LIMULUS. 



297 



The trabeculae are most highly developed along the lateral margins of the 

 thoracic and branchial shields, and in the cornua. In these places the spaces 

 between the dorsal and ventral walls are filled with dense masses of this tissue. 

 (Fig. 199 C.) In the cornua the trabecular network of each wall is united to the 

 other by long columns with branching ends. (Fig. 199, C.) 



In the margins of the thoracic shield (Fig. 202, A) one sees how the new 

 trabeculse arise at separate points, unite, and later form new supports which 

 gradually lift the older bars off the surface. 



Fig. 203. — Section of the dermal skeleton of Limulus, tangential to the surface, showing the cancellous spaces, 

 and the axial location of the lacunae in the trabecule. Margin of thoracic shield. 



There are similar deposits of this tissue under the lateral eyes, enclosing each 

 eye within a bony orbit (Fig. 201, D,E), another beneath the median eyes (Fig. 201, 

 B), and six or seven pairs of irregular patches arranged symetrically on the dorsal 

 wall of the abdomen along the median margin of the six pairs of entapophyses. 

 (Fig. 205.) 



^ipr'TiWi 



pc 



■I. c. tr 



Fig. 204. — Cross-section of the dermal skeleton of Limulus. Margin of thoracic shield. 



Minute Structure. — The chitenous trabeculas often form a nearly continuous 

 sheet spread out over the inner surface of the shell, and differing from it to a very 

 marked degree, in color, texture, and general appearance. (Figs. 201, A and 204.) 



The shell, in such places, is divided into several layers: 



I. The outer layer is strongly laminated, and traversed by two or three kinds 

 of rather large canals, which contain ducts of mucous glands, or nerve fibers, and 

 other tissue. All these canals reach the outer surface, and either open freely to 

 the exterior, or lead into spines or hairs of various shapes. Between them are 



