JPrqfessor C. E. Beecher — Structure of Trilohites. 



161 



of a trilobite would indicate the line of division between the two 

 main ventral bundles. The first pair of oblique ridges on each side 

 would delimit the main bundles and side strands, and show that 

 these strands joined the main bundles obliquely within the cavity 

 of the next anterior somite, as in ordinary Crustacea. This accounts 

 for the anterior truncation of the triangular area between the median 

 and lateral ridsres in the trilobite. 



Fig. 8. — Diagram of the axial portions of three segments; showing the ventral 

 abdominal muscles, the flexors, represented as two heavy longitudinal lines, 

 together with the lateral strands attached to the sternal plate in each somite and 

 continuing obliquely forward to their union with the main bundle in the cavity 

 of the next anterior somite. 



The natui-e of the outside pair of oblique ridges is not so plain. 

 They may serve to divide the side ventral strands of the flexors 

 from the bundle of muscles running fi'om the proximal joints of the 

 legs to the dorsal test, or they may simply mark the outside of the 

 lateral strands. 



The apodemes in general seem more strongly developed anteriorly 

 in the thorax. Possibly, this condition may be explained on the 

 basis that the ventral pair of the great flexor muscles received new 

 strands at each segment from behind forward, so that near the 

 cephalon they became large bundles for which progressively larger 

 apodemes were formed. 



It may be remarked, in conclusion, that a similar though 

 apparently much simpler apodemal arrangement would be developed 

 if the myology of the trilobites agreed with that of the theoretical 

 crustacean ancestor, or that existing in some Isopods,Amphipods, etc., 

 in which there are no large longitudinal bundles, but motion between 

 the somites is effected by strands running from one segment to the 

 next anterior. If viewed in this manner, there would necessarily 

 be two median and two lateral strands. The previous explanation 

 seems to be moi'e in accordance with the structures actually seen 

 in the trilobites, which in general possessed the power of eni'olment 

 to a high degree, and would be expected to have had a well-developed 

 and efficient system of ventral muscles. 



Summary. — -The ventral integument in trilobites is a thin uncalcified 

 membrane, which may be divided into pleurosternites and meso- 

 sternites, corresponding to the mesotergites and pleurotergites of 

 the dorsal test, and like them connected segmentally by an 

 interarticular membrane. 



The mesosternites are usually marked by five longitudinal ridges, 

 or buttresses, representing thickenings of the membrane, which 

 may be homologized with apodemal structures in other Crustacea, 

 and not with the appendicular system. 



DECADE IV. 



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