No. I.] STUDIES ON LIMULUS. 19 



made any decided approach in chemical composition towards 

 true bony or cartilaginous structures. 



We can readily understand how such a sub-dermal framework 

 as we see in Limtilus might lose its connection with the epi- 

 dermis, and the latter form a continuous layer above it. Then 

 the hard superficial chitinous armor would no longer be needed, 

 either for the attachment of muscles or for protection, and 

 might disappear altogether, thus obviating the dangers of peri- 

 odically shedding the old chitinous exoskeleton. 



Thus Linmlus appears to have laid the foundation for an 

 elaborate system of internal supports ; namely, the primordial 

 cranium, the branchial cartilages, the cartilages of the spinal 

 cord, and the sub-ectodermal structures which resemble dermal 

 bones. These parts have different modes of origin, and differ 

 widely histologically from one another, yet they agree in all 

 essential features with the corresponding parts of the vertebrate 

 skeleton. In vertebrates we so frequently see these structures 

 welded together into a common framework for the whole body, 

 that we underestimate the importance of certain facts of verte- 

 brate ontogeny, which indicate more and more clearly that 

 the vertebrate skeleton is also composed of several parts quite 

 different in structure and origin. 



W. Patten. 



