The evolution of the sternal plastron and its impact on internal anatomy
The presence of a broad sternal plastron seems to be a prerequisite for the compaction and anterior shift of the cephalothoracic ganglion brought about by the disappearance of the barriers formed by the endosternites. The cephalothoracic ganglion in Aegla, Kiwa, Lomis and thus in the ground pattern of Australopoda is similarly compact as in Galatheoidea, but located more anteriorly than in Galatheoidea (Keiler et al., 2015a). Its compactness sets it apart from the more elongated form found in other decapods such as homarid lobster or axiid mud shrimp (see Bouvier, 1889). In lobsters, crayfish and mud shrimps, in which a broad plastron is not present, the endophragmal skeleton forms a narrow scaffold (see e.g., Secretan, 1998) restricting the leg nerves and thus exerting a constraint on the form of the cephalothoracic ganglion. The leg arteries in the ground pattern of Australopoda emanated separately and at roughly equal distances from each other, similarly to the situation in Kiwa (Fig. 5E) and galatheoid squat lobsters (see Keiler et al., 2015a). The fusion of the posterior leg arteries only appeared in the lineage leading towards Aegla. The position of and distances between the roots of leg arteries p1a-p4a correlate roughly with the position of the cephalothoracic ganglion and the degree of compaction of the neuropils (Fig. 5E). Though hemolymph supply to the cephalothoracic ganglion is affected by the ascending arteries which run between the roots of the leg arteries, the ascending arteries and the position of leg arteries p1a-p4a seem, at least to some degree, to be structurally dependent from, i.e. coherent with, each other. In other words, if the cephalothoracic ganglion and thus the ascending arteries became shifted anteriorly along the ventral vessel, the roots of leg arteries p1a-p4a became also shifted anteriorly. Or alternatively, the anterior shift in the joint between the ventral vessel and the descending artery (which is constrained by the need to puncture the cephalothoracic ganglion) possibly necessitated an anterior shift in the leg artery roots to ensure sufficient hemolymph supply to the legs by shortening the distance the hemolymph is required to flow.