70 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



five segments in the glabella of certain trilobites. In his table (p. 165) he has listed the 

 segments with their appendages as follows: I. Acron, with hypostoma; 2, rostrum (epis- 

 toma), with free cheeks; 3, first frontal lobe, with (?) antennules; 4, second frontal lobe, 

 with antennae; 5, mandibles; 6, first, or pre-maxillae ; 7, second maxilla; 8, occipital seg- 

 ment with maxillipeds. 



Jaekel refused to believe that the antennae of trilobites were really entirely simple, and 

 so homologized them with the antennae and not the antennules of other Crustacea. In this 

 he was obviously incorrect, but it accounts for his homology of the remainder of the cephalic 

 appendages. 



It is, at present, impossible to demonstrate the actual number of somites in the cephalon 

 of the trilobite, but I believe that Beecher was correct in holding that the glabella was 

 composed of four segments. There are, it is true, a number of trilobites (Mesonacidse, Para- 

 doxidae, Cheiruridae, etc.) which show distinctly four pairs of glabellar furrows, indicat- 

 ing five segments in the glabella. This is, however, probably due to a secondary division 

 of the first lobe. 



The correspondence of the five segments on the dorsal side with the five pairs of appen- 

 dages makes it unlikely that any pair of limbs has been lost. The condition in Marrella, 

 where a trilobite-like cephalon bears five pairs of appendages, the second pair of which are 

 tactile antenme, is favorable to the above interpretation. In spite of the apparent degener- 

 ation of the first two pairs of appendages in Calymene, no limbs are actually missing, and 

 if some are dropped out in the later trilobites it would not affect the homology of those 

 now known. I therefore agree with Beecher in homoldgizing the appendages, pair for pair, 

 with those of the higher Crustacea. 



FUNCTIONS OF THE APPENDAGES. 



Antennules. 



The antennules were obviously tactile organs, probably freely movable in most trilo- 

 bites, but in the case of Triarthrus perhaps rather rigid, judging from the great numbers of 

 specimens which show the characteristic sigmoid curve made familiar by Professor Beecher's 

 restoration. The proximal end of the shaft of each antennule of Triarthrus is hemispheric 

 and doubtless fitted into a socket, thus suggesting great mobility of the whole organ. In 

 spite of this, I have seen no specimens in which they did not turn in toward each other and 

 cross the anterior margin very near the median line. In front of the margin, various 

 specimens show evidence of flexibility, but from the proximal end to the margin the position 

 is the same in all specimens. 



In all the few specimens of Cryptolithus retaining the antennules, these organs are 

 turned directly backward, but it is entirely within the range of probabilities that while its 

 burrowing habits made this the more usual position, the animal had the power of turning 

 them around to the front when they could be used to advantage in that direction. 



Exopodites. 



It has been the opinion of most observers that the exopodites of trilobites were swim- 

 ming organs, while others have thought that they functioned also in aerating the blood. 

 To the present writer it seems probable that the chief function was that of acting as gills, 

 for which the numerous thin, flattened or blade-like setae are particularly adapted. That 



