TRILOBITA. 



523 



thoracic limbs as well. The abdominal or pygidial rings carried 

 appendages also, a pair to each segment, but these do not appear 

 to have differed in form from the thoracic limbs. With regard, fin- 

 ally, to the appendages of the head, the mouth is situated behind 

 the hypostome, and is bounded by four pairs of jointed manduca- 

 tory appendages, the basal joints of which are, partly or wholly, 

 modified to act as jaws. The hindmost cephalic appendages (fig. 

 379, b) are larger than the anterior ones, and may be regarded as 

 foot-jaws. 



In connection with the exoskeleton of the Trilobites, a few words 

 may be said as to the minute structure of the crust. Thin vertical 

 sections (fig. 380, b) show that the crust is traversed by vertical 

 canals or tubes of different sizes, the general tissue being finely 



Fig. 378. — Transverse section of the thorax of Calymenc seuaria, partially restored (after C. 

 D. Walcott). a, Dorsal crust ; b, Visceral cavity, continued laterally to the pleural margins of 

 the dorsal crust; c, Legs, restored; d, Epipodite; e, Spiral gills. Enlarged six times. 



tubulated so as to resemble somewhat the dentine of teeth, and 

 there being usually a system of comparatively large vertical tubes as 

 well. Tangential sections (fig. 380, a and c) may only exhibit the 

 openings of the larger set of tubes, or may show the minute and 

 close-set apertures of the small tubuli as well. Where a large set of 

 tubes is clearly developed, the minute structure of the crust is very 

 similar to that of the shell of the recent Limulus ; but the presence 

 of these large tubes cannot be regarded as a distinctive feature, since 

 similar canals are present in the shell of the Lobster, in parts at any 

 rate. 



With regard to the development of the Trilobites, the eggs have 

 been noticed by both Barrande and Walcott. They are spheroidal 

 or cylindroidal in shape, and mostly about a twenty-fifth to a fiftieth 

 of an inch in diameter, and they seem to have been deposited in 

 clusters. The larval condition of the Trilobites is only known in 



