CHIRONOMUS PRASINUS. 73 



their optical sections on either side give at first sight the impres- 

 sion of a row of cells, but on more careful examination their true 

 character is revealed. They are, I believe, simple, fusiform, 

 nucleated muscle cells, each extending for a considerable distance 

 round the intestine. 



It is said * that the muscular tunic of the alimentary canal of 

 the Arthropoda presents an exception to the general rule, in that 

 its fibres are of the striped variety. If these fibres are striated, 

 the striation is very faint indeed ; so much so, that I doubt 

 whether it be striated,t and the more so as it contrasts strongly 

 with a cincture of undoubtedly striated fibres which surrounds the 

 rectum, whose elements resemble in all respects those of the 

 sub-cuticular muscles. There is a distinct cell membrane envelop- 

 ing the contractile substance, which forms the contour of the 

 cell-like optical sections shown in my drawing, in which, here and 

 there, a nucleus may be distinguished occupying the centre of the 

 section, which is therefore evidently placed in the midst of the 

 contractile substance and not on the surface. In certain places 

 the pointed ends of the fibres may be seen, but I have not been 

 able to trace the whole extent of any single fibre. Each of them 

 however, extends, I think, at least half round the canal ; at one 

 part of the small intestine this muscular layer is double. 



At times I have, I think, detected evidence of a fifth mem- 

 branous investment to the canal surrounding the muscular coat, 

 and as this observation exactly tallies with one of Newport's, I will 

 quote what he says upon the subject J : — " The peritoneal coat or 

 layer is an exceedingly transparent white, shining, and delicate 

 membrane, and is observed only with great difiiculty. It covers 

 the outer surface of the muscular coat throughout the whole 

 course of the canal. It is seen most distinctly extending along 

 the sides of the canal, directly across the angles formed by the 

 contraction of some part of the muscular coat, where this is 

 thrown into folds or depressions." Whether or not this membrane 

 is correctly described as peritoneal, will depend upon the view 

 that is taken of the homologies of the ccelom or body cavity in 



* Ranvier, Lecons d'Anatomie Generale, Paris, 1880, page 462. 



"t Since writing this, I have satisfied myself that these fibres are striated, 



+ Todd's Cyclopoedia of Anatomy. Art. Insecta, page 99. 



