He attributes this remarkable resemblance to a physiological necessity that similarity of function necessitates similarity of structure, for he considers it out of the question to suppose any near relationship between arthropods and vertebrates.
Truly an interesting remark, with the one fallacy that relationship is out of the question.
The evidence so far has consistently pointed to some member of the palæostracan group as the ancestor of the vertebrates—a group which had affinities both to the crustaceans and the arachnids; indeed, many of its members resembled scorpions much more than they resemble crustaceans. The olfactory organs of the scorpions and their allies are, therefore, more likely than any others to give a clue to the position of the desired olfactory organs. In these animals and their allies paired olfactory antennæ are not present, either in the living land-forms or the extinct sea-scorpions, for all the antennæ-like, frequently chelate, appendages seen in Pterygotus, etc. (Fig. [8]), represent the cheliceræ, and correspond, therefore, to the second pair of antennæ in the crustaceans.
What, then, represents the olfactory antennæ in the scorpions? The answer to this question has been given by Croneberg, and very striking it is. The two olfactory antennæ of the crustacean have combined together to form a hollow tube at the base of which the mouth of the animal is situated, so that the food passes along this olfactory passage before it reaches the mouth. This organ is often called after Latreille, the camerostome, sometimes the rostrum; it is naturally median in position and appears, therefore, to be an unpaired organ; its paired character is, of course, evident enough, for it is innervated by a pair of nerves, and these nerves, as ought to be the case, arise from the supra-œsophageal ganglia. In Galeodes it is a conspicuously paired antennæ-like organ (Fig. [94]).
Croneberg has also shown that this rostrum, or camerostome, arises embryologically as a pair of appendages similar to the other appendages. This last observation of Croneberg has been confirmed by Brauer in 1894, who describes the origin of the upper lip, as he calls it, in very similar terms, without, however, referring to Croneberg's paper. Croneberg further shows that this foremost pair of antennæ not only forms the so-called upper lip or camerostome, but also a lower lip, for from the basal part of the camerostome there projects on each side of the pharynx a dependent accessory portion, which in some cases fuses in the middle line, and forms, as it were, a lower lip. The entosclerite belonging to this dependent portion is apparently the post-oral entosclerite of Lankester and Miss Beck.
Fig. 94.—Dorsal View of Brain and Camerostome of Galeodes.
cam., camerostome; pr. ent., pre-oral entosclerite; l.l., dependent portion of camerostome; ph., pharynx; al., alimentary canal; n. op., median optic nerves; pl., plastron; v.c., ventral nerve chain; 2, 3, second and third appendages.
At the base of the tubular passage formed by this modified first pair of antennæ the true mouth is found opening directly into the dilated pharynx, the muscles of which enable the act of suction to be carried out. The narrow œsophagus leads out from the pharynx and is completely surrounded by the supra- and infra-œsophageal nerve masses.
Huxley also describes the mouth of the scorpion in precisely the same position (cf. o, Fig. [96]).