THE THORACIC EXO-SKELETON. 157 



Audouin's ideal segment in its simplest form represents the 

 type of a wing - bearing somite, but if we seek the more 

 primitive condition, it must be amongst terrestrial forms in 

 which the skeletal ring in each somite consists of a dorsal 

 and ventral arch, each developed from two lateral halves. 



Macleay [75] supposed that each thoracic segment consists 

 of four united sub-segmental annuli, but, so far as I can see, 

 this view is entirely unsupported by facts, and the whole 

 evidence of development is adverse to it. 



Another, and perhaps more tenable, view is held by Patten 

 [48] . He regards each segment as the result of the fusion of 

 two primary somites or metameres. This hypothesis origi- 

 nated from the fact that the ventral lateral appendages of the 

 somites are frequently bifurcate, or consist of an exo- and an 

 endopodite, in the more generalised Arthropoda, a character 

 which persists in the maxillae of insects, and, as I have 

 already observed, and shall show hereafter, also in the thoracic 

 legs of the fly at an early period of development. 



In support of his hypothesis, Patten [48] makes the following 

 statements : 



1. ' In Scolopendra each neuromere, or pair of segmental 

 ventral ganglia, has four pairs of nerves, two probably motor 

 and two sensory ; and in all arthropods, the neuromeres of 

 which have been carefully studied, each exhibits two transverse 

 commissures. 



2. * In Acilius each segment has two pairs of tracheal open- 

 ings, spiracles, one pair easily seen near its anterior margin, 

 and one pair very rudimentary and difficult to recognise near 

 its posterior edge. 



3. ' The double character of the segments of Julus is 

 evidenced according to Heathcote by the duplication in each 

 segment of the cardiac ostia, arteries, neuromeres, tracheae, 

 and legs ; whilst in Scorpio the neuromeres present a distinctly 

 double character.' 



Whilst admitting the apparent validity of Patten's arguments, 

 I would observe, however, that his hypothesis is not supported 

 by developmental evidence, so far as the Insecta are concerned. 





