MUSCULATURE. 91 



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 open- 

 ings 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 Isotclus. The endopodites of both Isotelus latus and /. maximus are fairly well pre- 

 served 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 arcnicola 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 



Fig. 28. Side view of a specimen of 

 Isotelus gigas Dekay, from which the test 

 of the pleural lobes has been broken to 

 show the position of the Panderian organs. 

 Natural size. Specimen in the Museum of 

 Comparative Zoology. 



Asaphidae may not have had internal gills, and whether some primitive member of the family 

 may not have given rise to tracheate arthropods. 



The explanation of the Panderian organs as openings of poison glands is less radical 

 than the one just set forth, and so possibly lies nearer the truth. One would expect poison 

 glands to lie at the bases of fangs, and so they do in specialized animals like chilopods and 

 scorpions, but the trilobites may have had the less effective method of pouring out the poison 

 from numerous glands. The purpose may have been merely to paralyze the brachiopod or 

 pelecypod which was incautious enough to open its shell in proximity to the asaphid. 



MUSCULATURE. 



This is. a field which is rather one for investigation than for exposition. Very little 

 has been done, though probably much could be. The chief obstacle to a clearer understand- 

 ing of the muscular system lies in the difficulty of getting at the inner surface of the test 

 without obscuring the faint impressions in the process. 



There exist in the literature a number of references to scars of attachment of muscles, 

 and any study of the subject should of course begin by the collection of such data. I shall 

 at this time refer to only a few observations on the subject. 



The structure and known habits of trilobites make it obvious that strong flexor and 

 extensor muscles must have been present, and some trace of them and of their points of 

 attachment should be found. It is likely that their proximal ends were tough tendons. The 

 muscles holding up the heart and alimentary canal would be less likely to reveal their pres- 



