266 BAUR, ON CHITINE. 



hollow of the new one. The new tendon, however, 

 remains hollow only for a short time; and when the old 

 one has been pulled out, its walls gradually close in upon 

 one another, and it soon puts on the appearance of a solid 

 body. 



The integuments of the crayfish consist, as Hackel has 

 correctly shown, of an outer layer of chitine and an inner 

 skin composed of connective tissue. These two, however, arc 

 never continuous with one another, but are always separated 

 by a number of cells, or rather, perhaps, of nuclei, which form 

 a single layer, and are specially evident at the time of moult- 

 ing. The new layer of chitine is formed between the old one 

 and this layer of cells, and it is therefore evidently produced 

 by them. The connective tissue, which is of variable thick- 

 ness, has therefore no chitinogenous function, and serves only 

 as a substratum for the true chitinogenous layer. At the 

 time of moulting another layer, that is to say the new chitine, 

 is added to the three layers always present ; this is equally 

 true for the tendon as for the mandible, but with this differ- 

 ence, that the latter being a projection, and the former an 

 inversion of the skin, the order of sequence of the layers is 

 reversed. This structure is not confined to the main stem 

 of the tendon, but is repeated in its branches ; each one of 

 these presents the same arrangement, but the outer con- 

 nective tissue increases in size at the expense of the chitine, 

 which finally disappears where the muscle commences, so that 

 the sheath of the muscle is formed by the expansion of the 

 layer of connective tissue only. 



Hackel denies that the pore-canals, which are so character- 

 istic of true chitine, exist in chitinous tendons. Certainly in 

 a longitudinal section no trace of them can be perceived ; but 

 if the tendon is cut transversely, besides the laminar structure 

 a radial shading may be perceived, like that which is seen in 

 sections made at other parts of the chitine skeleton, and 

 which is referred to the presence of pores. Here, however, 

 evidently no pores are present ; and without wishing to deny 

 that they do occur in other parts, it may at least be asked 

 whether the appearance presented has not been, in this 

 respect, misunderstood. The cell-impressions are also absent 

 in the chitine of tendons, as indeed in some other parts of 

 the skeleton, but certainly these two differences are not 

 sufficient to prove that the tissue in question is not true 

 chitine. 



The principal conclusions to which M. Banr arrives are 

 that the so-called chitine tendons are inwardly projecting, ori- 

 ginally tubular, subsequently solid, and more or less branched 



