LIMULUS. 



299 



The lacunae are best seen in old crabs where several trabeculae unite. At such 

 points they are very numerous and apparently vary a good deal in shape. (Fig. 

 206, B.) This is largely due to the fact that they are turned in various directions, 



FIG. 205. Inner surface of the shield of Limulus, showing the muscle markings and the distribution of the 

 bony trabeculae, the entapophyses, and the principal muscle markings. 



so that some are cut crosswise, others lengthwise. The lacunae are usually filled 

 with air and appear jet black. As the canaliculi enter the darker axial core 

 (Fig. 206, B.a.b.}, they become sinuous, and many side branches arise which 

 terminate in minute lacunae. Some of the latter appear to increase in size and 



