40 THE EARLY STAGES OF TABANED^ 



is no more easily visible in the final form. This h3T>othesis is, accord- 

 ing to Paoli, the only one which explains satisfactorily the manner 

 in which this strange organ is formed, and it is sufiSciently supported 

 by observation. In fact, the whole organ is covered by the hypo- 

 derm, the latter not being very different from that which covers the 

 body integument internally. 



In a preparation of a whole larva which had molted recently 

 Graber's organ was made up of five pairs of black pedunculate bodies, 

 and one pair, anteriorly located, had the pedunculate bodies already 

 formed but entirely colorless; this capsule was evidently recently 

 formed and its pedunculate bodies were not yet completely chitinized 

 and fully colored. 



The further fact, that the number of pairs of pedunculate bodies 

 increases with the age of the larva, clearly demonstrates that they 

 are formed successively, and inasmuch as we have to deal with an 

 organ dependent on the integument, these successive formations must 

 have something to do with the molts as modifications of the integu- 

 ment. All the cells and nuclei described by Graber as surrounding 

 the cyst and the tube consequently are merely cells and nuclei of the 

 hypoderm which surrounds the whole organ. 



Paoli discusses further the muscles and ligaments observed by 

 Graber. It is found that the muscles as well as the nerves are at- 

 tached to the anterior capsule, which alone forms the living and 

 functioning part of the organ, while the remaining part is to be con- 

 sidered dead, having functioned in previous stages of development. 



While Graber described only one pair of muscles, Paoli observed 

 four pairs and stated that probably Graber's second pair of nerves 

 corresponds to the second pair of muscles. ,Two large anterior 

 muscles are attached to the anterior part of the capsule with a large 

 surface of insertion extending from the dorsal posterior side of the 

 capsule upwards almost to contact with one another. These muscles 

 are those seen by Graber who assumed that they were attached 

 with the other end in the fifth abdominal segment, the seventh as 

 he erroneously assumed. According to PaoH, they are shorter and 

 inserted in the sulcus between the sixth and seventh abdominal 

 segments; they diverge anteriorly passing across the two large tracheal 

 trunks. By the contraction of these muscles the organ is displaced 

 towards the head. 



