Mr Sinclair^ Note on abnormal pair of appendages, etc. 235 



Note on the abnorrtial pair of appendages in Lithobius. 

 By F. G. Sinclair, M.A., Trinity College. 



[Received 3 June 1909.] 



The presence of an abnormal extra pair of appendages is of 

 interest to me, as I have examined a great number of abnormalities 

 in the Mj'riapoda, but I am inclined to give a different inter- 

 pretation to the facts from that given in the note referred to by 

 Doncaster*, in which the appendage is taken to be a reduplication 

 of the poison claw. 



The abnormalities which are very frequent in Myriapods fall, 

 in all cases that are familiar to me, into two classes. A re- 

 duplication in the transverse axis of the body or appendage, or a 

 reduplication in the longitudinal axis of the body or appendage. 

 The former is the more frequent — an example being a reduplication 

 of part of the antenna or leg forming a bifurcation. 



The description of the small processes described as teeth, 

 jointed to the base of the appendage and the drawing of them, 

 remind me strongly of two small processes which are commonly 

 present on the end of the larval appendages in Myriapods. They 

 are moveable by muscular fibres, and, as the animal grows up, one 

 of them usually degenerates and disappears while the other changes 

 to a claw or spike. 



In the reduplication of a part in the longitudinal axis I should 

 think it more likely that the abnormal part would be posterior to 

 that of which it was a copy, following the order in which the 

 normal segments are formed. 



It results from these considerations that I take the abnormal 

 appendage described in the note to be a reduplication, not of the 

 poison claw, but of the second maxilla. The basal joint with the two 

 small processes I believe to be an imperfectly formed transverse 

 reduplication of the other branch, so that if the development 

 had gone a step further there would have been two similar 

 branches on either side. 



According to my view the undivided plate at the base would 

 be — not the coalesced basal joints of the poison claws, but a 

 metamorphosed sternal plate. 



Of course if a poison gland and duct had been found present 

 it would have upset this view, but that does not seem to have 

 been the case. It is worth remembering that in the Myriapoda 

 portions of proliferous tissue from which complete segments are 

 formed normally are present till comparatively late in life. The 

 complete number of segments in Diplopoda is not present for 

 a long period. 



* Proe. Camb. Phil. Soc. xv. p. 178. 



