208 STUDIES IN EVOLUTION 



evidently the gnathobases, as will be seen at once from a com- 

 parison with Triarthrus (Plate VIII, figure 1). 



Organs in the Median Line. 



The Hypostoma. There is nothing peculiar in the hypos- 

 toma of Triarthrus, since it presents features commonly 

 found in many other genera. It is longer than wide, and 

 extends more than half the length of the head. The posterior 

 end is narrowly rounded, margined by a slight doublure, and 

 often presents a transverse elevation near the apex, as shown 

 on Plate VIII, figure 9. This may represent a correspond- 

 ing hollow on the inner side to allow for movements of the 

 manducatory organs. 



In considering the exact location of the appendages of the 

 head, it must be understood that in their present positions 

 they are probably somewhat displaced. During the process 

 of fossilization the whole inner tissues of the animal were 

 removed without replacement, allowing the ventral membrane 

 to come more or less in contact with the under side of the 

 dorsal crust, and thus causing some stretching of the mem- 

 brane and consequent displacement of the organs. The 

 hypostoma, being more rigid and attached in front to the 

 margin of the head, doubtless was not shifted, but dropped 

 down into the cavity of the glabella. When raised to its 

 natural position, it probably extended a little over the mouth 

 parts. The fact that the mouth and lower lip do not come 

 opposite the organs correlated as mandibles may be due in 

 part to the unequal stretching of the membrane over the un- 

 even inner surface of the dorsal crust. The extended gnatho- 

 bases directed obliquely backward and lying in the axial 

 hollow cause the appendages to appear as though originating 

 further back than is really the case. Nevertheless the pos- 

 terior position of the second and third pairs of appendages, or 

 the antennae and mandibles, with respect to the mouth, does 

 not offer any serious difficulty. As shown by Lankester,* 



* Observations and Reflections on the Appendages and on the Nervous 

 System of Apus cancriformis. Quar. Jour. Mic. Sci., XXI, 1881. 



