We have, therefore, two distinct possible positions for the genital ducts among the group of extinct scorpion-like animals, the one from the cephalic region to the operculum, and the other from the abdominal region to the operculum.
{ 209}THE GENERATIVE GLANDS OF LIMULUS AND ITS ALLIES.
The whole argument, so far, has in every case ended with the conclusion that the original scorpion-like form with which I have been comparing Ammocoetes resembled in many respects Limulus rather than the present-day scorpions, and therefore in the case also of the generative organs, with which the thyroid gland or palæo-hysteron was in connection, it is more probable that they were cephalic in position rather than abdominal. If this were so, then the duct on each side, starting from the median ventral uterus, would take a lateral and dorsal course to reach the huge mass of generative gland lying within the prosomatic carapace, just as I have represented in the figure of Eurypterus (Fig. 79), a course which would take much the same direction as the ciliated groove in Ammocoetes.
We ought, therefore, on this supposition, to expect to find the remains of the invertebrate generative tissue, the ducts of which terminated in the thyroid, in the head-region, and not in the abdomen.
Upon removal of the prosomatic carapace of Limulus, a large brownish glandular-looking mass is seen, in which, if it happens to be a female, masses of ova are very conspicuous. This mass is composed of two separate glands, the generative glands and the hepatico-pancreatic glands--the so-called liver--and surrounds closely the central nervous system and the alimentary canal. From the generative glands proceed the genital ducts to terminate on the posterior surface of the operculum. From the liver ducts pass to the pyloric end of the cephalic stomach, and carry the fluid by means of which the food is digested, for, in all these animals, the active digesting juices are formed in the so-called liver, and not in the cells of the stomach or intestine.
It is a very striking fact that the brain of Ammocoetes is much too small for the brain-case, and that the space between brain and brain-case is filled up with a very peculiar glandular-looking tissue, which is found in Ammocoetes and not elsewhere. Further, it is also striking that in the brain of Ammocoetes there should still exist the remains of a tube extending from the IVth ventricle to the surface at the _conus post-commissuralis_, which can actually be traced right into this tissue on the outside of the brain (see Fig. 13, _a-e_, Pl. XXVI., in my paper in the _Quarterly Journal of Microscopical Science_). { 210}This, in my opinion, is the last remnant of one of the old liver-ducts which extended from the original stomach and intestine into the cephalic liver-mass. This glandular-looking material is shown surrounding the pineal eye and its nerve, in Fig. 31, also in Fig. 22, and separately in Fig. 92. It is composed of large cells, with a badly staining nucleus, closely packed together with lines of pigment here and there between the cells; this pigment is especially congregated at the spot where the so-called liver-duct loses itself in this tissue. The protoplasm in these large cells does not stain well, and with osmic acid gives no sign of fat, so that Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue is not true; peculiar it certainly is, but fatty it is not.