MUSCULATURE. 9 1 
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 Isotelus. 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 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 
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. 
Asaphida; 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- 
