100 F. M. BALFOUR. 



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 

 separated compartments. There appears, however, to be 

 left a free axial space for the alimentary tract, the meso- 

 blastic 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 

 mammillse. Between these two bands are placed the nervous 

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

 PI. X, fig. 20 d, which cuts the abdomen horizontally and 

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

 nervous bands within them at ah. 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 

 in the thorax [vide fig. 20 c). There are, in addition to these, 

 intrinsic transverse fibres on the ventral side of the thorax. 

 Besides these muscles there are in the thorax, attached to 

 the suctorial extremity of the stomodaeum, three powerful 

 muscles, which I believe to be derived from the somatic me- 

 soblast. One of these passes vertically down from the dorsal 

 surface, in the septum the commencement of which was 

 described in the last stao^e. The two other muscles are 

 lateral, one on each side (PI. IX, fig. 20 c). 



The heart has now, in most respects, reached its full 

 development. It is formed of an outer muscular layer, within 

 which is a doubly- contoured lining, containing nuclei at 

 intervals, which is probably of the nature of an epithelioid 

 lining (PI. X, fig. 22 ht). In its lumen are numerous 

 blood-corpuscles (not represented in my figure). The heart 



