NOTES ON THE DEVELOPMENT OF THE ARANEINA. 687 



lished are transversely directed channels passing from the heart 

 outwards. They probably constitute the venous spaces, and 

 perhaps also contain the transverse aortic branches. 



In the intervals between these venous spaces the somatic and 

 splanchnic layers of mesoblast are in contact with each other. 



I have not been able to work out satisfactorily the later 

 stages of development of the septa, but I have found that 

 they play an important part in the subsequent development 

 of the abdomen. In the first place they send off lateral off- 

 shoots, which unite the various septa together, and divide up 

 the cavity of the abdomen into a number of partially sepa- 

 rated compartments. There appears, however, to be left a 

 free axial space for the alimentary tract, the mesoblastic walls 

 of which are, I believe, formed from the septa. 



At the present stage the splanchnic mesoblast, apart from 

 the septa, is a delicate membrane of flattened cells (fig. 22, sp}. 

 The somatic mesoblast is thicker, and is formed of scattered 

 cells (so). 



The somatic layer is in part converted, in the posterior 

 region of the abdomen, into a delicate layer of longitudinal 

 muscles, the fibres of which are not continuous for the whole 

 length of the body, but are interrupted at the lines of junc- 

 tion of the successive segments. They are not present in the 

 anterior part of the abdomen. The longitudinal direction of 

 these fibres, and their division with myotomes, is interesting, 

 since both these characters, which are preserved in Scorpions, 

 are lost in the abdomen of the adult Spider. 



The original mesoblastic somites have undergone quite as 

 important changes as the dorsal mesoblast. In the abdominal 

 region the somatic layer constitutes two powerful bands of 

 longitudinal muscles, inserted anteriorly at the root of the 

 fourth ambulatory appendage, and posteriorly at the spinning 

 mammillae. Between these two bands are placed the nervous 

 bands. The relation of these parts are shewn in the section 

 in PL 32, fig. 20 d, which cuts the abdomen horizontally and 

 longitudinally. The mesoblastic bands are seen at m., and the 

 nervous bands within them at ab. g. In the thoracic region 

 the part of the somatic layer in each limb is converted into 

 muscles, which are continued into dorsal and ventral muscles 



