MUSEUM OF COMPARATIVE ZOOLOGY. 149 



III. Histology. 



1. Body Wall. 



In the body wall may be found the following layers, beginning with 

 the surface : (1) a cuticula, (2) a hypodermis, (3) a cutis, (4) the mus- 

 cular system, covered internally by (5) a delicate peritoneal membrane. 



a. Cuticula and Hypodermis. 



The cuticula consists of a substance optically like chitin, but differing 

 from this, as has often been pointed out, in being soluble in boiling 

 KOH. It is further aberrant in the absence of cellulose, which has 

 been shown by Ambronn ('90) to be characteristic of true chitin. Tests 

 with chloriodide of zinc showed neither any trace of blue nor the sub- 

 sequent pleochroismus described by that author for true chitin. This 

 layer is undoubtedly the product of the underlying hypodermal cells, 

 which are everywhere found in a single layer, and normally display a 

 sac-like form, although, as mentioned by Vogt und Yung ('88, p. 383), 

 they may by contraction or compression of the body wall be drawn out 

 into the form of spindles. This has given rise, as they mention, to the 

 erroneous interpretation of such groups of elongated cells as being 

 sensory organs. In contradistinction to these authors, I do not find 

 the proximal ends of these cells ordinarily continuous with fibres which 

 extend to the muscular layer, and cannot agree with them in regarding 

 the entire mass external to the muscles as one layer. For if one exam- 

 ines a transverse section of the body wall as seen in Figure 5, the major- 

 ity of the hypodermal cells are seen to be clearly marked off from the 

 underlying tissue by the cell wall. The fibres of this subjacent tissue, to 

 be described later, often extend up to the bases of the hypodermal cells ; 

 but close examination in favorable 'regions shows the connection to be 

 merely apparent. Often when these cells are crowded and distorted by 

 near-lying glands, one is inclined to believe in an actual continuity of 

 cell and fibre which cannot be demonstrated, and which, so far as I could 

 find, is not present in less confused regions. 



Lying partly in the hypoderm, but mostly below it, are the dermal 

 bodies (Hautkbrper), which are of three sorts. A description of these 

 will be given in the account of the cutis, with which they are most 

 closely associated. No further specialized cells of any kind were found, 

 neither sensory cells nor peculiar nerve endings of any sort, and I am 

 inclined to regard the claims of their presence as founded upon the ex- 



