184 
F. M. BALFOUR. 
of the abdomen. In the first place they send ofi* 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 ( 50 ). 
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. XXI, fig. 20 c?, 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 stomodseum, 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 stage. The two other muscles are 
lateral, one on each side (PI. XX, 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. XXI, fig. 22 ht). In its lumen are numerous 
blood-corpuscles (not represented in my figure). The heart 
