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 junction 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 mammillæ. Between these two bands are placed the nervous bands. The relation of these parts are shewn in the section in Pl. 32, fig. 20d, 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 in the thorax (vide fig. 20c). 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 stomodæum, three powerful muscles, which I believe to be derived from the somatic mesoblast. 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 (Pl. 31, fig. 20c.).
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 (Pl. 32, fig. 22, ht). In its lumen are numerous blood-corpuscles (not represented in my figure). The heart lies in a space bound below by the splanchnic mesoblast, and to the sides by the somatic mesoblast. This space forms a kind of pericardium (fig. 22, pc), but dorsally the heart is in contact with the epiblast. The arterial trunks connected with it are fully established.
The nervous system has undergone very important changes.
In the abdominal region the ganglia of each side have fused together into a continuous cord (fig. 21, ab.g). In fig. 20, in which the abdomen is cut horizontally and longitudinally, there are seen the two abdominal cords (ab.g) united by two transverse commissures; and I believe that there are at this stage three or four transverse commissures at any rate, which remain as indications of the separate ganglia, from the coalescence of which the abdominal cords are formed. The two abdominal cords are parallel and in close contact.
In the thoracic region changes of not less importance have taken place. The ganglia are still distinct. The two cords formed of these ganglia are no longer widely separated in median line, but meet, in the usual way, in the ventral line. Transverse commissures have become established (fig. 20c) between the ganglia of the two sides. There is as little trace at this, as at the previous stages, of an ingrowth of epiblast, to form a median portion of the central nervous system. Such a median structure has been described by Hatschek for Lepidoptera, and he states that it gives rise to the transverse commissures between the ganglia. My observations shew that for the spider, at any rate, nothing of the kind is present.
As shewn in the longitudinal section (Pl. 32, fig. 21), the ganglion of the cheliceræ has now united with the supra-œsophageal ganglion. It forms, as is shewn in fig. 20b (ch.g.), a part of the œsophageal commissure, and there is no sub-œsophageal commissure uniting the ganglia of the cheliceræ, but the œsophageal ring is completed below by the ganglia of the pedipalpi (fig. 20c, pd.g.).
The supra-œsophageal ganglia have become completely separated from the epiblast.
I have unfortunately not studied their constitution in the adult, so that I cannot satisfactorily identify the parts which can be made out at this stage.
I distinguish, however, the following regions: