288 THE APOD I D^ PART n 



and in their general organisation, but in the posses- 

 sion of a sternal plate or entosternitc. The same 

 explanation given in this essay of this sternal plate 

 in Apus and Limulus must be applied here. It is due 

 to a massing together of the ventral longitudinal 

 muscle bands of a certain number of anterior seg- 

 ments, so that their muscular elements disappear, 

 leaving the sinewy elements for the attachment of 

 transverse muscles. In the primitive Crustacea, 

 the longitudinal muscles of these segments were 

 rendered useless by the bending of the body. In the 

 Arachnida, however, they were rendered useless by 

 the axial fusing of the segments ; while the muscular 

 elements degenerated, the sinewy elements were 

 retained to form the entosternite. This seems to 

 show that the Arachnida were differentiated from 

 the Tracheatan-Annelid at a stage when the 

 Annelidan segments were still of the typical form, 

 i.e. before the ventral longitudinal muscle bands had 

 become specialised in adaptation to new modes of 

 life. 



In conclusion, it may be interesting to see how 

 Peripatus compares with Apus as a transition form. 

 The Annelidan characteristics of Peripatus are 

 certainly more visible than^ are those of Apus, where 

 they are all more or less disguised or transformed. 

 On the other hand, Peripatus is a development by 

 itself, and can hardly be shown to have given rise to 

 any group of the Tracheata. It is indirectly a most 

 remarkable transition form, having preserved so many 

 characteristics of the common racial Tracheatan- 



