The Origin of Vertebrates

by Walter Holbrook Gaskell

It is, however, to be remembered that the operculum of Scorpio covers only the terminal genital apparatus, and does not, therefore, resemble the operculum of the presumed ancestor of Ammocœtes, which, as already argued, must have resembled the operculum of Thelyphonus with its conjoint branchial and genital apparatus, rather than that of Scorpio. Before, therefore, making too sure of the insuperable character of this difficulty, we must examine the uterus of the Pedipalpi, and see the nature of its opening.

The nature of the terminal genital organs in Thelyphonus has been described to some extent by Blanchard, and more recently by Tarnani. The ducts of the generative organs terminate, according to the latter observer, in the large uterus, which is found both in the male and female; he describes the walls of the uterus in the female as formed of elongated glandular epithelium, with a strongly-developed porous, chitinized intima. In the male, he says that the epithelium of the uterus masculinus and its processes is extraordinarily elongated, the chitin covering being thick. In these animals, then, the common chamber or uterus into which the genital ducts empty, which, like the corresponding chamber in the scorpion, occupies the middle region of the operculum, is a large and conspicuous organ. Further, and this is a most striking fact, the uterus masculinus does not open direct to the exterior, but into the genital cavity, "which lies above the uterus, so that the latter is situated between the lower wall of the genital cavity and the outer integument." The opening, therefore, of the uterus is not external but internal, into the large internal space known as the genital cavity. The arrangement is shown in Fig. 91, taken from Tarnani's paper, which represents a diagrammatic sagittal section through the exit of the male genital duct. Yet another most striking fact is described by Tarnani. This genital cavity is continuous with the pulmonary or gill cavities on each side, so that instead of a single opening for the genital products and one on each side for each gill-pouch, as would be the case if the arrangement was of the same kind as in the scorpion, there is a single large chamber, the genital chamber, common to both respiratory and genital organs.

Fig. 91.—Sagittal Median Diagrammatic Section through the Operculum of the Male Thelyphonus. (From Tarnani.)

The thick line is the operculum, composed of two segments, I. and II. Ut. Masc., uterus masculinus; Gen. Ch., genital chamber; Int. Op., internal opening; Ext. Op., external opening common to the genital and respiratory organs.

This genital chamber, according to Tarnani, opens to the exterior by a single median opening between the operculum and the succeeding segment; similarly, a communication from side to side exists between the second pair of gill-pouches. I have been able to examine Hypoctonus formosus and Thelyphonus caudatus, and in both cases, in both male and female, the opening to the exterior of the common chamber for respiration and for the genital products was not a single opening, as described by Tarnani in Thelyphonus asperatus, but on each side of the middle line, a round orifice closed by a lid, like the nest of the trapdoor spider, led into the common genital chamber (Gen. Ch.) into which both uterus and gills opened. In Fig. [77] I have endeavoured to represent the arrangement of the genital and respiratory organs in the male Thelyphonus according to Tarnani's and my own observations.

If we may take Thelyphonus as a sample of the arrangement in those scorpions in which the operculum was fused with the first branchial appendage, among which must be included the old sea-scorpions, then it is most significant that their uterus should open internally into a cavity which was continuous with the respiratory cavity. Thus not only the structure of the gland, but also the arrangement of the internal opening into the respiratory, or, as it became later, the pharyngeal cavity, is in accordance with the suggestion that the thyroid of Ammocœtes represents the uterus of the extinct Eurypterus-like ancestor.

Into this uterus the products of the generative organs were poured by means of the vasa deferentia, so that there was not a single median opening or duct in connection with it, but also two side openings, the terminations of the vasa deferentia. These are described by Tarnani in Thelyphonus as opening into the two horns of the uterus, which thus shows its bilateral character, although the body of the organ is median and single; these ducts then pass within the body of the animal, dorsal to the uterus, towards the testes or ovaries as the case may be, organs which are situated in these animals, as in other scorpions, in the abdomen, so that the direction of the ducts from the generative glands to the uterus is headwards. If, however, we examine the condition of affairs in Limulus, we find that the main mass of the generative material is cephalic, forming with the liver that dense glandular mass which is packed round the supra-œsophageal and prosomatic ganglia, and round the stomach and muscles of the head-region. From this cephalic region the duct passes out on each side at the junction of the prosomatic and mesosomatic carapace to open separately on the posterior surface of the operculum, near the middle line, as is indicated in Fig. [75].

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.

The Generative Glands of Limulus and its Allies.