fibrils are continuous with the 

 fibrous spongy network of the epithelial cells (Leydig), or the 

 muscle is attached directly to the chitinous exoskeleton, the hypo- 

 dermal epithelium being absent at the point of attachment (Janet, 

 Hecht, Holmgren). The usual mode of attachment, however, 

 is by means of an intervening tendon composed of straight fibrils 

 whose length is equal to the thickness of the liypodermal layer 

 within which they are located. The tendon fibrils are so numer- 

 ous that little remains of the original hypodermal cells but a 

 more or less distorted nucleus surrounded by a very small quan- 

 tity of cytoplasm. Without ontogenetic studies it is, of course, 

 almost impossible to determine whether these fibrils are within 

 the hypodermal cells or between them. The former view is the 

 one accepted by most authors, but Frenzel, Nicholas, Ide, Pantel 

 and Hecht claim that the tendon fibrils are between the hypoder- 

 mal cells. This is undoubtedly true in such cases as Holmgren 

 describes for some of the Diptera, where the tendon is divided 

 into several fine bundles of fibrils which pass between or spin 

 around large epithelial cells having definite cell boundaries. 

 However, this condition must be very exceptional, as most authors 

 describe the hypodermis in the region of muscle insertion as being 



381 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO. 4 

 SEPTEMBER, 1<> 12 



