In Crustacea the mesoblast usually originates from the walls of the invagination, which gives rise to the mesenteron.
It does not become divided into two distinct bands, but forms a layer of scattered cells between the epiblast and hypoblast, and does not usually break up into somites; and though somites are stated in some cases to be found they do not resemble those in the Tracheata.
The proctodæum is usually formed in Crustacea before and rarely later[216] than the stomodæum. The reverse is true for the Tracheata. In Crustacea the proctodæum and stomodæum, especially the former, are very long, and usually give rise to the greater part of the alimentary tract, while the mesenteron is usually short.
In the Tracheata the mesenteron is always considerable, and the proctodæum is always short. The derivation of the Malpighian bodies from the proctodæum is common to most Tracheata. Such diverticula of the proctodæum are not found in Crustacea.
[210] The nature of the inner membrane is obscure. It is believed by Packard to be moulted after the formation of the limbs, and to be equivalent to the amnion of Insects, while by Dohrn it is regarded as a product of the follicle cells.
[211] Dohrn finds at first only five appendages, but thinks that the sixth (the anterior one) may have been present but invisible.
[212] Dohrn believes that he has succeeded in shewing that the first pair of appendages of Limulus is innervated in the embryo from the supra-œsophageal ganglia. His observations do not appear to me conclusive, and arguing from what we know of the development of the Arachnida, the innervation of these appendages in the adult can be of no morphological importance.
[213] The biflagellate antennæ of Pauropus amongst the Myriapods can hardly be considered as constituting an exception to this rule.
[214] I hope to shew this in a paper I am preparing on the anatomy of Peripatus.
[215] Stecker’s description of an invagination in the Chilognatha cannot be accepted without further confirmation; vide p. [388].