The proctodæum is formed considerably later than the stomodæum. It is a comparatively shallow involution, which forms the rectum of the adult. It is dilated at its extremity, and two Malpighian vessels early grow out from it.
The mesenteron is formed in the interior of the yolk. Its walls are derived from the cellular elements of the yolk, and the first section to be formed is the hinder extremity, which appears as a short tube ending blindly behind in contact with the proctodæum, and open to the yolk in front. The later history of the mesenteron has not been followed, but it undoubtedly includes the whole of the abdominal section of the alimentary canal of the adult, except the rectum, and probably also the thoracic section. The later history of the yolk which encloses the mesenteron has not been satisfactorily studied, though it no doubt gives rise to the hepatic tubes, and probably also to the thoracic diverticula of the alimentary tract.
The general history of the alimentary tract in Scorpio is much the same as in Spiders. The hypoblast, the origin of which as mentioned above is somewhat uncertain, first appears on the ventral side and gradually spreads so as to envelop the yolk, and form the wall of the mesenteron, from which the liver is formed as a pair of lateral outgrowths. The proctodæum and stomodæum are both short, especially the former (vide [fig. 207]).
Summary and general conclusions.
The embryonic forms of Scorpio and Spiders are very similar, but in spite of the general similarity of Chelifer to Scorpio, the embryo of the former differs far more from that of Scorpio than the latter does from Spiders. This peculiarity is probably to be explained by the early period at which Chelifer is hatched; and though a more thorough investigation of this interesting form is much to be desired, it does not seem probable that its larva is a primitive type.
The larvæ of the Acarina with their peculiar ecdyses are to be regarded as much modified larval forms. It is not however easy to assign a meaning to the hexapodous stage through which they generally pass.
With reference to the segments and appendages, some interesting points are brought out by the embryological study of these forms.
The maximum number of segments is present in the Scorpion, in which nineteen segments (not including the præ-oral lobes, but including the telson) are developed. Of these the first twelve segments have traces of appendages, but the appendages of the six last of these (unless the pecten is an appendage) atrophy. In Spiders there are indications in the embryo of sixteen segments and in all the Arachnida, except the Acarina, at the least four segments bear appendages in the embryo which are without them in the adult. The morphological bearings of this fact are obvious.
It deserves to be noted that, in both Scorpio and the Spider, the cheliceræ are borne in the embryo by the first post-oral segment, and provided with a distinct ganglion, so that they cannot correspond (as they are usually supposed to do) with the antennæ of Insects, which are always developed on the præ-oral lobes, and never supplied by an independent ganglion.
The cheliceræ would seem probably to correspond with the mandibles of Insects, and the antennæ to be absent. In favour of this view is the fact that the embryonic ganglion of the mandibles of Insects is stated (cf. Lepidoptera, Hatschek, p. [340]) to become, like the ganglion of the cheliceræ, converted into part of the œsophageal commissure.