In the formation of the alimentary tract, again, the differences between the Crustacea and Tracheata are equally marked, and the Arachnida agree with the Tracheata. There is generally in Crustacea an invagination, which gives rise to the mesenteron. In Tracheata this never occurs. The proctodæum is usually formed in Crustacea before or, at any rate, not later than the stomodæum[474]. 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 organs are not found in the Crustacea.
With reference to other points in my investigations, the evidence which I have got that the cheliceræ are true postoral appendages supplied in the embryo from a distinct postoral ganglion, confirms the conclusions of most previous investigators, and shews that these appendages are equivalent to the mandibles, or possibly the first pair of maxillæ of other Tracheata. The invagination, which I have found, of part of a groove of epiblast in the formation of the supra-œsophageal ganglia is of interest, owing to the wide extension of a similar occurrence amongst the Tracheata.
The wide divarication of the ventral nerve cords in the embryo renders it easy to prove that there is no median invagination of epiblast between them, and supports Kleinenberg's observations on Lumbricus as to the absence of this invagination. I have further satisfied myself as to the absence of such an invagination in Peripatus. It is probable that Hatschek and other observers who have followed him are mistaken in affirming the existence of such an invagination in either the Chætopoda or the Arthropoda.
The observations recorded in this paper on the yolk cells and their derivations are, on the whole, in close harmony with the observations of Dohrn, Bobretzky, and Graber, on Insects. They shew, however, that the first formed mesoblastic plate does not give rise to the whole of the mesoblast, but that during the whole of embryonic life the mesoblast continues to receive accessions of cells derived from the cells of the yolk.
Araneina.
1. Balbiani, “Mémoire sur le Développement des Araneides,” Ann. Sci. Nat., series v, Vol. XVII. 1873.
2. J. Barrois, “Recherches s. l. Développement des Araignées,” Journal de l'Anat. et de la Physiol., 1878.
3. E. Claparède, Recherches s. l'Evolution des Araignées, Utrecht, 1860.
4. Herold, De Generatione Araniorum in Ovo, Marburg, 1824.