78 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Jaekel (1901, p. 168, fig. 28) has produced a very different restoration. His discus- 

 sion of this point seems so good, and has been so completely overlooked, that I will append 

 a slightly abridged version of a translation made some years ago for Professor Beecher. 

 The idea was, however, not original with Jaekel, as it was suggested by Bernard (1894, p. 

 417), but not worked out in detail. 



While considering the problem as to what organ could hav; lain beneath the glabella of the trilobite, and 

 while studying the organization of living Crustacea for the purpose of comparison, I found in the collections 

 of the Geological Institute preparations of Limulus which seemed to me to directly solve the entire question. 



From the mouth, which lies at about the middle of the head shield, the oesophagus bends forward, swells 

 out at the frontal margin of the animal at a sharp upward bend in order to take a straight course backward 

 after the formation of an enlarged stomach. Still within the head shield there branch out from each' side 

 of the canal two small vessels which pass over into the richly branched mass of liver lying under the broad 

 lateral parts of the head shield. After seeing this specimen, I no longer had the least doubt that the head 

 shield of the trilobites is to be interpreted in a similar manner. The position of the hypostoma and 

 gnathopods makes it necessary to assume that the position of the mouth of the trilobite lay pretty far back. 

 If, therefore, this depends upon the secondary ventral deflection of the oral region, as seems to be the case, 

 then it is a priori probable that the anterior part of the canal has also shared in this ventral inflection. 



The posterior part of the canal in the region of the segmented thorax and pygidium is comparatively 

 narrow, as shown long ago by Beyrich; he represents only a thin tube which shows no swellings whatever, 

 and such are usually missing in Arthropoda. 



As the glabella of most trilobites is regularly convex, there must lie beneath it an organ running from 

 front to back, which presses the bases of the cephalic legs away from each other and down from the dorsal 

 test. An organ so extensive and unpaired, running thus from front to back, can, among the Arthropoda, be 

 regarded only as an alimentary canal, for the swellings of the cephalic ganglia and the heart are by far too 

 small to produce such striking elevations on the front and upper surface of the glabella. The canal might 

 then have consisted of a gizzard belonging to the oesophagus, and a stomach proper or main digestive canal. 



. . . Among the trilobites there are two pairs of vessels on both sides of the glabella which have 

 precisely the same position with reference to the supposed course of the alimentary canal as the ducts of 

 the hepatic lobes in Limulus. One observes in numerous trilobites, although in different degrees of clearness 

 and under various modifications, a dendritic marking of the inner surface of the cheeks which takes its 

 rise at the lateral margins of the glabella and spreads thence like a bush over the entire surface of the 

 . cheeks. Exactly the same position is taken by the richly branched hepatic lobes of Limulus on the lower 

 surface of the head shield; a fact of special weight in favor of the homology and similar significance of the 

 two phenomena, is that in the trilobites also, the anterior of the two main ducts is the larger, the posterior 

 the smaller. The striking similarity of the two structures is shown by a comparison of the head shield of 

 Eurycare [Elyx] from the Cambrian of Sweden, in which the course of the canals is shown with remarkable 

 clearness [with those of Limulus]. 



I have been able to convince myself that the existence of the two canals on each side is also the rule in 

 other genera, even though the posterior pair is frequently but feebly developed or completely obscured by 

 the anterior pair. In Dionide formosa, for example, I find only the anterior pair, which is very large and 

 divided into two principal branches. From all these considerations it seems to me no longer doubtful that 

 the median elevation was caused by the stomach and gizzard, and that the cheeks have principally served to 

 cover the hepatic appendages of the alimentary canal. 



The cause of the incomplete development of the glabellar lobes lies, hence, in the intrusion of the 

 alimentary canal, and it makes naturally the most effect where the gizzard spreads out and bends into the 

 stomach. This spot lies behind the frontal lobe, which is hence increased in size according as the stomach 

 increases in size; in this way not only the foremost segments of the glabella become enlarged, but also the 

 following ones more or less pressed aside. This process is easily followed phylogenetically and ontogenetically. 



From the latter point of view, the development of Paradoxides is very instructive. In a head shield 

 2.5 mm. long the whole anterior part of the glabella is broadened, but the five pairs of lateral impressions 

 are clearly marked and the six segments of the head bounded by them are all of about the same size. In a 

 head shield about 13 mm. long, the foremost segment is very much increased in size, the jaw lobes pressed 

 still further apart; in adult forms both anterior segments are combined into the frontal swellings of the 

 glabella. In other groups this process proceeds phylogenetically still further, so that among the Phacopidae 

 and in Trinucleus, behind the frontal swelling of the glabella only the last cephalic segment retains a certain 

 independence. The frontal lobe is thus no definite part, although it is as a rule composed of the mesotergites 

 of the first two cranidial segments. 



