The Appendages, Anatomy, and Relationships of Trilobites

by Percy E. Raymond

If the spines were protective, it would not be surprising if some of them, hollow and open at the top, were poisonous also, and had glands at the base. These are, however, purely matters of speculation so far.

Renal excretory organs.—Nothing has been seen of any such organs, unless the genal cæca may possibly be of that nature. The main trunks of these always lead to the sides of the anterior glabellar lobe, which is not the point of attachment of either antennæ or biramous limbs, so that there seems little chance that they will bear this interpretation.

Reproductive organs.—Nothing is yet positively known about the reproductive organs or the position of their external openings. If the "exites" of Neolenus could be interpreted as brood-pouches, which does not seem probable, then the genital openings were located near the base of some pair of anterior thoracic appendages.

The Panderian Organs: Internal Gills or Poison Glands?

At a meeting of the Mineralogical Society at St. Petersburg, Volborth (1857) announced that Doctor Pander had two years before discovered certain organs on the lower side of the doublure of the pleural lobes of the thorax of a specimen of Asaphus expansus. These organs were oval openings in the doublure, one near the posterior margin of the cephalon, and one on each thoracic segment of the half-specimen figured by Volborth in 1863. They were explained by Volborth and by Eichwald (1860, 1863) as the points of attachment of appendages. Billings (1870) described and figured the "Panderian organs" of "Asaphus platycephalus" and stated that he had seen them in Asaphus [Ogygites] canadensis and A. megistos [Isotelus maximus] as well. He thought some sort of organ was attached to them, but could not suggest its function. Woodward (1870) thought that the openings were "only the fulcral points on which the pleuræ move." Their position outside the fulcra shows that this explanation is impossible.

So far as I am aware, the Panderian organs have been seen only in the Asaphidæ. Barrande figured them in "Ogygia" [Hemigyraspis] desiderata (1872) and Schmidt in two species of Pseudasaphus. They seem to occupy the same position in Bohemian, Russian, and American specimens. There is always one pair of openings on each thoracic segment, and one pair in line with them on the posterior margin of the cephalon. They occur near the anterior margin of the segment, and near the inner end of the doublure. In some cases they are surrounded by a ventrally projecting rim, while in others they have a thin edge. There seem to be no markings on the interior of the shell which are connected with them.

While thinking over the trilobites in connection with the origin of insects, it occurred to me that these hitherto unexplained Panderian organs might possibly be openings to internal gills and that the Asaphidæ might have been tending toward an amphibious existence. On mentioning this to Doctor R. V. Chamberlin of the Museum of Comparative Zoology, he called my attention to the possibility that they might be openings similar to those of the repugnatorial glands of Diplopoda. While no definite decision as to the function can be made, the explanation offered by Doctor Chamberlain seems more plausible than my own, and has suggested still a third, namely, that they might be the openings of poison glands.

If one were to argue that these apertures are really connected with respiration, it might be pointed out that they are ventral in position, while the foramina repugnatoria are always dorsal or lateral, even in diplopods with broad lateral expansions. If offensive secretions were poured out beneath a concave shell like that of a trilobite, they would be so confined as to be but slightly effective against an enemy. This would indicate that if these openings were the outlets of glands, the substance secreted might be a poison used to render prey helpless. On the other hand, openings to gills are normally ventral in position, and if the pleural lobes were folded down against the body, they would be brought very close to the bases of the legs.

A further curious circumstance is that so far no traces of exopodites have been found on Isotelus. The endopodites of both Isotelus latus and I. maximus are fairly well preserved in the single known specimen of each, yet no authentic traces of exopodites have been found with them. Moreover, Walcott sliced specimens of Isotelus from Trenton Falls and found only endopodites. It may also be recalled that the finding of the specimen of Isotelus arenicola at Britannia and the tracks which I attributed to it, suggested to me that it was a shore-loving animal (1910). It offers a field for further inquiry, whether the Asaphidæ may not have had internal gills, and whether some primitive member of the family may not have given rise to tracheate arthropods.