458 W. s, NICKERSON, 



In character these libres are the same as the luugitudiual muscles, 

 but they are less numerous (Fig. 9). 



The determination of the histological structure of these muscles 

 is not easy. The ditlerent cellular elements, as seen in sections of 

 prepared material, are so intimately united as to make the correct- 

 ness of any analysis of the tissue which is not confirmed by sub- 

 sequent study of maceration i)reparations somewhat open to question. 



Concerning the outer, circular muscle fibres I am unaljle to give 

 any satisfactory explanation. I have not been able to find nuclei 

 either in these fibres or connected with them. Somewhat more satis- 

 factory results were obtained in studying the longitudinal and oljlique 

 fibres. 



Just beneath the zone of muscles and among the cortical cells of 

 the body parenchyma occur certain nuclei which may be recognized 

 as slightly smaller than those of the ordinary parenchymatous cells. 

 In especially favorable conditions such nuclei may occasionally be seen 

 lying embedded in a homogeneous mass of substance that is in close 

 contact with a muscle fibre, as shown in PI. 30, Fig. 11. The homo- 

 geneous mass has the form of a low rounded elevation on one side 

 of the fibre not far from the middle of its length, and I interpret it 

 as the undifferentiated remnant of the muscle cell, and hence regard 

 these smaller nuclei as probably belonging to the muscle fibres of the 

 longitudinal and diagonal sets. Whether certain of them are connected 

 with the outer, circular fibres may be open to question, but I have 

 seen no indication of such a relation. 



In one preparation the longitudinal fibres present the appearance 

 shown in PI. 30, Fig. 16; each fibre appears to be a tube with nodes 

 of more deeply staining substance filling the lumen at intervals. The 

 exact significance of this condition is somewhat doubtful. It seems 

 probable that the tube is formed by what was the more contractile 

 outer part of the fibre, which was ditterentiated from the less hyaline 

 interior portion. The tubular appearance may very i)robably be due 

 to imperfect preservation, which has allowed the breaking down of 

 the axial part of the fibre. 



Besides the muscles of the body-wall there are scattered paren- 

 chymatous muscle fil)res, many of which extend from the intestine to 

 the i)eriphery of tlie body. These are nearly radial in direction and 

 distributed rather uuiformely through all jjurts of a cross section and 

 in all regions of the body. At tlie anterior end of the body, however, 

 they are more largely develoi)i'd and there constitute the i)rotractor 



