MUSEUM OF COMPAEATIVE ZOOLOGY. y 



cuticula, or whether they are confined to its surface, is difficult to say. 

 The production of the cuticula is such a uniform process that one would 

 naturally expect to find that the marking extended through it, for the 

 successive layers would be similarly marked, and thus bands would be 

 established extending from its deep to its superficial face. Concerning 

 the vertical extension of the bands between the facets there is no ques- 

 tion, for in transverse sections of the cuticula (Fig. 1, x) they reappear 

 in their proper positions, and extend from one surface to the other. 

 Owing to the roughness of the cut face, they are much less readily de- 

 tected in sections than when viewed from the outer surface of the cutic- 

 ula (compare Figs. 1 and 2). The diagonal band and its central spot 

 have not been observed in transverse sections, even when the plane of 

 section is in the most advantageous position for demonstrating these 

 structures. Notwithstanding their apparent absence, both may be pres- 

 ent, although indiscernible. For even in the superficial view, when the 

 outline of the facet was so readily visible, the diagonal band was only 

 faintly seen. In transverse sections, where the distinct boundary of the 

 facet is visible with difficulty, one should not expect to see the much 

 fainter diagonal. On comparing the diagonal band and the boundary 

 of the fiicet by focusing through the corneal cuticula, I was unable to 

 distinguish a greater vertical extension in the one than in the other. 

 Since it has been shown that the boundary^ of the facet extends through 

 the cuticula, this observation supports the conclusion that the diagonal 

 band also extends through it. 



Patten ('86, pp. 626, 627) has described in the facet of Penseus a 

 band which has many resemblances to the diagonal band in the lobster. 

 It is not diagonal, however, but transverse, and divides the square facet 

 into two equal rectangles, in which the sides are in the proportion of 

 one to two. I have already given my reason for believing that the 

 diagonal band in the coniea of the lobster extends through the sub- 

 stance of the cuticula. Patten states that the transverse band in 

 Penaeus is only a superficial structure, and says ('86, p. 627) that in 

 cleaning the cuticula " when the treatment with caustic potash has been 

 carried to excess, all markings disappear except the contours of the 

 facets." I have subjected the corneal cuticula of a lobster to a boiling 

 solution of potassic hydrate (75%) for a quarter of an hour, and, al- 

 though the potash completely cleaned the cuticula, the outlines of the 

 facets, the diagonal band, and- its spot were as readily visible after this 

 treatment as before. A second and third trial with the same piece of 

 cuticula did not noticeably eff"ect the markings. In this respect, then, 



